Unit 7- Heredity
What is the difference between a character and a trait? Explain using an example.
A character is a heritable feature that varies among individuals, such as flower color. A trait is each variant for a character such as purple or white color for flowers.
crossing over
As a consequence of the independent assortment of chromosomes during meiosis, each of us produces a collection of gametes differing greatly in their combinations we inherited from our two parents. In humans, one to three crossovers occur per chromosome. Crossing over produces chromosomes with new combinations of maternal and paternal alleles
Which type of reproduction will result in genetically identical offspring?
Asexual reproduction
parental cross (testcross)
Breeding an organism of unknown genotype with a recessive homozygote (used to check the genotype of the unknown individual)
When Thomas Hunt Morgan mated his first white-eyed male fly with a red-eyed female fly, he came to the startling conclusion that the trait for eye color was located on the chromosome that determines sex. What was the phenotypic result of this cross in the F1 and F2 generations? Show this cross.
F1 Cross Phenotypic ratio: 100% red F2 Cross Phenotypic ratio: 3 red: 1 white (white only on males)
Where are the gametes of an animal produced? Be specific as to male and female gametes.
Female gametes (eggs) are produced in the ovaries. Male gametes (sperm) are produced in the testes
Aneuploidy of Sex Chromosomes: XXX, sex? Physical Traits?
Female, Generally healthy and have no unusual physical features other than being slightly taller than average. Are at risk for learning disabilities
Aneuploidy of Sex Chromosomes: X0, sex? Physical Traits?
Female, Phenotypically female, but are usually sterile because their sex organs do not mature. Typical intelligence
Note that Morgan made a major breakthrough in thinking about inheritance. He showed that genes are located on specific chromosomes. What would Mendelian genetics predict for the results of the F1 cross described in question 5?
For the F1 cross, Mendelian genetics would predict that all offspring would have red eyes.
Explain Mendel's law of independent assortment.
Mendel explains independent assortment in dihybrid experiments as: two or more genes assort independently --that is, each pair of alleles segregates independently of any other pair of alleles--during gamete formation.
By what process are the damaged cells in a wound replaced?
Mitosis
You should have identified mitochondrial DNA as the correct response to question 39 above. What other organelle has its own genes? These are extranuclear genes.
Plastids (chloroplasts, chromoplasts, etc.)
What is pleiotropy? Explain how this is significant in cystic fibrosis and sickle-cell disease.
Pleiotrophy is a property that describes the fact that most genes have multiple phenotypic effects. In humans, for example, pleiotrophic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.
Suppose alleles M and n are linked on one chromosome, and m and N are linked on the homologous chromosome. Individuals homozygous for M and n are mated with individuals homozygous for m and N. Their offspring are test crossed and the following results are recorded: Mn/mN 232 mN/mN 240 MN/mn 15 mn/mn 13 How many units apart are these genes on the chromosome?
Recombination frequency = 28 recombinants / 500 total offspring = 0.056 x 100 = 5.6% 5.6% = 5.6 map units apart
What unusual result of phenotypes was seen in the F2 generation of that cross?
The Mendelian genetics would predict that both males and females are equally likely to end up with the white-eyed phenotype with the ratio 3:1. However, this was not the case because only males had white-eyes.
What is a Barr body? Why do human females show a Barr body in their cells?
The barr body is the inactive X in each cell that condenses into a compact object that lies along the inside of the nuclear envelope. Females show a Barr body in their cells because they have two copies of X chromosomes and in order to not produce twice the number of proteins, the cells inactivated on X in embryonic development.
In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?
There are exactly two chromosomes in each homologous pair because one chromosome is inherited from the maternal parent and the other is inherited from the paternal parent.
What is the F2 genotypic ratio?
1: 2: 1
Alfred H. Sturtevant, a student of Thomas Hunt Morgan, used assumptions from observations of crossovers to map genes. What is a linkage map?
A genetic map based on recombination frequencies is called a linkage map
Do a AB x O cross. Write genotypes and describe the genotypic ratio
Draw it on paper
Quantitative variation usually indicates polygenic inheritance. What is a good example of this?
Quantitative characters are traits that are expressed as gradients in the population. Polygenic inheritance is an additive effect of two or more genes on a single phenotypic character. Height is a good example of polygenic characters: study of 250,00 people found 700 genetic variations in over 180 genes that affect height.
A female carrier of color-blindness genotype
XNXn (subscript)
Notate the following genotypes for a female fruit fly: a fly homozygous for white eyes
XwXw (subscript)
polyploidy
a chromosomal alteration in which an organism has more than two complete chromosome sets in all somatic cells. Ex: 3n or 4n
female gamete
eggs
What would be the phenotypic ratio of a testcross between a wild-type dihybrid female fly with gray body and normal wings, and a homozygous recessive fly with black body, vestigial wings if the genes were not linked? Show the four possible phenotypes and the expected ratio below.
gray body, normal wings: 1 gray body, vestigial wings: 1 black body, normal wings: 1 black body, vestigial wings: 1 1:1:1:1
Is the F1 female homozygous or heterozygous?
heterozygous
Translocation (chromosomal)
moves a segment from one chromosome to a nonhomologous chromosome. In a reciprocal translocation, the most common type, nonhomologous chromosomes exchange fragments Less often, a non reciprocal translocation occurs: A chromosome transfers a fragment but receives non in return
chiasmata
one or more X shaped regions on homologous chromosomes where crossovers have occurred.
locus
A gene's specific location along the length of a chromosome
n important idea for you to understand is that new alleles arise by changes in the DNA or mutation, but genetic diversity also occurs when the deck is simply reshuffled. So, there are three ways that sexually reproducing organisms "shuffle the deck." They are listed below. Explain what occurs in each, and how this increases diversity.
Independent assortment of chromosomes, crossing over, and random fertilization
What is the purpose of meiosis?
It is a type of cell division that reduces the number of sets of chromosomes from two in the parent cell to one in each gamete, counterbalancing the doubling that occurs at fertilization.
What is the SRY gene? Where is it found, and what does it do?
It's a gene on the Y chromosome. It is found at the sex-determining region of Y and is required for the development of testes. In the absence of SRY gene, the gonads develop into overies, even in an XY embryo.
Neither Tim nor Shonda has Duchenne muscular dystrophy, but their firstborn son does. Make a Punnett square to show the parental genotypes as well as all possible outcomes for their children. Then use your results to answer the questions that follow. What is the probability if the second child is a girl?
0
An event that is certain to occur has a probability of __ , and an event that is certain not to occur has a probability of __.
1, 0
How does a somatic cell compare to a gamete in terms of chromosome number?
A somatic cell has twice the number of chromosomes compared to a gamete (two chromosomes in each 23 types of chromosomes for humans = 46). This is because meiosis makes the gamete have half the number of chromosomes compared to the somatic cell.
heterozygous
An organism that has two different alleles for a gene. Ex: pp
In meiosis, what follows the duplication of chromosomes?
Cell divisions: meiosis I and meiosis II
Prophase I of Meiosis I
Chromosome duplication; chiasma form after crossing over, pair of duplicated homologs are held together by chiasma and sister chromatid cohesion
Sex chromosome, description and # in Human Cells
Chromosomes that deal with sex determination. Includes X and Y chromosomes, 2
For the following human inherited conditions, describe the condition and give the pattern of inheritance: achondroplasia
Dominant disorder.form of dwarfism one in 25,000 people. Heterozygous have the dwarf phenotype. All people who dont have this condition, are homozygous recessive. Recessive allele much more prevalent
Which generation is heterozygous?
F1
Which generation has both heterozygous and homozygous offspring?
F2
Understand the blood groups chart in your notes
Go memorize it
One of the keys to success for Mendel was his selection of pea plants. Not only were there many varieties, they also had short generation time and large numbers of offspring were produced. How did Mendel control the crosses so he was always certain of their parentage?
He controlled the crosses so he was always certain of their parentage by removing the immature stamens of a plant before they produced pollen and then dusted pollen from another plant onto the altered flowers.
Explain epistasis.
In epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a second locus.
X-0 system of sex determination
In grasshoppers, cockroaches, and some other insects, there is only one type of sex chromosome, the X. Females are XX; males only one sex chromosome (X0). Sex of the offspring is determined by whether the sperm cell contains an X chromosome or no sex chromosome
X-Y system of sex determination
In mammals, the sex of an offspring depends whether the sperm cell contains and X chromosome or a Y
Review meiosis. When does crossing over occur?
In meiosis I, crossing over occurs during prophase I before the tetrads line up at the metaphase plate.
Why is a particular trait recessive? Note the difference shown between the DNA nucleotide sequences for the two different color alleles. Explain, at the molecular level, why purple or white flowers result.
In purple-flowered plants, they inherit a DNA sequence in their chromosomes that codes for the synthesis of an enzyme. That enzyme then helps synthesize the purple pigment that makes the flower purple. For white flowers, their DNA sequence results in the absence of the enzyme, so they remain white.
How many times does the cell divide in meiosis?
The cell divides twice in meiosis
What is meant by the wild type?
The phenotype for a character most commonly observed in natural populations
Law of Segregation
The two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes
genotype
an organism's genetic makeup. Ex: PP or Pp
Which generation is haploid, and which is diploid?
Sporophyte is the diploid generation and the gametophyte is the haploid generation.
What happens to the chromosome number in meiosis?
The chromosome number is cut in half
Notate the following genotypes for a female fruit fly: a fly heterozygous for red eyes
Xw+X (subscript)
monohybrid cross
a cross that tracks the genetic outcomes of one characteristic
somatic cells
all cells of the body except the gametes and their precursors
Is the parental female homozygous or heterozygous?
homozygous
Duplication (chromosomal)
repeats a segment
male gamete
sperm
What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order? Show your calculation.
½ x ½ x ½ x ½ = 1/16
F2 generation
(second filial generation) the offspring of the F1 generation
What is the F2 phenotypic ratio?
3: 1
F1 generation
(the first filial generation, filial is Latin for "son") the hybrid offspring of the P generation
P generation
(the parental generation) the true-breeding parents
The first evidence that a specific gene was associated on a specific chromosome came from the work of Thomas Hunt Morgan in the early 1900s. He selected Drosophila melanogaster as his experimental organism. List at least three reasons why the fruit fly is an excellent subject for genetics studies.
-Fruit flies are prolific breeders -a single mating will produce hundreds of offspring -a new generation can be bred every two weeks -It has only 4 pairs of chromosomes which are easily distinguishable with a light microscope
What is the alternative to the wild type phenotype?
Mutant phenotypes - they are due to alleles assumed to have originated as changes, or mutations, in the wild-type allele
amniocentesis: 3 steps
1. A sample of amniotic fluid can be taken starting at 15th or 16th week of pregnancy 2.Genetic and biochemical tests are performed on the amniotic fluid and fetal cells in it 3.If indicated, fetal cells from either amniocentesis or CVS can be cultered and then karyotyped to check for large-scale chromosomal abnormalities
Steps in chorionic villus sampling
1. A sample of chorionic villus tissue can be taken as early as the 10th or 11th week of pregnancy 2.Genetic and biochemical tests are performed on the fetal cells 3.If indicated, fetal cells from either amniocentesis or CVS can be cultered and then karyotyped to check for large-scale chromosomal abnormalities
Neither Tim nor Shonda has Duchenne muscular dystrophy, but their firstborn son does. Make a Punnett square to show the parental genotypes as well as all possible outcomes for their children. Then use your results to answer the questions that follow. What is the probability if the second child is a boy?
1/2
Neither Tim nor Shonda has Duchenne muscular dystrophy, but their firstborn son does. Make a Punnett square to show the parental genotypes as well as all possible outcomes for their children. Then use your results to answer the questions that follow. What is the probability that a second child will have the disease?
1/4
Monosomy and trisomy are estimated to occur in _________ of human conceptions and are the main reason for pregnancy loss.
10-25%
Therefore, because humans have 46 chromosomes or 23 homologous pairs, what is the number of possible gametes that can be formed due to independent assortment of chromosomes?
2^23 = 8.4 million
Now, this is the number of unique gametes your mom could have made. Your father could have made the same number. To see the effect of random fertilization, multiply the number of gametes one parent could make by the number of unique gametes the other parent could make.
2^23 x 2^23 = about 70 trillion
The number of different gametes that can be formed because of independent assortment is
2n, where n = the number of homologous pairs
How many daughter cells are formed in meiosis?
4 daughter cells
How many chromosomes are in human cells? What is a chromosome?
46 chromosomes. A chromosome is the combination of proteins and DNA that are condensed in an organized matter.
Two heterozygous dihybrid phenotypic ratio
9: 3 : 3 : 1
crossing over
The DNA molecules of nonsister chromatids are broken (by proteins) and are rejoined to each other of maternal and paternal homologous chromosomes
When you are given the frequency of crossing over between genes, it is possible to map their location on a chromosome. (As a hint, place the two genes that have the greatest frequency of crossing over at either end and work from there.) What is the sequence of these genes on the chromosome if the frequency of crossing over between linked genes A and B is 35 percent; between B and C, 10 percent; between C and D, 15 percent; between C and A, 25 percent; between D and B, 25 percent?
A D C B
What is the definition of a sex-linked gene?
A gene located on either sex chromosome. Ex: X-linked genes vs. Y-linked genes
What is a karyotype? How is it prepared?
A karyotype is a display of condensed chromosomes arranged in pairs. It can be used to screen for defective chromosomes or abnormal numbers of chromosomes to find certain diseases like down syndrome. Karyotypes are prepared from isolated somatic cells, which are treated with a drug to stimulate mitosis and then grown in culture for several days. When chromosomes in the cell are most highly condensed--at metaphase-- they are taken, stained, and examined. A digital computer organizes chromosomes into pairs.
Let's take a moment to consider the question "why sex?" Any individual passes on more of its genes through asexual reproduction, so there must be some advantage to sexual reproduction, as it is almost universal among animals. Under what conditions is sexual reproduction most advantageous, and why?
A population evolves through the differential reproductive success of its variant members (Darwin). On average, those individuals best suited to the local environment leave the most offspring, thereby transmitting their genes. Thus, natural selection results in the accumulation of genetic variations favored in certain environment. If the environment changes, some will survive and those will reproduce. The reason sexual reproduction is advantageous is because of the ability to generate genetic diversity and progress evolution (survival too).
What is a map unit?
A type of unit that expresses the distances between genes, one map unit is equivalent to a 1% recombination frequency.
4 steps of crossing over during prophase I
After interphase, the chromosomes have been duplicated, and sister chromatids are held together by proteins called cohesins (purple). Each pair of homologs associate along their length. The DNA molecules of two nonsister chromatids are broken at precisely corresponding points. The chromatin of the chromosomes starts to condense A zipper-like protein complex, the synaptonemal complex (green), begins to form, attaching one homolog to the other. The chromatin continues to condense. The synaptonemal complex is fully formed; the two homologs are said to be in synapsis. During synapsis, the DNA breaks are closed up when each broken end is joined to the corresponding segment of the nonsister chromatid, producing crossovers After synaptonemal complex disassembles, the homologs move slightly apart from each other but remain attached because of sister chromatid cohesion, even though some of the DNA may no longer be attached to its original chromosome. The points of attachment where crossovers have occurred show up as chiasmata. The chromosomes continue to condense as they move toward the metaphase plate
What are alleles? Give an example.
Alleles are different versions of genes at corresponding loci on homologs (each version is called an allele). Ex: one chromosome has an allele for freckles, but another homologous chromosome has an allele for the absence of freckles at6 the same locus.
Your study of plants this year will include knowing that they exhibit alternation of generations. a. What does this mean?
Alternation of generations is a second type of life cycle. This type includes both diploid and haploid stages that are multicellular.
Mendel Model's 4 Concepts
Alternative versions of genes account for variations in inherited characters. For each character, an organism inherits two versions (that is, two alleles) of a gene, one from each parent If the two alleles at a locus differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance The two alleles for a heritable character segregate (in other words, separate from each other) during gamete formation and end up in different gametes
What are the strengths and weaknesses of each fetal test?
Amniocentesis - Strengths: Some genetic disorders can be detected from the presence of certain proteins or hormones in the amniotic fluid itself. Other disorders (thousands) are tested for such as cystic fibrosis tests are performed on DNA from fetal cells sloughed off into the amniotic fluid using DNA microarray that tests for many alleles at the same time. Karyotype can reveal chromosomal abnormalities Weaknesses: can cause complications 1 in 1000, might have to consider terminating pregnancy if there are defects CVS - Strengths: DNA can be analyzed for the presence of genetic disorders. Can be performed very early at the 10th or 11th week of pregnancy. Weaknesses: can cause complications 1 in 500, might have to consider terminating pregnancy if there are defects
There are a number of tests that can be done to look for genetic disorders in a fetus. Imaging using ultrasound and analysis of fetal DNA that has escaped into the mother's blood are two noninvasive procedures. ____________ and _______ are two widely used methods for testing a fetus for genetic disorders. Use the following unlabeled diagram to explain the three main steps in the two processes
Amniocentesis and chorionic villus sampling (CVS)
Describe what you think is medically important to know about the behavior of recessive alleles.
An allel that causes a genetic disorder (allele a) coes for either a malfunctioning protein or no protein at all. In the case of disorders classified as recessive, heterozygous (Aa) typically have the normal phenotype because one copy go the normal allele (A) produces a sufficient amount of a specific protein. Thus, a reccesively inherited disorder shows up only in the homozgous indivduals (aa) who inherit a recessive allele from each parent. Although phenotypically normal, heterozygous may transmit the recessive allele to their offspring as carriers.
Explain the potential influence of the environment on phenotypic expression and cite an example.
Another difference from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype. For humans, nutrition influences height, excercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence. Identical twins even accumulate phenotypic differences due to environment
Why can the parental female produce only one type of gamete but the phenotypically identical F1 female produce four types of gametes?
Because for the parental gamete, she only has two alleles that are the same for each chromosome. This means she can only give the same two alleles to her offspring making one type of gamete, while the F1 female can associate her four different alleles (two for each gene) in 4 ways.
Compare and contrast codominance with incomplete dominance.
Codominance is where the two alleles each affect the phenotype in separate, distinguishable ways. Ex: MN blood group where there are M and N molecules on the red blood cell, otherwise it would only be M or only be N. In this case both traits are dominant and visible. On the other hand, incomplete dominance is a blend of the two alleles and non allele is dominant over the other.
Alleles can show different degrees of dominance. Explain how incomplete dominance is different from complete dominance and give an example of incomplete dominance.
Complete dominance occurs when one of the two alleles displays their phenotype over the other, while incomplete dominance occurs when neither of the two alleles are completely dominant over the other. Ex: a red snapdragon is bred with a white snapdragon to produce a pink snapdragon that is heterozygous.
Why must crossing over occur with a nonsister chromatid?
Crossing over must occur with a nonsister chromatid because otherwise the crossing over would not exchange maternal and paternal DNA. A sister chromatid is just a copy of either maternal or paternal DNA and that copy would just exchange its DNA with another copy sister chromatid. There would be no change in the DNA then.
What causes Down syndrome? What are four characteristics of Down syndrome?
Down syndrome is an aneuploid condition and it occurs when there is an extra chromosome for chromosome 21. This results in trisomic cells and down syndrome is often called trisomy 21. 4 Characteristics: -Characterisitic facial features -Short stature -Correctable heart defects -Developmental delays
Independent assortment of chromosomes occurs in meiosis. Refer to Figure 13.11, p. 265, in your text. Explain the event in meiosis when independent assortment occurs.
During metaphase I of meiosis I, the arrangement of homologous chromosomes is when independent assortment occurs. During this time, it is equally likely for the maternal and paternal chromosomes to line up towards each pole. The direction that the maternal and paternal chromosomes line up, causes genetic variation.
Mitosis: Role in the animal body Number of DNA replications Number of divisions Number of daughter cells Chromosome number of daughter cells Are the daughter cells identical to or different from the parent cell?
Enables multicellular animal, fungus, or plant (gametophyte or sporophyte) to arise from a single cell; produces cells for growth, repair, and, in some species, asexual reproduction; produces gametes in the plant gametophyte 1 1 2 Diploid Identical
What are linked genes? Do linked genes sort independently?
Genes located near each other on the same chromosome tend to be inherited together in genetic crosses; such genes are said to genetically linked and are called linked genes. No, they tend to be transmitted together. They can rarely break linkage however.
independent assortment of chromosomes
Genetic variation is caused by the random orientation of pairs of homologous chromosomes at metaphase of meiosis I. At metaphase I, the homologous pairs, each consisting of one maternal and one paternal chromosome, are situated at the metaphase plate. Each pair may orient with either its maternal or paternal homolog closer to a given pole -- its orientation is as random as the flip of a coin. Thus, there is a 50% chance that a particular daughter cell of meiosis I will get the maternal chromosome of a certain homologous pair and 50% chance that it will get the paternal chromosome. Because each pair of homologous chromosomes is positioned independently of the other pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair, called independent assortment. 2^n power gives the chromosome possibilities (n=haploid chromosome number).
Explain what is meant by homologous chromosomes.
Homologous chromosomes (homologs) are the pair of chromosomes that have the same length, centromere position, and staining pattern. Both chromosomes of each pair carry genes controlling the same inherited characters.
What are homologs (homologous chromosomes)?
Homologs (homologous chromosomes) are chromosomes that are paired together because they code for the same genes located at certain loci. They look almost identical, but they might have different alleles at the loci. One of each homologous chromosome comes from the mother, the other comes from the father.
Anaphase I and Telophase I of Meiosis I
Homologs separate during anaphase I; sister chromatids remain attached at the centromere
Humans and other mammals have two types of sex chromosomes, designated X and Y. Describe how they differ from each other and make a small sketch to illustrate this.
Humans and other mammals have two types of sex chromosomes, designated X and Y. Describe how they differ from each other and make a small sketch to illustrate this.
trisomy
If a chromosome is present in triplicate in the zygote (2n+1 chromosomes), the anueploid cell is triosmic
Explain why the dihybrid cross, detailed in Figure 14.12 in your text, has four yellow Labrador retrievers instead of the three that would have been predicted by Mendel's work.
In Labrador retrievers, black coat color is dominant to brown. B and b are the two alleles for this character. For a lab to have brown fur, its genotype must be bb. A second gene determines whether or not pigment will be deposited in the hair. Dominant allele E, results in the deposition of of either black or brown pigment depending on the genotype at the first locus. But if the Lab is homozygous recessive for the second locus (ee), then the coat is yellow, regardless of the genotype at the black/brown locus. If, BbEe x BbEe is mated, there is a forth dog with a yellow coat (bbee) for this reason despite the ratio predicted by Mendel to be 9:3:3:1. This is epitasis
Z-W system of sex determination
In birds, some fishes, and some insects, the sex chromosomes present in the egg(not the sperm) differ, and thus determine the sex of offspring. The sex chromosomes are designated Z and W. Females are ZW and males are ZZ.
random fertilization
In human's each male and female gamete represents one of about 8.4 million (2^23) possible chromosome combinations due to independent assortment. The fusion of a male gamete with a female gamete during fertilization will produce a zygote with any of about 70 trillion (2^23 x 2^23) diploid combinations. If variation due to crossing over is factored in, the number or possibilities increases even more.
There are two divisions in meiosis. What will separate in the first division in meiosis I?
In the first division in meiosis I, the two homologous chromosomes of each pair separate
There are tests to identify carriers that allow couples with a family history of a disorder to make informed decisions about having children. Genetic counseling for these families is based on Mendelian genetics and probability rules. Work through the example of hypothetical couple Tyler and Lily on p. 288 of your text to understand the mathematics involved. Then, solve the following problems. Lucia and Jared each have a sibling with cystic fibrosis; neither Lucia or Jared nor any of their parents have the disease. Calculate the probability that a child born to this couple will have cystic fibrosis. Show your work and explain your answer.
Lucia: AA or Aa Jared: AA or Aa Phenotypic ratio: 1 AA: 2Aa: 1 aa Lucia and Jared each have ⅔ chance of being carriers The chance that their child will have the disorder is: ⅔ (Lucia's chance) x ⅔ (Jared's chance) x ¼ (the chance of two carriers having a child with the disease) = 1/9 There is a 1/9 chance their child will have cystic fibrosis.
Study pedigree notes for tips
Make sure to understand them
Aneuploidy of Sex Chromosomes: XYY, sex? Physical Traits?
Male, Typical sexual development and don't exhibit any well-defined syndrome. Tend to be taller than average
Aneuploidy of Sex Chromosomes: XXY, sex? Physical Traits?
Male, male sex organs, but their testes are small and produce little to no sperm. Taller than average height, less muscle mass, and enlarged breast tissue. Learning disabilities
Although you inherited one chromosome of each pair from your mother and your father, you have inherited a group of genes from your mother only. What genes are these?
Maternal mitochondrial DNA
By what process are eggs formed?
Meiosis
By what process are gametes produced?
Meiosis
Which process reduces the chromosome number of daughter cells?
Meiosis
During which division is the chromosome number reduced?
Meiosis I
Explain how Mendel's simple cross of purple and white flowers did the following: b. determined dominant and recessive characteristics:
Mendel questioned if the white-flowered plants' genetic contribution to the hybrids were lost. If it were lost, then the F1 plants could produce only purple-flowered offspring in the F2 generation. But when the F1 plants cross-pollinated, the white-flower trait reappeared in the F2 generation. He reasoned that the white trait was recessive while the purple trait was dominant.
Explain how Mendel's simple cross of purple and white flowers did the following: c. demonstrated the merit of experiments that covered multiple generations:
Mendel used very large sample sizes and kept accurate records of his results: 705 of F2 plants purple and 224 white-flowered. The data fit a ratio of approx. 3 purple to 1 white.
By what process does a zygote develop into a multicellular organism?
Mitosis
In which process are identical daughter cells produced?
Mitosis
What are the symptoms of phenylketonuria (PKU)? How is newborn screening used to identify children with this disorder? Why is newborn screening important?
PKU is a reccessively inherited disorder that occurs in about one out of 10,000 to 15,000 births. Children with this disease cannot properly metabolize the amino acid phenylalanine. This compound and its byproduct, phenylpyruvate, can accumulate to toxic levels in the blood, causing severe intellectual disability. It is screened for in the blood from the baby's heel in the hospital. However, if PKU is detected in a newborn, a special diet low in alanine will allow for typical development. This diet will excludes aspartame, an artificial sweetener, which contains phenylalanine.
Metaphase I of Meiosis I
Pairs of homologous chromosomes line up at the metaphase plate
Meiosis: Role in the animal body Number of DNA replications Number of divisions Number of daughter cells Chromosome number of daughter cells Are the daughter cells identical to or different from the parent cell?
Produces gametes (in animals) or spores (in fungi and in plant sphorophytes); reduces number of chromosome sets by half and introduces genetic variability among the gametes or spores 1 2 4 Haploid Different
It is easy to have the misconception that the chromosomes inherited from our parents are exclusively maternal or paternal. The chromosomes we inherit are actually recombinant chromosomes. Explain what this means and clarify the misconception.
Recombinant chromosomes are individual chromosomes that carry genes (DNA) from two different parents. This means that the inherited chromosomes are not exclusively maternal or paternal, but a combination of both.
Name and describe three human sex-linked disorders.
Red-green colorblindness is a sex-linked disorder. This is because the male can only inherit the recessive allele from their mother, and the Y chromosome from the father. For females, they can inherit and X chromosome with the recessive allele from a carrier mother and colorblind father. Chance of male colorblindness is higher Duchenne muscular dystrophy - one out 5,000 males. Disease involves progressive weakening of the muscles and loss of coordination. Life expectancy mid-20s. Absenxe of a key muscle protein dystrophin on a specific locus on the X chromosome Hemophilia - X-link recessive disorder defined by the absence of one or more of the proteins required for blood clotting. When person injured, bleeding is prolonged because a firm clot is slow to form. Common in royal family
In pea plants, T indicates the allele for tall plants, and t is the allele for dwarf plants. If you have a tall plant, demonstrate with a testcross how it could be determined if the plant is homozygous tall or heterozygous tall.
Since tt is the genotype for dwarf plants, t is recessive. By crossing a recessive plant with either a homozygous or heterozygous dominant plant (T for tall), then by the offspring you will know the mystery genotype. This is because if the genotype is Tt, heterozygous and bred with tt, then the offspring will have 50% tall and 50% dwarf. If the genotype is TT, then all offspring will be tall with a 100% Tt.
What are the two generations?
Sporophyte - the multicellular diploid state Gametophyte - the cell with the haploid chromosomes divides mitotitically and produces more cells that are haploid like the starting cell. Multicellular haploid stage Sporophyte generation produces a gametophyte as its offspring and the gametophnyte generation produces a sporophyte. This alternates.
X inactivation maintains the proper gene dosage. How is the X chromosome inactivated?
The active XIST gene in the the randomly designated inactive X turns on to inactive the whole X during embryonic development. Which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation. As a result, there are two types of cells those with the active X from the father and those with an active X from the mother. After an X chromosome is inactivated in a cell, all mitotic descendants of that cell have the same inactive X. Thus, if a female is heterozygous for a sex-linked trait, about half her cells will express one allele the other will express another.
State the addition rule and give an original example.
The addition rule states that the probability that any one of two or more mutually mutually exclusive events will occurs is calculated by adding their individual probabilities. Ex: driving up to traffic light, equal probability of getting a red light, green light, or yellow light, what's the probability of getting either a green light or red light = ⅓ + ⅓ = ⅔
The figure below shows metaphase I of meiosis. How is the arrangement of chromosomes here different from the metaphase of mitosis?
The arrangement of chromosomes in metaphase I of meiosis is different from metaphase of mitosis because in mitosis two sister chromatids are pulled apart. But in metaphase I of meiosis, two homologous chromosomes are pulled apart. The sister chromatids of each chromosome are kept intact.
Explain how Mendel's simple cross of purple and white flowers did the following: a.refuted the "blending" hypothesis:
The blending hypothesis believed that genetic material from both parents mixed like blue and yellow paint, making green. However, if the blending model were correct, the F1 hybrids from Mende's cross between purple-flowered and white-flowered pea plants would have been pale purple flowers. This was not the case, the F1 hybrids were purple like the purple parent.
What is the chromosome number of the daughter cells of meiosis?
The chromosome number of daughter cells of meiosis are haploid
How does the chromosome theory of inheritance provide a physical explanation for Mendelian inheritance?
The chromosome theory of inheritance found that Mendelian genes have specific loci (sites) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.
How many times do the chromosomes duplicate?
The chromosomes duplicate once during interphase
Figure 15.4 in your text is very important, as it ties together Mendel's laws, which you studied in Chapter 14, and the chromosome theory of inheritance. Notice that each resultant gamete has one long chromosome and one short chromosome. Which Mendelian law explains this?
The law of Independent assortment
Each resultant gamete has one of each homologous chromosome pairs. Which Mendelian law explains this?
The law of segregation
What occurs in nondisjunction?
The meiotic spindle makes an error in distributing chromosomes to the daughter cells. What happens is the members of a pair of homologous chromosomes do not move apart properly during meiosis I or sister chromatids fail to separate during meiosis II. In nondisjunction, one gamete receives two of the same type of chromosome and another gamete receives no copy. The other chromosomes are usually distributed normally.
State the multiplication rule and give an original example.
The multiplication rule states that to determine the probability of one event and the other occurring, we multiply the probability of one event by the probability of the other event. Ex: the probability of having two girls in a row ½ x ½ = 1/4
Autosome, description and # in Human Cells
The other chromosomes that are not sex chromosomes, 44
In probability, what is an independent event?
The outcome of any particular trial is is unaffected by what has happened on previous trials.
What is the purpose of meiosis?
The purpose of meiosis is to reduce the number of chromosomes by half so when the offspring is formed, the zygote does not have double the number of chromosomes. Furthermore, meiosis allows for sexual reproduction which allows offspring to have varied genetic traits.
Chapter Summary Question: Why does meiosis result in four haploid gametes?
The reason meiosis results in four haploid gametes because the DNA of the maternal and paternal sets of chromosomes are duplicated. Then the duplicated homologous chromosomes are separated to form 2 haploid cells. Then in Meiosis II, the sister chromatids are pulled apart to form 4 haploid daughter cells
During meiosis I, homologous chromosomes separate. What separates during meiosis II?
The sister chromatids separate during meiosis II
Haplo-diploid system of sex determination
There are no sex chromosomes in most species of bees and ants. Females develop from fertilized eggs and are thus haploid. Males develop from unfertilized eggs and are haploid; they have no fathers
Why can you say that all calico cats are females?
These cats have two sex-linked alleles for fur color: black and orange. Only females can have both alleles with two X chromosomes. If she is heterozygous, she has the tortoiseshell phenotype. So all calico cats have to be female.
If two genes are linked on the same chromosome, we call this combination the parental combination. These genes will be transmitted as a unit and will not sort independently. However, during meiosis, crossing over occurs between homologous chromosomes, and the linked genes can become "unlinked." In general, the farther two genes are from each other along the chromosome, the more often they will come "unlinked." Genetic recombination is the process during which linked genes become unlinked. What do geneticists call the offspring that show these new combinations?
These offspring display nonparental phenotypes They are called Recombinant types (recombinants for short)
Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.
This is not always true because there is a rare dominant allele where every 1 in 400 babes is born with extra fingers or toes (polydactyly). The recessive allele is far more common and is what causes 5 digits per appendage.
Homologous chromosomes may have alleles that are the same or they may have alleles that are different. Explain how this can be possible. Figure 15.1 will help with your answer.
This is possible because each individual inherits a paternal and maternal set of chromosomes and the chromosomes from each parent may have different alleles for certain characters.
Chapter Summary Question: The offspring of purple and white-flowering plants all produce purple flowers, but still are able to form gametes for white-flowering offspring. What principle does this demonstrate?
This principle demonstrates the idea of dominant and recessive alleles. The purple pigment is dominant to the allele of no pigment white.
What does this suggest considering Hunt got only males with white eyes for the F2 cross?
This suggests that the white-eyed gene is located on the X chromosome and there is no corresponding locus on the Y chromosome. Because the wild type allele was not available on the Y chromosome, the X chromosome's white eye trait could not be masked.
A number of genes will cause a variation in phenotype, depending on whether the gene came from the father or the mother. This variation occurs because of genomic imprinting. Explain genomic imprinting.
Variation in phenotype depnding on whether an allele is inherited from the male or female parent. (Most imprinted genes are on autosomes). Genomic imprinting occurs during gamete formation and results in the silencing of a particular allele of certain genes. Because these genes are imprinted differently in sperm and eggs, the offspring expresses only one allele of an imprinted gene, the one that has been inherited from a specific parent--either the female or male parent, depending on the particular gene.The imprints are then transmitted to all body cells during early development. In each generation, the old imprints are "erased" in gamete producing cells, and the chromosomes of the developing gametes are newly imprinting according to the sex of the individual forming the gametes.
Explain what is meant when a gene is said to have multiple alleles. Blood groups are an excellent human example of this.
When a gene is said to have multiple alleles, it means that the gene has more than two alleles. For example, blood groups are made up of the alleles IA, IB, and i. These form the blood types: A, B, AB, or O.
A female who is a not a carrier and has normal vision genotype
XNXN (subscript)
A male with normal vision genotype
XNY (subscript)
A female with color-blindness genotype
XnXn (subscript)
A male with color-blindness genotype
XnY (subscript)
Notate the following genotypes for a female fruit fly: a fly homozygous for red eyes
Xw+Xw+ (subscript)
What are two things that can be determined from a karyotype?
You can see if defective chromosomes or abnormal numbers of chromosomes are present. This can help determine congenital disorders like Down syndrome.
As you start to work word problems in genetics, two things are critical: the parent's genotype must be correct, and the gametes must be formed correctly. Using Figure 14.8 in your text as your guide, explain how the gametes are derived for the following cross. (You should have four different gametes. This is Mendel's law of independent assortment in action.)
YyRr × YyRr For the two characteristics, each allele has an equal chance of arranging with the other. Therefore gametes are formed accordingly: YR, Yr, yR, yr
aneuploidy
a condition in which the zygote has an abnormal number of a particular chromosome
monosomy
a condition when a zygote with aneuploidy (an abnormal number of chromosomes) has a missing chromosome (2n-1 chromosomes)
dihybrid cross
a cross that tracks the genetic outcomes of two characteristics
asexual reproduction
a single individual (like a yeast cell or an amoeba) is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes.
What is meiosis?
a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell, as in the production of gametes and plant spores.
homozygous
an organism that has a pair of identical alleles for a gene encoding a character. Ex: PP or pp
phenotype
an organism's appearance or observable traits. Ex: purple flowers
During prophase I: Synapsis
an process in which the DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid. Thus, a paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point, and vice versa.
For the following human inherited conditions, describe the condition and give the pattern of inheritance: Huntington's disease
caused by a lethal dominant allel eand is a degenerative disease of the nervous system. Has no phenotypic effect until the individual is about 35 to 45. Once deterioration begins, its irreversible and fatal. By then, the parent may have reproduced and passed the disease of the dominant allele on. 1 in 10,00
gene
coded information in the form of hereditary units
For the following human inherited conditions, describe the condition and give the pattern of inheritance: sickle-cell disease
common of African descent, one out of 400. Caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells. In homozygous individuals, all hemoglobin is of the sickle-cell (abnormal variety). The presence of one sickle-cell allele can affect the phenotype too (incomplete dominance), genetically (codominant).
genome
consists of the genes and other DNA that make up the chromosomes we inherited from our parents
You will see that plants have a life cycle that involves spores, which form as a result of meiosis, so these spores are haploid. Notice also that both haploid and diploid cells can divide by mitosis. However, meiosis always begins with cells that are ________, and as a result of meiosis, daughter cells are formed that are always ________. These cells can be gametes (in animals) or spores (in plants).
diploid, haploid
For the following human inherited conditions, describe the condition and give the pattern of inheritance: Tay-Sachs disease
disorder with issues with lipid accumulation in the brain, very high among Ashkenazic Jews. One out of 3,600 births recessive
In the third concept of Mendel's model, he explains that the ___________ determines the organism's appearance if the two alleles at a locus differ while the ___________ has no noticeable effect on the organism's appearance.
dominant allele, recessive allele
Describe the chromosomes of the daughter cells.
each daughter cell has half as many chromosomes as the parent cell--one set, rather than two
How many daughter cells are formed at the conclusion of meiosis?
four daughter cells
After Anaphase I and Telophase I (the end of the first meiotic division), are the daughter cells haploid or diploid?
haploid
For the following human inherited conditions, describe the condition and give the pattern of inheritance: cystic fibrosis
lethal genetic disease. One out of every 2,500 people in European descent. One out of 25 are carriers. Normal alles codes for a membrane protein that functions in the transport of chloride ions between certain cells and the extracellular fluid. These chloride channels are absent or defective of children who are recessive = abnormally high intracellular chloride which cause an uptake of water due to osmosis, leads to excessive mucus build up that is deadly.
For the following human inherited conditions, describe the condition and give the pattern of inheritance: albinism
recessive disorder with the lack of skin pigment, inherited aa form Aa x Aa parents with a ¼ chance
Deletion (chromosomal)
removes a chromosomal segment
gametes
reproductive cells
Inversion (chromosomal)
reverses a segment within a chromosome
sexual reproduction
two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
