Bio Unit 5
Select the characteristics that are to be expected of genetic material (DNA)
- able to store information - stable
A mRNA transcript
- can be used for translation - was produced by RNA polymerase - was made from a DNA template
Select the characteristics of translation
- results in a protein product - involves a mRNA molecule
What is the probability that two heterozygous individuals will produce a child that is recessive
1/4
What blood types and ratios can we expect the offspring to have if the parents have blood times AB and O
1: A, 1:B
If 30% of an organism's DNA is thymine, then
20% is guanine.
A condon is composed of ____ nucleotides, also called a triplet
3
If a non-disjunction event occurs during Meiosis I, how many daughter cells of the original parent cell that completed meiosis will have an abnormal chromosome number
4
What phenotypic ratio would you expect if the parents were TT and tt
4:0
Freckles (F) is dominant over no freckles (f). What percentage of offspring would be predicted to have freckles if the father was eheterozygous for freckles and the mother did not have freckles (Ff x ff)
50%
The genetic material for life must be ____ so it can be accurately replicated over and over again
A stable molecule
Arrange the steps of translation in order
A strand of mRNA attaches to a ribosome tRNA molecules bring amino acids that match with each mRNA codon A completed protein is released from the ribosome The protein folds into a specific shape based on the amino acids
Zygote
A zygote is a diploid cell resulting from the fusion of two haploid gametes.
Methemoglobinemia is a(n)
Autosomal recessive disorder
Each gamete contributes a single copy of each
Chromosome
An experimental cross in which the parents differ in two traits is called a ______ cross
Dihybrid
The probability of heads - tails - heads sequence in three successive coin tosses is ONE in
EIGHT
Trisomy 18
Edwards syndrome
During the phase of translation called _____, the polypeptide increases in size and one amino acid at a time
Elongation
The probability of a tails - heads sequence in two successive coin tosses is ONE in
FOUR
Eukaryotic DNA Replication
In eukaryotes, DNA replication begins at numerous origins of replication along the length of the linear chromosome, and replication bubbles spread bidirectionally until they meet. Notice in Figure 12.8b that there is a V shape wherever DNA is being replicated. This is called a replication fork.
Structure of Ribosomal RNA
In eukaryotes, ribosomal RNA (rRNA) is produced from a DNA template in the nucleolus of a nucleus. The rRNA is packaged with a variety of proteins into two ribosomal subunits, one of which is larger than the other. The subunits then move separately through nuclear envelope pores into the cytoplasm, where they join together at the start of translation (Fig. 12.16a). Once translation begins, ribosomes can remain in the cytoplasm, or they can become attached to endoplasmic reticulum.
A heterozygote displaying an intermediate variation of a trait--a phenotype in between that of individuals homozygotic for both alleles--- is an example of
Incomplete Dominance
Which description below explains epistatic inheritance
Inheritance in which one gene can override another
The first stage of translation, during which ribosomes assemble around mRNA is called
Initiation
Which of the following would contain the most nucleotide bases, if all from the same eukaryotic gene
Introns + exons
When a trait exists in several allelic forms within a population, the trait is referred to as being controlled by ___ ____
Multiple Alleles
Autosomal number disorders such as down syndrome are often the result of
Nondisjunction
Trisomy 13
Patau syndrome
Which human sex chromosome anomalies result form an individual having two or more X chromsomes
Poly-X syndrome Klienfelter syndrome
Ribosome
Site of translation
What happens if you set a low voltage during the electrophoresis
The migration of the DNA bands will be slow
Wild-type definition
The most common phenotype observed in a natural population for a given characteristic
In the term tRNA, the t stands for
Transfer
Polyribosomes are strings of ribosomes simutaneously _____ strand
Translating an mRNA
Which cross could produce a child with type A blood
Type A x Type O Type AB x Type AB Type AB x Type A
What mRNA sequence would be created from the DNA sequence ATCCGCTACGA
UAGGCGAUGCU
In fruit flies and other species, the ____ ____ is the most common phenotype of particular characteristic that is observed in nature
Wild-type
Cystic fibrosis is an example of a/an _________ trait.
autosomal recessive
The code for storing genetic information in DNA is based on the sequence of _____ in the DNA molecule
bases
Simple dominant and recessive patterns don't apply to most traits. Another genetic interaction is ___ ___ where heterozygous genotype is a mix between the alleles. Yet another example is ___ ___ traits that are found on the X and Y chromosomes.
blended inheritance sex linked
DNA replication is said to be semiconservative because
both of the new DNA molecules contain one new strand and one old strand.
Individuals on a pedigree with a half-colored shape are called
carriers
Addition of which reagent caused precipitation of DNA
cold ethanol
Across a DNA molecule, the nitrogenous bases form ___ pairs
complementary
RNA primarse
creates a starting point for replication
Cellular and nuclear membranes are emulsified in the filtrate by the addition of
detergent
two different alleles for a given gene
heterozygous
In order, the three steps of DNA replication are
initiation, elongation, termination
codominance
multiple dominant alleles
polygenic
multiple genes control a single trait
The monomers that make up DNA molecules are called
nucleotides
Phenotype
physical expression of alleles
A single gene produces a variety of phenotypes.
pleiotropic gene
XXX
poly-x syndrome
Multiple genes are involved, and the distribution resembles a bell-shaped curve.
polygenic trait
Chromosomes involved in determining the gender of an individual are called ____ chromosomes, all other chromosomes are referred to as ____
sex; autosomes
Anticodons are found on which nucleic acid
tRNA
During the phase of translation called ____, a completed polypeptide is released
termination
degrees of freedom (d.f.) is defined as
the number of classes (phenotypes) minus 1
Sister chromatids
the two copies of a duplicated chromosome
Why was electrophoresis buffer added to the gel
to facilitate conduction of the electrical current
TNF alpha - is a protein involved in the inflammatory and immune responses
true
Which of the following genotypes are homozygous
tt AA aa
Topoisomerase
unwinds the DNA molecule
Helicase
unzips the DNA molecule
Mendel's success was based on
use of the pea plant as a model organism. his ability to apply statistics to his studies. careful planning of his experiments.
allele
variations of a trait
RNA polymerase has all the following in common with DNA polymerase
- nucleotides are joined together - a growing strand runs in a 5' to 3' direction - nucleotides are added to the 3' end
What voltage is applied to the gel
1-5 volts per centimeter of gel
Gamete
A gamete is a haploid male or female germ cell which is able to unite with another of the opposite sex in sexual reproduction to form a zygote.
A standard gel apparatus for electrophoresis is
A rectangle that is open on top
Which of the following genotypes are heterozygous
Aa Qq
What gamete pairs could a parent contribute if their genotype is Aabb?
Ab ab
List the four nitrogenous bases found in DNA in alphabetical order
Adenine, Cytosine, Gaunine, Thymine
mRNA
Holds code for protein
What is the role of DNA polymerase in the DNA replication
It synthesizes a daughter strand that is complementary to the template strand It "proof reads" new daughter strands to be sure the bases are in the correct order
XYY
Jacob syndrome
genotype
Jj
An image of chromosomes arranged in pairs by shape and size is called a(n)
Karyotype
RNA Carries the Information
Like DNA, RNA (ribonucleic acid) is a polymer composed of nucleotides. The nucleotides in RNA, however, contain the sugar ribose and the bases adenine (A), cytosine (C), guanine (G), and uracil (U). In RNA, the base uracil replaces the thymine found in DNA (Table 12.3). Finally, RNA is single-stranded and does not form a double helix in the same manner as DNA (Fig. 12.9).
Metamorphosis
Metamorphosis is the process of transformation from an immature form to an adult form that goes through different stages.
In a ___ ___, only single base is altered. One example is a __ ___ where one base is changed. Another is a ___ ___ where a base is added or deleted
Point mutation Base substitution Frameshift mutation
Pre-mRNA
RNA that has not been process yet
Ribosomes attached to the endoplasmic reticulum are synthesizing proteins destined for
Secretion from the cell
What served as the control
TNF1 TNF2
The original parent strand from which a complementary strand is made is called the ___ strand
Template
Ribosomes
Translation
A triplet of nucleotides in tRNA is called a(n)
anti-codon
The list of alleles, such as Gg is known as
genotype
The term ___ refers to the specific genetic makeup of an individual
genotype
The term ____ refers to the specific genetic makeup of an individual
genotype
The total set of alleles for a trait is called a
genotype
Single-stranded binding protein (SSB)
keeps single stranded DNA apart
An observable or measurable characteristic of an individual organism is called a
phenotype
The physical expression of the genotype is the
phenotype
Purpose of ethanol
precipitation
The major types of RNA are tRNA, mRNA, and ___
rRNA
Pleitropy
single gene controls many traits
DNA polymerase
synthesizes DNA, removes RNA and proofreads DNA
DNA primase
synthesizes short RNA strands
What is the function of the promoter
- defines the start of transcription - defines the direction of transcription - guides RNA polymerase to the strand to be transcribed
Which of the following are true of transcription but not translation
- involves RNA polymerase - occurs in eukaryotic nuclei
Select the characteristics of translation
- involves a mRNA molecule - results in a protein product
Duchenne muscular dystrophy is an X-linked recessive disorder. If a mad with this disease has children with a women who is homozygous dominant for this allele, what percentage of their offspring would be expected to have the disease phenotype
0%
Unattached earlobes (E) is dominant over attached earlobes (e). What percentage of the offspring would be expected to have unattached earlobes if the father was heterozygous for this trait and the mother was homozygous dominant (Ee x EE).
100%
If a non-disjunction event occurs during meiosis I, how many daughter cells of the original parent cell that underwent meiosis will have an extra chromosome
2
The expected phenotypic ratio of the offspring of Aa x Aa cross is
3:1
The expected phenotypic ratio of the offspring of a CcDD x Ccdd cross is
3:1
In the genotype "AaBb" with alleles are dominant
A B
Arrange the steps of transcription in order
A cell is signaled to make a protein RNA polymerase attaches to the gene for the protein RNA polymerase creates a complementary strand of mRNA A completed mRNA strand is sent to the ribosomes
Which of the following statements are true for phenotypes
A characteristic that can be measured, such as blood type A characteristic that can be observed, such as eye color Based upon which combination of alleles an individual has inherited
Chromosome
A chromosome is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
Gene
A gene is a unit of heredity that is transferred from a parent to offspring and determines some characteristics of that offspring.
What is the anticodon that would match the mRNA codon UAG
AUC
If the sequence of bases in the coding strand of a DNA molecule is TAGC, then the sequence of bases in the mRNA will be
AUCG.
Which of the following statements are true of alleles
Alleles are found on homologous chromosomes Alleles determine traits Alleles are variations of genes
What is the best explanation as to why the number of guanines in an organism's DNA is equal to its number of cytosines
Because cytosine pairs with guanine in DNA
If two individuals appear normal for a genetic condition, but they produce a child who expresses the condition, the parents would be called _____ for that condition
Carriers
In an autosomal recessive disorder, healthy males will appear as _____ __________ on a pedigree
Clear Squares
Introduction
Deoxyribonucleic acid, or DNA, is the genetic material of all life on Earth. Each of the organisms in the photo above, and all of the estimated 2.3 million species on the planet, use a combination of four nucleotides (A, C, G, and T) to code for 20 different amino acids. The combination of these amino acids produces the tremendous variation that gives us the diversity of life.
DNA is structured as ______ _____. The monomers of DNA are _____
Double Helix Nucleotides
DNA is structured as a __ - ___, but RNA is a single ____ of nucleotides. Instead of thymine as one of the bases, RNA has ____.
Double Helix Strand Uracil
Trisomy 21
Down syndrome
Why must the DNA bands in the gel be viewed under the UV light
Ethidium bromide can only be seen under UV light
The coding portions of a gene that are joined during mRNA processing are called
Exons
Which of the following contain proper complementary DNA base pairs
G-C A-T
Why is it important to allow the gel used to cool completely
Gel must be solid to separate fragments of different sizes
Gene Expression
Gene expression is how genes are read to make an organism.
Gene Expression
Gene expression is the appearance in a phenotype of a characteristic or effect attributed to a particular gene.
Epistasis
Genes control expression of eachother
Genetic Modification
Genetic modification is an organism that contains genetic material that has been artificially altered so as to produce a desired characteristic.
Which of the following genotypes can be determined by a one-trait testcross
Heterozygous Homozygous Dominant
An organism carrying two identical alleles for a particular gene would be considered ____ for that trait
Homozygous
The genotype "PP" can be best described as
Homozygous
Which of the following terms are used with the term genotype
Homozygous Heterozygous Recessive Dominant
Epistatic Interactions
However, there is another gene, HERC2, that can override the instructions of the OCA2 gene. If individuals are homozygous for a recessive allele of HERC2, they will have blue eyes regardless of the genotype associated with OCA2. Geneticists call this type of interaction, where one gene can override another, an epistatic interaction.
Initiation
Initiation is the step that brings all the translation components together. Proteins called initiation factors are required to assemble the small ribosomal subunit, mRNA, initiator tRNA, and the large ribosomal subunit for the start of protein synthesis. Initiation is shown in Figure 12.17. In prokaryotes, a small ribosomal subunit attaches to the mRNA in the vicinity of the start codon (AUG). The first, or initiator, tRNA pairs with this codon. Then, a large ribosomal subunit joins to the small subunit (Fig. 12.17). Although similar in many ways, initiation in eukaryotes is much more complex. As already discussed, a ribosome has three binding sites for tRNAs. One of these is called the E (for "exit") site, second is the P (for "peptide") site, and the third is the A (for "amino acid") site. The initiator tRNA binds to the P site, even though it carries only the amino acid methionine (see Fig. 12.11). The A site is where tRNA carrying the next amino acid enters the ribosome, and the E site is for any tRNAs that are leaving a ribosome. Following initiation, translation continues with elongation and then termination.
The process by which sexually producing organisms reduce their chromosome number in half during the production of gametes is called
Meiosis
During the termination stage a completed ____ is released
Polypeptide
Strings of ribosomes simultaneously translating a mRNA strand are called
Polyribosomes
In the 1940s, scientists had concluded that the genetic material was most likely either ____ or ____
Protein / DNA
A triplet code for amino acids is made up of
RNA nucleotides
What is the name of the enzyme complex for synthesizing a new RNA molecule through nucleotide binding
RNA polymerase
Exons
RNA that will be used for translation
The independent assortment of allelic pairs during gamete formation, as described by mendels law of independent assortment, results from
Separation of homologous chromosomes during anaphase I
Which of the following occur in Meiosis I
Separation of homologs Crossing-over
Human X-Linked Disorders
Several X-linked recessive disorders occur in humans, including color blindness, Menkes syndrome, muscular dystrophy, adrenoleukodystrophy, and hemophilia.
Which of the following statements about meiosis are true
Sister chromatids separate during Anaphase II Nuclei reform in telophase II Homologous chromosomes separate during anaphase I
What is the relationship between voltage and migration rate
The higher the voltage the faster the movement
One-trait testcrosses are used ____
To determine if an individual is homozygous dominant or heterozygous for a particular trait
why is it important to wait a few minutes after adding meat tenderizer before continuing to the experiment
To give the meat tenderizer time to digest proteins
What is the purpose of the gel electrophoresis
To separate molecules in a solution by size
Widows peak (W) is an autosomal dominant hairline phenotype. A couple has six children, three with widows peak and three with straight hairline. If the mother is homozygous recessive for this trait, what is the genotype of the father
Ww
The double helix model of DNA resembles a twisted ladder in which the rungs of the ladder are
a purine paired with a pyrimidine.
The expected phenotypic ratio of the offspring of an Aa x AA cross is
all offspring have the dominant phenotype
A variation of a trait is called an
allele
What is the definition of a dominant allele
an allele whose phenotypic effect is expressed when present in one or two copies
Before pouring the liquid agarose solution into the casting tray, you must first
apply tape to the two open ends of the casting tray
Q1 and Q2 are different alleles of a single gene and are both completely expressed in a heterozygous individual. This type of inheritance is termed ___
codominant
An anticodon is the compliment of
codon
Each three letter sequence on mRNA is called a
codon
During protein synthesis, an anticodon on transfer RNA (tRNA) pairs with
messenger RNA (mRNA) nucleotide bases.
Anaphase II
sister chromatids are separated
During transcription, RNA polymerase uses the nucleotide sequence of the ____ to synthesize mRNA
template strand of DNA
A tRNA ___ - ___ allows the correct amino acid to be added based on the mRNA ____
Anti-codon Codon
In living organisms, information for making proteins flows from
DNA to RNA to protein
Genes are passed from parents to offspring through ____, which include sperm and eggs
Gametes
Which of the following are components of a DNA nucleotide
Nitrogenous base Phosphate Sugar
Recombinant DNA
Recombinant DNA is DNA that has been formed artificially by combining constituents from different organisms.
DNA polymerase
Replication
Jacob syndrome can occur as a result of nondisjunction in
Sperm formation
Filtrate must be kept cold to
Stabilize DNA
The parental strand of DNA serving as the guide for a complementary DNA daughter strand is called the ____ strand
Template
DNA size is identified by the number of base pairs (bp). A 100 bp DNA fragment and a 200 bp DNA fragment are separated by gel electrophoresis. Which band should be farther from the wells
The 100 bp fragment
What happens if ethanol is added to the test tube too quickly
The DNA precipitate will be broken up
Prokaryotic versus Eukaryotic Replication
The process of DNA replication is distinctly different in prokaryotic and eukaryotic cells (Fig. 12.8).
Overview of Transcription
The sequences of bases in a gene are transcribed into an mRNA molecule based on complementary base pairing: The T base in the DNA pairs with A in the mRNA, G with C, and A with U (note that uracil replaces T in the newly formed mRNA) (Fig. 12.12). When a gene is transcribed, a segment of the DNA helix unwinds and unzips, and complementary RNA nucleotides pair with DNA nucleotides of the strand opposite the gene. This strand is known as the template strand; the other strand is the nontemplate, or coding, strand. An RNA polymerase moves along the template strand in the 5′ direction. Like DNA polymerase, an RNA polymerase adds a nucleotide only to the 3′ end of the polymer under construction.
When observing the gel at the end of the run period, the DNA that is closest to the positive electrode is the
The smallest in size (length)
In a DNA molecule, adenine always pairs with ____ and guanine always pairs with ____
Thymine Cytosine
During DNA replication, the enzyme ____ unwinds the DNA. The enzyme ____ breaks the bonds between nitrogenous bases. The enzyme that actually reads the DNA adds the new correct bases is called ____ ____. The enzyme __ _____ creates a location for the replication to begin
Topoisomerase Helicase DNA Polymerase RNA primase
A ____, such as eye color, is a characteristic of an organism determined by its ____
Trait Genes
RNA polymerase
Transcription
If the sequence of DNA on the template strand of a gene is AAA, the mRNA codon produced by transcription will be _______ and will specify the amino acid _______ .
UUU, phenylalanine
Dominant trait
appear every generation
Sex-linked trait
appear mostly in either males or females
The complex formed by DNA wrapped around histone proteins is called
chromatin
Prophase I
chromosomes condense
DNA is a molecule found in the nucleus of ___ cells
eukaryotic
The underlying code for any trait is called a
gene
When only one copy of a chromosome is present in a diploid organism, such as the single X chromosome of human males, the condition is referred to as
hemizygous
_____ is an example of an Xlinked chromosome that produces a bleeding disorder
hemophilia
Why is it important to spin the glass rod in one direction only
it helps prevent DNA from breaking
The process by which individuals that have two copes of each gene, form gametes that contain only one copy of each gene is called
meiosis
Which of the following were direct contributions of the work of James Watson and Francis Crick
- Determined that nucleotides were paired inside the double helix - discovered the double helix shape of deoxyribonucleic acid - determined that sugars and phosphates formed the backbones in the DNA
Which of the following is/are true of both transcription and translation
- RNA is involved - several, identical products can be produced from one strand - involve nucleic acids - occur in eukaryotes and prokaryotes
Select the choices indicating similarities between RNA and DNA
- both are nucleic acids - both contain 4 types of nucleotide bases - both are polymers of nucleotides
Which of the following are characteristics of the genetic material that early scientists were seeking
- storage of genetic information - able to undergo mutations - a stable molecule
If a single a non-disjunction event occurs during Meiosis II, how many daughter cells of the original parent cell that underwent meiosis will have an extra chromosome
1
If a single a non-disjunction event occurs during Meiosis II, how many daughter cells of the original parent cell that underwent meiosis will have be missing a chromosome
1
A certain badger is heterozygous for two genes: he has sharp claws and bad breath. He mates with another badger that is homozygous for both genes, with sharp claws and minty-fresh breath. What are the chances that they have an offspring with sharp claws and minty-fresh breath? (Assume simple, mendelian inheritance)
1/2
In humans the dominant free earlobe phenotype is encoded by the F allele and Widows peak hairline is encoded by the W allele. what would be the expected phenotypic ratio for this characteristic in the offspring of an FfWw father and an ffww mother
1:1:1:1 free earlobes, widows peak: attached earlobes, straight hairline; free earlobes, widows peak; attached earlobes, straight hairline
If a non-disjunction event occurs during meiosis I, how many daughter cells that underwent meiosis will be missing a chromosome
2
A male and female are both carriers for a recessive disorder. What are the chances their child will have the disorder?
25%
Hemophilia B, a blood clotting disorder, is an X-linked recessive trait. What is the expected percentage of offspring with hemophilia that would be produced by a father who does not have hemophilia and a mother who is heterozygous for this trait
25%
On each strand of the DNA molecule, replication proceed from the __ prime end to the ___ prime end
3 5
Codon
3-base sequence on mRNA
Anti-codon
3-base sequence on tRNA
What is the expected phenotype ratio of a mendelian monohybrid cross
3:1
If the sequence of bases in the template strand of DNA is 5′ TAGCCT 3′, then the sequence of bases in the coding strand is
3′ ATCGGA 5′.
A male who has a dominant disorder, but is heterozygous for the trait, wants to have a child with a woman who does not have the disorder. What are the chances their child will have the disorder?
50%
A male who has a recessive disorder wants to have a child with a woman who is a carrier for the disorder. What are the chances their child will have the disorder?
50%
Color blindness is an X-linked recessive trait (X^b). What is the expected ratio of color blind children produced by a color blind man and a woman who is heterozgyous for this trait
50%
Next, when Mendel crossed two of the purple flowered offspring with each other, _____% of their offspring ended up having purple flowers and _____% ended up having white flowers
75 25
Accuracy of Replication
A DNA polymerase is very accurate and makes a mistake approximately once per 100,000 base pairs at most. This error rate, however, would result in many errors accumulating over the course of several cell divisions. DNA polymerase is also capable of checking for accuracy, or proofreading the daughter strand it is making. It can recognize a mismatched nucleotide and remove it from a daughter strand by reversing direction and removing several nucleotides. Once it has removed the mismatched nucleotide, it changes direction again and resumes making DNA. Overall, the error rate for the bacterial DNA polymerase is only 1 in 100 million base pairs!
What is the definition of a phenotype
A characteristic of an individual that can be observed or measured
Which of the following can be true about a phenotype
A characteristic that can be observed A characteristic that can be observed or measured A sequence of nucleotides that encodes a specific protein or RNA molecule
Summary of Gene Expression
A gene has been expressed once its product, a protein (or an RNA), is made and is operating in the cell. For a protein, gene expression requires transcription and translation (Fig. 12.20). Often, modifications to the protein occur after translation so as to prepare the protein for its specific function in the cell (see Section 13.2)
Gene
A gene is a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.
Genome
A genome is a haploid set of chromosomes. It is an organism's complete set of DNA and contains all the information needed to build and maintain an organism.
RNA Molecules Undergo Processing
A newly formed RNA transcript, called a pre-mRNA, is modified or processed before leaving the eukaryotic nucleus. For example, the molecule receives a cap at the 5′ end and a tail at the 3′ end (Fig. 12.14). The cap is a modified guanine (G) nucleotide that helps tell a ribosome where to attach when translation begins. The tail consists of a chain of 150-200 adenine (A) nucleotides. This poly-A tail facilitates the transport of mRNA out of the nucleus, helps initiate loading of ribosomes and the start of translation, and delays degradation of mRNA by hydrolytic enzymes. When the mRNA is first made by RNA polymerase from the gene, it is in a rough form. Called pre-mRNA, it contains a mix of exons (protein-coding regions) and introns (non-protein-coding regions), particularly in multicellular eukaryotes. Because only the exons of the pre-mRNA will be contained in the mature mRNA, the introns, which occur in between the exons, must be spliced out. An easy way to remember this is with the phrase "exons are expressed and introns are in the way." In lower eukaryotes, introns are removed by "self-splicing"—that is, the intron itself has the capability of enzymatically splicing itself out of a pre-mRNA. In higher eukaryotes, the RNA splicing is done by spliceosomes, which contain small nuclear RNAs (snRNAs). By means of complementary base pairing, snRNAs are capable of identifying the introns to be removed. A spliceosome utilizes a ribozyme (enzyme made of RNA rather than just protein) to cut and remove the introns. Following splicing of the exons together and the addition of the 5′ cap and 3′ poly-A tail, an mRNA is ready to leave the nucleus and be translated into a protein.
Which of the following would contain the fewest base pairs, if starting with the same size eukaryotic gene
A post-modification mRNA transcript
which of the following are true of a gene
A sequence of DNA that encodes a functional product A unit of heredity
Alleles are
Alternate forms of a gene
The order of nucleotides in the DNA molecule encodes the order of ____ ____ in cellular proteins
Amino Acids
For genes that code for proteins, the mRNA sequence determines the sequence of ____ in a protein
Amino acids
Allele
An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
What is the definition of recessive
An allele whose phenotypic effect is exerted when present in two copies
Which of the following statements are true for genotypes
An individuals complete genetic makeup The specific alleles that determine a particular phenotype
During which stages of meiosis can nondisjunction occur
Anaphase I Anaphase II
When two molecular strands are connected but orientated in opposite directions, the strands are said to to be ____
Antiparallel
Replication of DNA
As soon as Watson and Crick developed their double helix model, they commented, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material." The term DNA replication refers to the process of copying a DNA molecule. Following replication, there is usually an exact copy of the parental DNA double helix. A template is most often a mold used to produce a shape complementary to itself. During DNA replication, each DNA strand of the parental double helix serves as a template for a new strand in a daughter molecule. In the 1950s, Matthew Meselson and Frank Stahl used radioactive isotopes of nitrogen to determine that DNA replication is an example of semiconservative replication because each daughter DNA double helix contains an old strand from the parental DNA double helix and a new strand.
Prokaryotic DNA Replication
Bacteria have a single circular loop chromosome, whose DNA must be replicated before the cell divides. In some circular DNA molecules, replication moves around the DNA molecule in one direction only. In others, as shown in Figure 12.8a, replication occurs in two directions. The process always occurs in the 5′ to 3′ direction. The process begins at the origin of replication, a specific site on the bacterial chromosome. The strands are separated and unwound, and a DNA polymerase enzyme binds to each side of the opening and begins the copying process. When the two DNA polymerases meet at a termination region, replication is halted, and the two copies of the chromosome are separated.
The Watson and Crick Model
Based on previous work of other scientists, Watson and Crick knew that DNA is a polymer of nucleotides, but they did not know how the nucleotides were arranged within the molecule. However, by constructing models, they deduced that DNA is a double helix with sugar-phosphate backbones on the outside and paired bases on the inside (Fig. 12.5c). This arrangement fits the mathematical measurements provided by Franklin's X-ray diffraction data for the spacing between the base pairs (0.34 nm) and for a complete turn of the double helix (3.4 nm). According to Watson and Crick's model, the two DNA strands of the double helix are antiparallel, meaning that the sugar-phosphate groups that are chained together to make each strand are oriented in opposite directions. As seen in Figure 12.5d, each nucleotide possesses a phosphate group located at the 5′ position of the sugar. Nucleotides are joined together by linking the 5′ phosphate of one nucleotide to a free hydroxyl (-OH) located at the 3′ position on the sugar of the preceding nucleotide, giving the molecule directionality. Antiparallel simply means that while one DNA strand runs 5′ to 3′, the other strand runs in a parallel but opposite direction. This model also agreed with Chargaff's rules, which state that A = T and G = C. Figure 12.5d shows that A is hydrogen-bonded to T, and G is hydrogen-bonded to C. This complementary base pairing means that a purine (large, two-ring base) is always bonded to a pyrimidine (smaller, one-ring base). This antiparallel pairing arrangement of the two strands ensures that the bases are oriented properly, so that they can interact. The consistent spacing between the two strands of the DNA was detected by Franklin's X-ray diffraction pattern, because two pyrimidines together are too narrow, and two purines together are too wide.
What is the best explanation as to why the number of adenines if an organism's DNA is equal to the number of thymines
Because adenine pairs with thymine in DNA
When following traits through generations of a plant lineage, true- ____ means that the offspring are alike and have the same traits as the parents
Breeding
A sample mixed with bromophenol blue is run using agarose gel at a standard voltage and time. The results yield bands of 200 bp, 50bp and bromophenol blue. Which of the three will migrate the shortest distance
Bromophenol blue
Structure of DNA
By the early 1950s, DNA was widely accepted as the genetic material of all living organisms. However, the structure of DNA was not known. How can a molecule with only four different nucleotides produce the great diversity of life on Earth? To understand the structure of DNA, we need to understand how the bases in DNA are composed. Investigators knew that DNA contains four different types of nucleotides: two with purine bases, adenine (A) and guanine (G), which have a double ring; and two with pyrimidine bases, thymine (T) and cytosine (C), which have a single ring (Fig. 12.3). Erwin Chargaff used new chemical techniques developed in the 1940s to analyze in detail the base content of DNA. A sample of Chargaff's data (Table 12.1) shows that while some species—E. coli and Zea mays (corn), for example—do have approximately 25% of each type of nucleotide, most do not, and that the DNA of various species differs. For example, in humans the A and T percentages are about 31%, but in fruit flies these percentages are about 27%. Therefore, the nucleotide content of DNA is not fixed across species, and DNA does have the variability between species required for it to be the genetic material. Within each species, however, DNA was found to have the constancy required of the genetic material—that is, all members of a species have the same base composition. Also, the percentage of A always equals the percentage of T, and the percentage of G equals the percentage of C. It follows that if the percentage of A + T equals 40%, then the percentage of G + C equals 60%. These relationships are called Chargaff's rules. Although only one of four bases is possible at each nucleotide position in DNA, the sheer number of bases and the length of most DNA molecules are more than sufficient to provide for variability. For example, it has been calculated that each human chromosome typically contains about 140 million base pairs. This provides for a staggering number of possible sequences of nucleotides. Because Page 212any of the four possible nucleotides can be present at each nucleotide position, the total number of possible nucleotide sequences is 4(140 × 106), or 4140,000,000. This amount of variability explains how each species has its own unique base percentages.
DNA: The Transforming Substance
By the time the next group of investigators, led by Oswald Avery in the 1940s, began their work, it was known that the genes are on the chromosomes and that the chromosomes contain both proteins and nucleic acids. Investigators were having a very heated debate about whether protein or DNA was the genetic material. Many thought that the protein component of chromosomes must be the genetic material because proteins contain up to 20 different amino acids that can be sequenced in any particular way. On the other hand, nucleic acids—DNA and RNA—contain only four types of nucleotides as basic building blocks. Some argued that DNA did not have enough variability to be able to store information and be the genetic material.
tRNA
Carries amino acids to ribosome
Enter the right order of events that transforms a genetic code into a specific protein sequence in a eukaryotic cell
Coding DNA strand Transcription Complementary mRNA strand Translation Synthesized protein
A(n) ______ is a three-nucleotide sequence in mRNA that codes for a particular acid
Condon
Which of the following is a likely source of error resulting from the failure to remove the adult flies from the experiment after the eggs were deposited
Counting the adults would have increased the number of WT wing flies in the observed, resulting in skewed data
Cystic Fibrosis
Cystic fibrosis (CF) is the most common lethal genetic disease among Caucasians in the United States (Fig. 11.12). About 1 in 20 Caucasians is a carrier, and about 1 in 2,000 newborns has the disorder. CF patients exhibit a number of characteristic symptoms, the most obvious being extremely salty sweat. In children with CF, the mucus in the bronchial tubes and pancreatic ducts is particularly thick and viscous, interfering with the function of the lungs and pancreas. To ease breathing, the thick mucus in the lungs has to be loosened periodically, but still the lungs frequently become infected. The clogged pancreatic ducts prevent digestive enzymes from reaching the small intestine, and to improve digestion, patients take digestive enzymes mixed with applesauce before every meal.
In living organisms, which nucleic acid functions as the genetic material
DNA
The molecule that holds the genetic code is called
DNA
The enzyme responsible for separating double-stranded DNA into single-stranded DNA is
DNA helicase.
The enzyme that links DNA nucleotides together to form a new daughter strand is called
DNA polymerase
Which enzyme is used in DNA replication
DNA polymerase
In the ladder analogy, which of the following form the sides of the ladder
Deoxyribose Sugar Phosphate groups
What are the parts of a nucleotide from the sides of a DNA molecule
Deoxyribose sugar Phosphate
An individual genotype QqBb bred with an individual of genotype QqBb is an example of a(n) ____ cross
Dihybrid
a ___ cross is mating between parents who have non-identical alleles for two different genes of interest
Dihybrid
Transformation of Bacteria
During the late 1920s, the bacteriologist Frederick Griffith was attempting to develop a vaccine against Streptococcus pneumoniae (pneumococcus), which causes pneumonia in mammals. In 1931, he performed a classic experiment with the bacterium. He noticed that when these bacteria are grown on culture plates, some, called S strain bacteria, produce shiny, smooth colonies and others, called R strain bacteria, produce colonies that have a rough appearance. Under the microscope, S strain bacteria have a capsule (mucous coat) that makes them smooth, but R strain bacteria do not. Finally, Griffith injected the mice with a mixture of heat-killed S strain and live R strain bacteria. Most unexpectedly, the mice died—and living S strain bacteria were recovered from the bodies! Griffith concluded that some substance necessary for the bacteria to produce a capsule and be virulent must have passed from the dead S strain bacteria to the living R strain bacteria, so that the R strain bacteria were transformed (Fig. 12.1d). This change in the phenotype of the R strain bacteria must have been due to a change in their genotype. Indeed, couldn't the transforming substance that passed from S strain to R strain be genetic material? Reasoning such as this prompted investigators at the time to begin looking for the transforming substance to determine the chemical nature of the genetic material.
Gene Expression: Transcription
During transcription, a segment of the DNA serves as a template for production of an RNA molecule. Although all types of RNA are produced by transcription, we will focus in this section on mRNA transcription, since this is the type of RNA that eventually leads to the production of a protein.
Elongation
Elongation is the stage during protein synthesis when a polypeptide increases in length one amino acid at a time. In addition to the necessary tRNAs, elongation requires elongation factors, which facilitate the binding of tRNA anticodons to mRNA codons within a ribosome.
Fertilization
Fertilization is the process in sexual reproduction that involves the union of male sperm and female ovum gametes, each with a single, haploid set of chromosomes, to produce a diploid zygote.
Place the stages of the fruit fly life cycle in the correct order
Fertilized egg First instar larvae Second instar larvae Third instar larvae Pupa Adult
Autosomal Recessive Disorders
Figure 11.10 provides an example of the inheritance of an autosomal recessive trait in a family. Notice in the third generation that two closely related individuals (III-1 and III-2) have produced three children, two of which have the affected phenotype. In this case, a double line denotes consanguineous reproduction, or inbreeding, which is reproduction between two closely related individuals. This illustrates that inbreeding significantly increases the chances of children inheriting two copies of a potentially harmful recessive allele.
Functions of Introns
For many years, scientists thought that introns were simply wasted space within genes. Now, we realize they serve several key functions in the cell. The presence of introns allows a cell to choose which exons will go into a particular mRNA (see Section 13.2). Just because an mRNA has all the exons in its pre-mRNA doesn't mean they will all make it to the final product. For example, if a gene has three exons, then depending on cell need and environmental conditions, it may produce an mRNA with exons 1 and 2 only, or 1 and 3 only, or 1, 2, and 3. This ability is called alternative mRNA splicing, and it increases the flexibility and efficiency of the cell. The snRNAs of the spliceosomes that excise the introns play an important role in alternative splicing in eukaryotes.
The father of modern genetics was _____ ____. He studied traits in ____ such as seed shape and flower color. Mendel produced ___ ___ strains fro each trait that always produced the same variety when bred to each other.
Gregor Mendel Peas True-breeding
___ studies how ___ are passed from parents to offspring
Heredity Traits
Heredity
Heredity is the passing of genetic factors from parent to offspring, or from one generation to the next. Heredity is the means by which an offspring acquires the qualities of its parents.
Blended inheritance
Heterozygote is a mix of alleles
If an organism carries two different types of alleles for the same gene, it is considered ____ for that gene
Heterozygous
Mendel's Experimental Design and Results
Figure 11.3 graphically represents Mendel's crosses. These diagrams are commonly referred to as Punnett squares, after the 20th century geneticist Reginald Punnett, who developed the diagrams to teach genetics to his students. In a Punnett square, all possible types of sperm are lined up vertically and all possible types of eggs are lined up horizontally (or vice versa), and every possible combination of gametes occurs within the squares. Notice that in Mendel's crosses, all the F1 offspring resembled the tall parent. Certainly, these results were contrary to those predicted by the blending theory of inheritance. Rather than being intermediate, all the F1 plants were tall and resembled only one parent. Did these results mean that the other characteristic (shortness) had disappeared permanently? Apparently not, because when Mendel allowed the F1 plants to self-pollinate, three-fourths of the next generation of offspring, or F2 generation, were tall and one-fourth were short, a 3:1 ratio (Fig. 11.3). Mendel inferred that the F1 plants were able to pass on a factor for shortness—it didn't disappear; it just skipped a generation. Because the F1 plants were tall but clearly still contained the shortness characteristic, Mendel deduced that tallness was dominant to shortness (Fig. 11.3). Mendel counted many plants in his plant height and other experiments. When he allowed the F1 pea plants (which were all tall but carried the characteristic for shortness) to self-fertilize and produce offspring, he counted a total of 1,064 plants, of which 787 were tall and 277 were short. This type of experiment is called a monohybrid cross (L. mono, "single"; hybrida, "mixture"), because it is a cross of a single trait (plant height) with organisms that are a hybrid (tall and short characteristics). In fact, in all monohybrid crosses that he performed for the traits shown in Figure 11.4, he found approximately a 3:1 ratio in the F2 generation. In a monohybrid cross of two heterozygotes, assuming a simple dominant/recessive relationship, the expected phenotypic ratio is 3:1.
The two strands of a DNA double helix are held together by
Hydrogen bonds
Overview of Gene Expression
In the genetic flow of information, two major steps are needed to convert the information stored in DNA into a protein that supports body function (Fig. 12.10). First, the DNA undergoes transcription (L. trans, "across"; scriptio, "a writing"), a process by which an RNA molecule is produced based on a DNA template. DNA is transcribed, or copied base by base, into mRNA, tRNA, and rRNA. Second, during translation (L. trans, "across"; latus, "carry or bear"), the mRNA transcript is read by a ribosome and converted into the sequence of amino acids in a polypeptide. Like a translator who understands two languages, the cell changes a nucleotide sequence into an amino acid sequence. Together, the flow of information from DNA to RNA to protein to trait is known as the central dogma of molecular biology.
List the steps of translation in the correct order
Initiation Elongation Termination
What is the purpose of the bromophenol blue dye in the samples
It allows you to visualize how far the sample has migrated
What is the purpose of the plexiglass lid on the UV gel viewer
It filters out UV wavelengths that can damage your eyes
Why did DNA form a white precipitate in the test tube
It is insoluable in cold ethanol
A certain portion of a protein has the amino acid sequence Lys-Gly-Leu. What was the original source of this particular sequence
It was directed by bases on the template DNA
XXY
Klienfelter syndrome
Function of a Ribosome
Like RNA polymerase during transcription, multiple ribosomes often attach and translate the same mRNA at one time. As soon as the initial portion of mRNA has been translated by one ribosome, and the ribosome has begun to move down the mRNA, another ribosome can attach to the mRNA. The entire complex of mRNA and multiple ribosomes is called a polyribosome (Fig. 12.16d), and it greatly increases the efficiency of translation.
75% dominant and 25% recessive
Ll X Ll
Which of the following is not correct for an autosomal recessive pedigree?
Males inherit the trait 50% of the time.
In which stage of meiosis are sister chromatids separated
Meiosis II
You have observed the four daughter cells produced by a parent cell that has undergone meiosis. You note that two of the daughter cells have the expected number of chromsomes, one daughter cell is missing one chromosome, and one daughter cell has an extra chromosome. Which of the following is the most likely explanation for this observation
Meiosis II
Meiosis
Meiosis is cell division necessary for sexual reproduction.
There are three major classes of RNA that function in protein synthesis.
Messenger RNA (mRNA) takes a message from DNA in the nucleus to the ribosomes in the cytoplasm. Transfer RNA (tRNA) transfers amino acids to the ribosomes. Ribosomal RNA (rRNA), along with ribosomal proteins, makes up the ribosomes, where polypeptides are synthesized.
Methemoglobinemia
Methemoglobinemia is a relatively harmless disorder that results from an accumulation of methemoglobin in the blood. Hemoglobin, the main oxygen-carrying protein in the blood, is usually converted at a slow rate to an alternate form called methemoglobin. Unlike hemoglobin, which is bright red when carrying oxygen, methemoglobin has a bluish color, similar to that of oxygen-poor blood. Although this process is harmless, individuals with methemoglobinemia are unable to clear the abnormal blue protein from their blood, causing their skin to appear bluish-purple (Fig. 11.11).
A genetic cross of a single trait with organisms that are a hybrid is called a
Monohybrid cross
What are the three components of nucleotide
Nitrogenous Base Phosphate Deoxyribose Sugar
Which part of the nucleotide forms the steps of a DNA molecule
Nitrogenous base
You have observed the four daughter cells produced by a parent cell that has undergone meiosis. You note that all of the daughter cells have either one extra chromosome, or are missing one chromosome. Which of the following is the most likely explanation for this observation
Nondisjunction in meiosis I
The Genetic Code
Now that we know that the DNA sequence within a gene is transcribed into an RNA molecule and, for genes that code for proteins, the mRNA sequence determines the sequence of amino acids in a protein, it becomes necessary to identify the specific genetic code for each of the 20 amino acids found in proteins. Although scientists knew that DNA somehow directed protein production, they did not initially know specifically how the code was translated. This discovery was made in the 1960s. Logically, the genetic code would have to be at least a triplet of nucleotides. This means that each coding unit, or codon, would need to be made up of three nucleotides. The reason is that fewer nucleotides would not provide sufficient variety to encode 20 different amino acids. In 1961, Marshall Nirenberg and J. Heinrich Matthei performed an experiment that laid the groundwork for cracking the genetic code. First, they found that a cellular enzyme could be used to construct a synthetic RNA (one that does not occur in cells), and then they found that the synthetic RNA polymer could be translated in a test tube that contained the cytoplasmic contents of a cell. Their first synthetic RNA was composed only of uracil, and the protein that resulted was composed only of the amino acid phenylalanine. Therefore, the mRNA codon for phenylalanine was known to be UUU. Later, they were able to translate just three nucleotides at a time; in that way, it was possible to assign an amino acid to each of the mRNA codons (Fig. 12.11). The genetic code is degenerate. This term means that most amino acids have more than one codon; leucine, serine, and arginine have six different codons, for example. The degeneracy (redundancy) of the code helps protect against potentially harmful mutations. The genetic code is unambiguous. Each triplet codon has only one meaning. The code has start and stop signals. There is only one start signal, but there are three stop signals. With a few exceptions, the genetic code (Fig. 12.11) is universal to all living organisms. In 1979, however, researchers discovered Page 218that the genetic code used within the mitochondria, chloroplasts, and some archaebacteria, differs slightly from the more familiar genetic code. The universal nature of the genetic code provides strong evidence that all living organisms share a common evolutionary heritage. Furthermore, since basically the same genetic code is used by all living organisms, it is possible to transfer genes from one organism to another. In Section 14.2 we will explore the implications of this with regards to the development of biotechnology products by many commercial and medicinal companies.
Which of the following apply to the process of transcription during protein synthesis
Occurs in nucleus RNA polymerase DNA --> mRNA
Ribosomes participating in production of proteins destined for secretion from the cell are located
On the endoplasmic reticulum
After many cross what will yield only puprle offspring (P)
PP x pp PP x Pp PP x PP
Translation Requires Three Steps
Protein synthesis involves three steps: initiation, elongation, and termination. Enzymes are required for each of the three steps to function properly. The first two steps, initiation and elongation, require energy.
Protein synthesis is also called gene expression. It is the process where DNA is read to create ____. The two phases of protein synthesis are ___ and ____
Proteins Transcription Translation
When Mendel crossed a true-breeding purple flowered pea with a true-breeding white flowered pea, all of the offspring (F1 generation) had ____ flowers. This showed the purple flower allele was ____ and the white flower allele was ____
Purple Dominant Recessive
If true-breeding purple flower was crossed with another true-breeding purple flower, they would produce offspring with ___ flowers. In his breeding experiments, Mendel called offspring produced from two true-breeding parents the F1 ____
Purple Generation
In Mendel's flower example: A pea with two alleles for purple flowers would have ____ flowers A pea with one allele for purple flowers and one allele for white flowers would have ____ flowers A pea with two alleles for white flowers would have ___ flowers
Purple Purple White
Thymine and cytosine are nitrogen-containing bases called
Pyrimidines
Introns
RNA that will be removed before translation
The process of copying a DNA molecule so that there are two copies of the same DNA is called DNA ____
Replication
The cellular basis of Mendels law of segregation is the
Separation of homologous chromosomes during Anaphase II of meiosis
Why must the tape be removed from the casting tray before placing it in the electrophoresis apparatus
Tape prevents buffer from surrounding the gel
Termination
Termination is the final step in protein synthesis. During termination, as shown in Figure 12.19, the polypeptide and the assembled components that carried out protein synthesis are separated from one another. Termination of polypeptide synthesis occurs at a stop codon—that is, a codon that does not code for an amino acid. Termination requires a protein called a release factor, which can bind to a stop codon and cleave the polypeptide from the last tRNA. After this occurs, the polypeptide is set free and begins to fold and take on its three-dimensional shape. The ribosome dissociates into its two subunits, which are returned to the cytoplasmic pool of large and small subunits, to be used again as necessary. Overall, proteins do the work of the cell, whether they reside in a membrane within the cell or are free in the cytoplasm. The field of biology called proteomics is dedicated to understanding the structure of proteins and how they function in metabolic pathways (see Section 14.4). One of the important goals of proteomics is to understand how proteins are modified in the endoplasmic reticulum and the Golgi apparatus.
Punnett Square
The Punnett Square is a square diagram that is used to predict the genotypes of a particular cross or breeding experiment. It is used to determine the probability of an offspring having a particular genotype.
The Genetic Material
The middle of the twentieth century was an exciting period of scientific discovery. On one hand, geneticists were busy determining that DNA (deoxyribonucleic acid) is the genetic material of all living organisms. On the other hand, biochemists were in a frantic race to describe the structure of DNA. The classic experiments performed during this era set the stage for an explosion in our knowledge of modern molecular biology. Able to store information that pertains to the development, structure, and metabolic activities of the cell or organism Stable, so that it can be replicated with high accuracy during cell division and be transmitted from generation to generation Able to undergo rare changes, called mutations, that provide the genetic variability required for evolution to occur
Why is DNA often described as having a structure that looks like a ladder
The pair bases resemble the rungs of a ladder The sugars and phosphates resemble the sides of a ladder
Overview of DNA Replication
The process of DNA replication requires three main events: the unwinding of the DNA molecule, complementary base pairing, and the joining of the new DNA strands (Fig. 12.7). At the molecular level, several enzymes and proteins participate in the synthesis of the new DNA strands (Table 12.2).
What were the purposes of this experiment?
The purpose of this experiment was to determine if brightly colored zebrafish are genetically modified to get their bright colors and if they can pass that trait onto their offspring. We tried for over six months just to get the zebrafish to breath. It took a long time, but finally got five, three-week old baby zebrafish. We used some normal colored zebrafish, grey with greyish-black stripes, and some fluorescent orange, yellow and pink zebrafish. There was also a mix of males and females. Supposedly there was a genetic modification that was added to the fluorescent fish to make them express those colors, but there was also a lot of rumors on the internet that it was not a genetic modification and that it was just dye that was injected into the zebrafish or that the fish are infertile and they can't even reproduce. So, we did an experiment where we crossed the fish in an attempt to figure out if they are genetically modified and/or can pass that trait onto their offspring.
What was the purpose of this experiment? Describe how it was performed. (metamorphosis, 1st bionow)
The purpose of this experiment was to take the larva from inside a piece of wood inside into warmer conditions and see if it would cause it to do whatever metamorphosis they do and change into an insect, and then see what insect it became. The warmth of being inside was supposed to help to trigger the larvae into completing the rest of the life cycle stages. The process of the experiment and how it was performed was relatively simple. The experiment was started by chopping wood, which was done in the winter to burn in the house. It was noticed that when the wood was being chopped, in the center there were little grubs or larvae. The wood was put in a wheelbarrow and a few pieces of wood were picked out and brought inside with the grubs in it. After the wood was brought in from the cold, after just a few minutes the grubs had already started to move around. The pieces of wood were placed into a big box and a cover was then placed on the top. Then after that, it was basically letting the larvae sit in the box while we wait to see what would happen and see if any insects would appear in the box. It can take years for organisms to complete the stages of the life cycle, and waiting years was not in the plan, but we did wait about 6 months due to forgetting about the box for a while. At the 6 month mark, there was a buzzing sound that was heard, which is when we realized we had forgotten about the box and that was when it was time to open it up and see what had happened after that amount of time to the larvae that was originally placed in the box.
Which of the following is true for genotypes
The specific alleles that determine a particular phenotype
The Role of Transfer RNA
There is at least one tRNA molecule for each of the 20 amino acids found in proteins. The amino acid binds to the 3′ end. The opposite end of the molecule contains an anticodon, a group of three bases that is complementary and antiparallel to a specific mRNA codon. For example, a tRNA that has the anticodon 5′ AAG 3′ binds to the mRNA codon 5′ CUU 3′ and carries the amino acid leucine. In the genetic code, 61 codons specify amino acids; the other 3 serve as stop sequences (see Fig. 12.11).
The process of forming mRNA from the template strand of DNA is called
Transcription
Stages of Transcription
Transcription begins when RNA polymerase attaches to a region of DNA called a promoter (Fig. 12.12). A promoter defines the start of transcription, the direction of transcription, and the strand to be transcribed. The binding of RNA polymerase to the promoter is the initiation of transcription. The RNA-DNA association is not as stable as the two strands in the DNA helix. Therefore, only the newest portion of an RNA molecule that is associated with RNA polymerase is bound to the DNA, and the rest dangles off to the side. Elongation of the mRNA molecule occurs as the RNA polymerase reads down the DNA template strand in the 3′ to 5′ direction and continues until RNA polymerase comes to a DNA stop sequence, where termination occurs. The stop sequence causes RNA polymerase to stop transcribing the DNA and to release the mRNA molecule, now called an mRNA transcript. It is not necessary for RNA polymerase (Fig. 12.13a) to finish making one mRNA transcript before it starts another. As long as they have access to the gene's promoter, many RNA polymerase molecules can be working one after the other to produce mRNA transcripts at the same time (Fig. 12.13b). This allows the cell to produce many thousands of copies of the same mRNA molecule, Page 219and eventually many copies of the same protein, within a shorter period of time than if a single mRNA copy were used to direct protein synthesis. This ability to rapidly express the gene enables the cell (and the organism) to better respond to changing environmental conditions and have a greater chance at survival.
What determines the sequence of an rRNA strand
Transcription from DNA template
Transcription
Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA. DNA safely and stably stores genetic material in the nucleic of cells as a reference, or template.
Match the following processes with the correct description
Transcription: DNA is used as a template to produce RNA Translation: RNA is used to produce protein Replication: DNA is used as a template to produce duplicate molecules of DNA
Translation
Translation is the process of translating the sequence of a messenger RNA molecule to a sequence of amino acids during protein synthesis. The genetic code describes the relationship between the sequence of base pairs in a gene and the corresponding amino acid sequence that it encodes.
Gene Expression: Translation
Translation, which takes place in the cytoplasm of eukaryotic cells, is the second step needed to express a gene into a protein. During translation, the sequence of codons (nucleotide triplets) in the mRNA is read by a ribosome, which connects the sequence of amino acids dictated by the mRNA into a polypeptide. The process is called translation because it requires the conversion of information from a nucleic acid language (DNA and RNA) into an amino acid language (protein).
Down syndrome, patu syndrome, and edwards syndrome are all caused by three copies of a given chromosome. the correct term for three copies of a chromosome is
Trisomy
(T/F) A change in the DNA code with result in a change in the RNA code
True
(T/F): homologous chromosomes have the same locus for a particular gene
True
(T/F): the genetic code is, with few exceptions, universal
True
In pea plants, the tall plant allele (T) is dominant over the short plant allele (t). What are parental genotypes for a cross that yields 50% tall and 50% short offspring
Tt X tt
Place the DNA replication stepns in order
Unwinding the DNA double helix Complementary base pairing Joining nucleotides to the growing DNA strand
Multiple Alleles and Codominance
When a trait is controlled by multiple alleles, the gene exists in several allelic forms within a population. For example, the gene associated with cystic fibrosis (CFTR) has over 1,000 known alleles, all of which have the ability to vary the phenotype. We will use the human ABO blood type to better understand the influence of multiple alleles on a phenotype. The ABO blood type is controlled by a single gene that is involved with placing antigens (see Section 33.4) on the surface of red blood cells. This gene has three different alleles within the human population that determine blood type.
Which of the following human chromosomes are sex-linked genes found
X & Y
The genotype of a human female is * matches with A
XX
The genotype of a male is * matches with B
XY
Which of the following genotypes would cause a person to have hemophilia?
X^h X^h X^hY
Which of the following genotypes represents someone who does not have hemophilia but carries the trait?
X^hX
What happens if the voltage is set too high during elecrophoresis
a high voltage can heat up the buffer and cause the gel to melt
What is the definition of a monohybrid cross
a mating between individuals that trace a single gene of interest
What is the definition of a hybrid cross
a mating between individuals who are heterozygous for two genes of interest
During the final stage of translation, a release factor binds to
a stop codon
Which of the following genotpyes are homozygous
aa AA tt
p=.20, reject or accept
accepted
DNA polymerase
adds new complementary bases
DNA ligase
attaches DNA fragments
to avoid cross contamination, it is important to
change the tip between samples
gene
code for specific trait
heterozygous
different alleles
A ___ ___ involves predicting the genotypes of two traits rather than one
dihybrid cross
____ perform the actual work of copying DNA
enzymes
Gene interactions that allow one gene to silence the expression of another gene.
epistatic interactions
All mutations are harmful (T/F)
false
Buffer is added to the surface of the gel after the samples have been loaded into the wells
false
Sex linked
found on X and Y chromosomes
A sequence of DNA that encodes a functional product such as a protein is called a(n)
gene
If wing color is bees was controlled by blended inheritance, WW produced black wings, and ww produced white wings, then a Ww bee would have ____ colored wings
grey
only one allele for a given gene
hemizygous
What is the correct genotype for an organism that expresses a recessive trait
hh
The proteins responsible for coiling of the DNA molecule in eukaryotes is called
histone
Anaphase I
homolgous chromosomes are separated
Two copies of the same allele for a given gene
homozygous
homozygous
identical alleles
The portion of the mRNA transcript that gets removed during RNA processing is the
introns.
The first step in the expression of genetic information in DNA is they synthesis of
mRNA
The type of nucleic acid bearing condons for the amino acid sequence is called
mRNA
Which of the following carries the instruction for making a polypeptide or protein from the nucleus to the cytoplasm
mRNA
Which step results in emulsification of filtrate
mixing with detergent
A cross between parents who have two different alleles for a single gene of interest is called a
monohybrid
Anything that increases the rate of mutations in DNA is called a _____. For example, chemicals or ultraviolet radiation from the sun.
mutagen
A phenotype that differs from the wild-type is referred to as a ____
mutant
DNA has a ___ charge
negative
__ is the failure of homolgous chromosomes or sister chromatids to separate during gamete formation
nondisjunction
rRNA strands are transcribed from a template strand of DNA found in the
nucleolous
Each group of three nucleotides acts like a "code word" representing
one amino acid
Each newly formed DNA molecule consists of
one parental strand and one daughter strand
Transformation occurs when
organisms receive foreign DNA and thereby acquire a new characteristic.
A ___ is a diagram that indicates the pattern of expression for a particular trait within the members of a family across several generations
pedigree
A ____ is a tool used to track inheritance across generations
pedigree
A ____ is a tree-like chart that shows a family history for a particular trait across several generations
pedigree
A ____ is an observable or measurable characteristic of an individual organism
phenotype
The environment may influence the phenotypic distribution of the trait.
polygenic trait
DNA will travel toward which electrode
positive
If the alleles for phenylketonuria are represented by the letter P which of the following genotypes belongs to an individual affected with this disorder
pp
In addition to linking together nucleotides to synthesize a daughter strand, DNA polymerase also
proofreads the order of nucleotides
DNA holds the code to build
proteins
DNA holds the code to build ____. They will not function properly if they are not built correctly
proteins
___ ___ is a tool used to predict the genotypes of offspring
punnett square
Adenine and guanine are nitrogen-containing bases called
purines
Which RNA molecule is associated most closely with both ribosomes and translation
rRNA
A ____ allele can only exert its phenotypic effect in a homozygote
recessive
An ____ allele can only exert its phenotypic effect on a homozygote (an individual with two identical alleles for a trait
recessive
The chi square analysis, p=.04. Should the null hypothesis be accepted or rejected
rejected
The activities of DNA replication occur at a location called the ___ ____
replication fork
Translation begins when mRNA becomes associated with a
ribosome
DNA replication is called ___ ____ because each new double-helix contains one old strand and one new strand
semi-conservative
DNA Helicase
separates double-stranded DNA into single strands
The cellular basis of Mendels law of segregation is the
separation of homologous pairs during anaphase I of meiosis
Genes found on the X and Y chromosome are referred to as ___ linked genes
sex
Genes that are carried by either sex chromosome are referred to as ____ linked genes
sex
___ linked genes are genes found on either the X chromosome or the Y chromosome, but not both
sex
Chromosomes that contain genes determining the gender of an individual are called
sex chromosomes
A change in a nucleotide sequence that results in no change in protein production is called a
silent mutation
During the S phase of the cell cycle, the DNA that makes up the chromosomes must be copied to make identical ___ ___
sister chromatids
Recessive trait
skip generations
the collection of DNA on a glass rod is called
spooling
100% recessive
ss X ss
The central dogma of molecular biology
states that DNA is a template for all RNA production.
Which of the following apply to the process of translation during protein synthesis
tRNA Amino Acids mRNA --> protein Occurs on ribosomes
When attaching electrodes to the gel chamber, which electrode should be closest to the sample wells
the negative electrode
centromeres
the region of close association that holds the two copies of a duplicated chromosome together
The null hypothesis states that
there is no significant difference between the expected and observed hypothesis
How many nucleotides form a codon that specifies a particular amino acid
three
XO
turner syndrome
In a chi square analysis of an experiment in which the number of classes (phenotypes) analyzed was three, the value for the degrees of freedom is
two
Homologous chromosomes
two copies of a given chromosome pair
Which of the following are the premises of Mendel's law of segregation
- random fusion of all possible gametes occurred upon fertilization, providing each gamete two copies of each factor - individuals have two factors for each trait - the factors separate during gamete formation - gametes contain only one of the two parental factors
There are ____ different types of DNA nucleotides
4
Because methemoglobinemia is an autosomal recessive disorder,
Only individuals who are homozygous recessive for the allele for this disorder are affected
In horses, B = black coat, b = brown coat, T = trotter, and t = pacer. A black trotter that has a brown pacer offspring has which of the following genotypes?
BbTt
If there are three possible alleles for the G gene, how many different possible genotypes exist in the population for this gene (assume none are lethal)
6
In order to conduct a testcross, one of the parents needs to be true-breeding _____
Homozygous recessive
The chart that shows the pattern of inheritance from one generation to the next is known as a ____ chart
Pedigree
In guinea pigs, smooth coat (S) is dominant over rough coat (s), and black coat (B) is dominant over white coat (b). In the cross SsBb × SsBb, what proportion of the offspring would you predict to have a smooth black coat?
9⁄16
The X and Y chromosomes are sex chromosomes while the rest of them are called
Autosomes
Incomplete Penetrance
A dominant allele may not always lead to the dominant phenotype in a heterozygote, even when the alleles show a true dominant/recessive relationship. The dominant allele in this case does not always determine the phenotype of the individual, so we describe these traits as showing incomplete penetrance. In other words, just because a person inherits a dominant allele doesn't mean he or she will fully express the gene or show the dominant phenotype. Many dominant alleles exhibit varying degrees of penetrance.
Autosomal Dominant Disorders
Autosomal dominant disorders are typically the result of a gain-of-function mutation in a gene, resulting in a dominant allele. Therefore, only a single copy of this allele is needed to show the phenotype.
An individual is considered ___ if they have one dominant and one recessive allele for a given trait
Heterozygous
Which two genotypes would express the same phenotype in a simple, mendelian system
Ff FF
Morgan's Experiment
From these results, he knew that red eyes are the dominant characteristic and white eyes are the recessive characteristic. He then crossed the F1 flies. In the F2 generation, there was the expected 3 red-eyed : 1 white-eyed ratio, but it struck him as odd that all the white-eyed flies were males: Notice that X-linked alleles have a different pattern of inheritance than alleles that are on the autosomes, because the Y chromosome is lacking for these alleles, and the inheritance of a Y chromosome cannot offset the inheritance of an X-linked recessive allele. For the same reason, males always receive an X-linked recessive mutant allele from the female parent—they receive only the Y chromosome from the male parent, and therefore sex-linked recessive traits appear much more frequently in males than in females.
The biological science of ___ explains the stability of inheritance and also variation between offspring from one generation to then next
Genetics
Huntington Disease
Huntington disease is a neurological disorder that leads to progressive degeneration of brain cells. The disease is caused by a mutated copy of the gene for a protein called huntingtin. Most patients appear normal until they are of middle age and have already had children, who may later also be stricken. Occasionally, the first sign of the disease appears during the teen years or even earlier. There is no effective treatment, and death comes 10 to 15 years after the onset of symptoms.
Color Blindness
In humans, the receptors for color vision in the retina of the eyes are three different classes of cone cells. Only one type of pigment protein is present in each class of cone cell; there are blue-sensitive, red-sensitive, and green-sensitive cone cells. The allele for the blue-sensitive protein is autosomal, but the alleles for the red- and green-sensitive pigments are on the X chromosome. About 8% of Caucasian men have red-green color blindness. Most of these see brighter greens as tans, olive greens as browns, and reds as reddish browns. A few cannot tell reds from greens at all. They see only yellows, blues, blacks, whites, and grays.
Osteogenesis Imperfecta
Osteogenesis (L. os, "bone"; genesis, "origin") imperfecta is an autosomal dominant genetic disorder that results in weakened, brittle bones. Although at least nine types of the disorder are known, most are linked to mutations in two genes necessary for the synthesis of type I collagen, one of the most abundant proteins in the human body. Collagen has many roles, including providing strength and rigidity to bone and forming the framework for most of the body's tissues. Osteogenesis imperfecta leads to a defective collagen I that causes the bones to be brittle and weak. Because the mutant collagen can cause structural defects even when combined with normal collagen I, osteogenesis imperfecta is generally considered to be dominant.
The condition in which a single mutant gene affects more than one distinct characteristic of an individual is called ___
Pleiotropy
Pleiotropic Effects
Pleiotropy occurs when a single mutant gene affects two or more distinct and seemingly unrelated traits. For example, persons with Marfan syndrome have disproportionately long arms, legs, hands, and feet; a weakened aorta; poor eyesight; and other characteristics (Fig. 11.15). All of these characteristics are due to the production of abnormal connective tissue.
Which genetic term refers to the physical appearance of an individual, which is determined by the proteins produced by the corresponding alleles
Phenotype
Mendel bred a plant with a dominant phenotype to an individual with the recessive phenotype. He then analyzed the offspring to determine the unknown genotype. This type of experiment is called a(n)
Ttestcross
Phenotype
The inheritance of the ABO blood group in humans is also an example of codominance, because both IA and IB are fully expressed in the presence of the other. A person who inherits chromosomes with IA and IB alleles will make fully functional A and B protein, and because these alleles are codominant, the resulting mixture of AB protein will give the red blood cell an AB phenotype. On the other hand, both IA and IB are dominant over i. Therefore, two genotypes are possible for type A blood, and two genotypes are possible for type B blood.
Testcrossess
To confirm that the F1 plants of Mendel's one-trait crosses were, in fact, heterozygous, he crossed his F1 generation tall pea plants with true-breeding short (homozygous recessive) plants; such a mating is termed a testcross. These crosses provided Mendel with further support for his law of segregation. For the cross in Figure 11.9, Mendel reasoned that half the offspring should be tall and half should be short, producing a 1:1 phenotypic ratio. His results supported the hypothesis that alleles segregate when gametes are formed. In Figure 11.9a, the homozygous recessive parent can produce only one type of gamete—t—and so the Punnett square has only one column. The use of one column signifies that all the gametes carry a t.The expected phenotypic ratio for this type of one-trait cross (heterozygous × recessive) is always 1:1.
When analyzing a pedigree, you notice that two unaffected parents have produced a child that is affected by the trait. This suggests which of the following patterns of inheritance?
autosomal recessive
The law of segregation states all of the following except
factors assort independently of each other by meiosis.
A cross of two heterozygous individuals produces an intermediate phenotype.
incomplete dominance
In humans, there are three possible alleles at the chromosomal locus that determine blood type.
multiple alleles
Mendel's work supported which of the following?
particulate theory of inheritance
In peas, yellow seed (Y) is dominant over green seed (y). In the F2 generation of a monohybrid cross that begins when a dominant homozygote is crossed with a recessive homozygote, you would expect
three plants with yellow seeds to every plant with green seeds.
An individual is referred to as a ____ of a genetic condition if they do not express that trait but can pass the condition to their offspring
Carrier
An allele that masks the expression of an alternate form of that same gene is called a ___ allele
Dominant
Polygenic traits are controlled by two or more sets of alleles; each ____ allele has a quantitative effect on an individual's phenotype, and these effects are additive
Dominant
Mendel and the Laws of Probability
How likely is it that an offspring will inherit a specific set of two alleles, one from each parent? Notice that the chances of each parent contributing an A allele is ½ and the chances of contributing an a allele is ½. The product rule of probability tells us that we have to multiply the chances of independent events to get the answer: (picture on page 9/24) The Punnett square helps us visualize these outcomes, because we can easily see that each of the combinations is ¼ of the total number of squares. The statement "Chance has no memory" is important when considering inheritance across offspring. Every time a couple produces an offspring, the child has the same chances of inheriting the different allele combinations. Thus, for a heterozygous (Aa) couple, each child has a 25% chance of having albinism (aa). Inheriting a recessive trait may not seem significant if we are considering phenotypes such as skin pigmentation. However, it becomes important when we consider a recessive genetic disorder such as cystic fibrosis, a debilitating respiratory illness. For a heterozygous couple, there is a 25% chance that their child will inherit two recessive alleles and exhibit the disease. And because each child is an independent event, it is possible that all their children—or none of them—will exhibit cystic fibrosis.
Autosomal Patterns of Inheritance
In a pedigree, males are designated by squares and females by circles. Shaded circles and squares are the affected individuals. The shaded shapes do not indicate whether the condition is dominant or recessive, only that the individual exhibits the trait. A line between a square and a circle represents a union. In the patterns above, a vertical line leads to a single child. If there are more children, they are lined up horizontally. In pattern I, the child is affected, but neither parent is; this can happen if the condition is recessive and both parents are Aa. Notice that the parents are carriers, because they appear normal (do not express the trait) but are capable of having a child with the genetic disorder. In pattern II, the child is unaffected, but the parents are affected. This can happen if the condition is dominant and the parents are Aa. Often, it is necessary to examine how a trait is being inherited across multiple generations to understand its pattern of inheritance. A pedigree is a graphical representation of the inheritance pattern of a single trait in a family. In a pedigree, generations are indicated by Roman numerals on the left side (Fig. 11.10). It is also common to identify individuals by number (from left to right) in the pedigree. For example, the first individual on the top generation of a pedigree would be given the identification I-1, the second in the same generation I-2, etc.
Incomplete Dominance
Incomplete dominance is exhibited when a heterozygote has an intermediate phenotype between that of either homozygote. In a cross between a true-breeding, red-flowered four-o'clock plant strain and a true-breeding, white-flowered strain, the offspring have pink flowers. Although this outcome might appear to be an example of the blending theory of inheritance, it is not. Although the phenotypes have blended, the individual alleles are not altered. How do we know? When the pink plants self-pollinate, the offspring plants have a phenotypic ratio of 1 red-flowered : 2 pink-flowered : 1 white-flowered. The reappearance of the three phenotypes in this generation makes it clear that we are still dealing with a single pair of alleles (Fig. 11.14). In humans, familial hypercholesterolemia (FH) is an example of incomplete dominance. The gene associated with this condition controls number of LDL-cholesterol receptor proteins in the plasma membrane. A person with two mutated alleles lacks LDL-cholesterol receptors. A person with only one mutated allele has half the normal number of receptors, and a person with two normal alleles has the usual number of receptors. People with the full number of receptors do not have familial hypercholesterolemia. When receptors are completely absent, excessive cholesterol is deposited in various places in the body, including under the skin. The presence of excessive cholesterol in the blood causes cardiovascular disease, which may result in death at a young age.
Mendelian Patterns of Inheritance and Human Disease
Many traits and disorders in humans, as well as other organisms, are genetic in origin and follow Mendel's laws. These traits are often controlled by a single pair of alleles on the autosomal chromosomes. An autosome is any chromosome other than a sex (X or Y) chromosome.
Mendel's Law of Independent Assortment
Mendel performed a second series of crosses in which true-breeding pea plants differed in two traits. For example, he crossed tall plants having green pods with short plants having yellow pods (Fig. 11.6). The F1 plants showed both dominant characteristics. As before, Mendel then allowed the F1 plants to self-pollinate. This F1 cross is known as a dihybrid cross (L. di, "two"), because the plants are hybrid in two ways. Mendel recognized that there were two possible results in the F2 generation of this cross:
Mendel's Conclusion
Mendel's mathematical approach led him to interpret his results differently than previous breeders. He knew that the same ratio was obtained among the F2 generation time and time again when he did a monohybrid cross involving one of the seven traits he was studying (Fig. 11.4). In each cross, one of the characteristics disappeared in the F1 generation, only to reappear in one-fourth of the F2 offspring. Eventually, Mendel arrived at this explanation: A 3:1 ratio among the F2 offspring was possible if (1) the F1 parents contained two separate copies of each hereditary factor, one of these being dominant and the other recessive; (2) the factors separated when the gametes were formed, and each gamete carried only one copy of each factor; and (3) random fusion of all possible gametes occurred upon fertilization. Only in this way could the missing trait reoccur in the F2 generation. Thinking this, Mendel arrived at the first of his laws of inheritance—the law of segregation—which is a cornerstone of his particulate theory of inheritance.
Mendel Worked with the Garden Pea
Mendel's preliminary experiments prompted him to choose the garden pea, Pisum sativum, as his experimental organism. The garden pea was a good choice for many reasons. The plants were easy to cultivate and had a short generation time. Although peas normally self-pollinate (pollen only goes to the same flower), they could be cross-pollinated by hand by transferring pollen from the anther (male part of a flower) to the stigma (female part of a flower). Many varieties of peas were available, and Mendel chose 22 for his experiments. When these varieties self-pollinated, over generations they became true-breeding—meaning that all the offspring were the same and exactly like the parent plants. Unlike his predecessors, Mendel studied the inheritance of relatively simple and discrete traits that were not subjective and were easy to observe, such as seed shape, seed color, and flower color. In his crosses, Mendel observed that the offspring did not possess intermediate characteristics but, rather, were similar in appearance to one of the parents. As we will see, this disproved the blending concept and supported the concept of a particulate theory of inheritance.
Mendel's Particulate Theory of Inheritance
Mendel's theory of inheritance is called a particulate theory because it is based on the existence of minute particles, or hereditary units, that we now call genes. Inheritance involves the reshuffling of the same genes from generation to generation. The two laws he proposed, the law of segregation and the law of independent assortment, which we will discuss shortly (see Section 11.2), describe the behavior of these particulate units of heredity as they are passed from one generation to the next. While Mendel had no knowledge of DNA or genetic material, his theories have been well supported by countless experiments of geneticists and molecular biologists.
Environmental Influences: Multifactorial Traits
Multifactorial traits are those controlled by polygenes subject to environmental influences. Many genetic disorders, such as cleft lip and/or palate, clubfoot, congenital dislocations of the hip, hypertension, diabetes, schizophrenia, and even allergies and cancers, are probably multifactorial, because they are likely due to the combined action of many genes plus environmental influences. The relative importance of genetic and environmental influences on the phenotype can vary, and often it is a challenge to determine how much of the variation in the phenotype may be attributed to each factor. This is especially true in complex polygenic traits for which there may be an additive effect of multiple genes on the phenotype. If each gene has several alleles, and each allele responds slightly differently to environmental factors, then the phenotype can vary considerably.
Solving X-Linked Genetics Problems
Notice that there are three possible genotypes for females but only two for males. Females can be heterozygous XRXr, in which case they are carriers. Carriers usually do not show a recessive abnormality, but they are capable of passing on a recessive allele for an abnormality. But unlike autosomal traits, males cannot be carriers for X-linked traits; if the dominant allele is on the single X chromosome, they show the dominant phenotype, and if the recessive allele is on the single X chromosome, they show the recessive phenotype. For this reason, males are considered hemizygous for X-linked traits, because a male possesses only one allele for the trait and, therefore, expresses whatever allele is present on the X chromosome.
Phenylketonuria
Phenylketonuria (PKU) is an autosomal recessive metabolic disorder that affects nervous system development. Affected individuals lack the enzyme needed for normal metabolism of the amino acid phenylalanine; therefore, it appears in the urine and the blood. Newborns are routinely tested in the hospital for elevated levels of phenylalanine in the blood. If an elevated level is detected, the newborn will develop normally if placed on a diet low in phenylalanine, which must be continued until the brain is fully developed, around the age of 7, or severe intellectual disabilities will develop. Some doctors recommend that the diet continue for life, but in any case, a pregnant woman with phenylketonuria must be on the diet to protect her unborn child.
Polygenic Inheritance
Polygenic inheritance (Gk. poly, "many"; L. genitus, "producing") occurs when a trait is governed by two or more sets of alleles. Examples include human height, skin color, and the prevalence of diabetes. The individual has a copy of all allelic pairs, possibly located on many different pairs of chromosomes. Each dominant allele has a quantitative effect on the phenotype, and these effects are additive. Therefore, a population is expected to exhibit continuous phenotypic variations, such as a wide variation in human height and weight. In Figure 11.17, a cross between genotypes AABBCC and aabbcc yields F1 hybrids with the genotype AaBbCc. A range of genotypes and phenotypes results in the F2 generation that can be depicted as a bell-shaped curve.
An allele is referred to as a(n) ____ allele if it produces a protein with altered or no function
Recessive
The separation of factors during the formation of gametes is known as mendel's law of
Segregation
X-Linked Inheritance
The X and Y chromosomes in mammals determine the gender of the individual. Females are XX, and males are XY. These chromosomes carry genes that control development; in particular, if the Y chromosome contains an SRY gene, the embryo becomes a male. The term X-linked is used for genes that have nothing to do with gender yet are carried on the X chromosome. The Y chromosome does not carry these genes and indeed carries very few genes. This type of inheritance was discovered in the early 1900s by a group at Columbia University headed by Thomas Hunt Morgan. Morgan performed experiments with fruit flies, Drosophilamelanogaster. Fruit flies are even better subjects for genetic studies than garden peas. They can be easily and inexpensively raised in simple laboratory glassware; after mating, females lay hundreds of eggs during their lifetimes; and the generation time is short, taking only about 10 days from egg to adult. Fruit flies have a sex chromosome pattern similar to that of humans, and therefore Morgan's experiments with X-linked genes apply directly to humans.
Gregor Mendel
The science of genetics explains the stability of inheritance (why you are human, as are your parents) as well as variations between offspring from one generation to the next (why you have a different combination of traits than your parents). Virtually every culture in history has attempted to explain observed inheritance patterns. An understanding of these patterns has always been important to agriculture, animal husbandry (the science of breeding animals), and medicine.
Genotype Versus Phenotype
The word genotype (Gk. genos, "birth, origin") refers to the alleles an individual receives at fertilization. Genotype may be indicated by letters (TT) or by short combinations of letters and numbers (CYP27A1) that represent the DNA sequence for a particular gene. Genotype TT is called homozygous dominant, and genotype tt is called homozygous recessive. Genotype Tt is called heterozygous. These refer to the different ways that alleles can be combined in a cell. The word phenotype (Gk. phaino, "appear") refers to the physical appearance of an individual, which is determined by the proteins produced by the corresponding alleles. A homozygous dominant (TT) individual and a heterozygous (Tt) individual both show the dominant phenotype and are tall, because they make fully Page 191functional proteins that build the tall trait, while a homozygous recessive individual that shows the recessive phenotype and makes less or nonfunctional protein for that trait is short. Thus, the DNA that makes up the genotype produces the proteins that make up the phenotype.
One-Trait Testcross
Today, a one-trait testcross is used to determine if an individual with the dominant phenotype is homozygous dominant (e.g., TT) or heterozygous (e.g., Tt). Because both of these genotypes produce the dominant phenotype, it is not possible to determine the genotype by observation. Figure 11.9b shows that if the individual is homozygous dominant, all the offspring will be tall. Each parent has only one type of gamete and, therefore, a Punnett square is not required to determine the results.
(T/F): each genetic outcome of a genetic cross between the same parent is independent of the others
True
(T/F): genetic probability for a couple having a girl as a second child is the same as it was for the first child
True
Mendel's Cross as Viewed by Modern Genetics
We now know that the traits Mendel studied are controlled by single genes. These genes occur on a homologous pair of chromosomes at a particular location, called the gene locus (Fig. 11.5). Alternative versions of a gene are called alleles (Gk. allelon, "reciprocal, parallel"). A dominant allele will mask the expression of a recessive allele when they are together in the same organism. The word dominant is not meant to imply that the dominant allele is better or stronger than the recessive allele. In both cases, these alleles represent DNA sequences that code for proteins. Often, the dominant allele codes for the protein associated with the normal function of the trait within the cell (such as the production of pigment), while the recessive allele represents a "loss of function," meaning that it codes for a protein that has an altered function or no function within the cell (such as a loss of pigment). In many cases, the dominant allele is identified by a capital letter, the recessive allele by the same letter but lowercase. Usually, the first letter designating a trait is chosen to identify the allele. Using the plant height example, there is an allele for tallness (T) and an allele for shortness (t). In Mendel's cross, the original parents (P generation) were true-breeding; therefore, the tall plants had two alleles for tallness (TT), and the short plants had two alleles for shortness (tt). When an organism has two identical alleles, as these had, we say it is homozygous (Gk. homo, "same"). Because the parents were homozygous, all gametes produced by the tall plant contained the allele for tallness (T), and all gametes produced by the short plant contained an allele for shortness (t). After cross-pollination between different pea plants, all the individuals of the resulting F1 generation had one allele for tallness and one for shortness (Tt). When an organism has two different alleles at a gene locus, we say that it is heterozygous (Gk. hetero, "different"). Although the plants of the F1 generation had one of each type of allele, they were all tall. The allele that is expressed in a heterozygous individual is the dominant allele. The allele that is not expressed in a heterozygote is the recessive allele. This explains why shortness, the recessive trait, skipped a generation in Mendel's experiment.
The Blending Concept of Inheritance
When Gregor Mendel began his work, most plant and animal breeders acknowledged that both sexes contribute equally to a new individual. They thought that parents of contrasting appearance always produced offspring of intermediate appearance. This concept, called the blending concept of inheritance, meant that a cross between plants with red flowers and plants with white flowers would yield only plants with pink flowers. When red and white flowers reappeared in future generations, the breeders mistakenly attributed this to instability in the genetic material.
Two-Trait Testcross
When doing a two-trait testcross, an individual with the dominant phenotype is crossed with one having the recessive phenotype. Suppose you are working with fruit flies with the following characteristics: (picture on page 11/24)
Hereditary Spherocytosis
With an incidence of approximately 1 in 5,000, hereditary spherocytosis is one of the most common hereditary blood disorders. Roughly one-fourth of these cases result from new mutations and are not inherited from either parent. Hereditary spherocytosis exhibits incomplete penetrance, so not all individuals who inherit the mutant allele will actually show the trait. The cause of incomplete penetrance in these cases and others remains poorly understood.
