Chapter 9 Reading Guide

What is the definition of a sex-linked gene?

A gene located on either sex chromosome. Most sex-linked genes are on the X chromosome and show distinctive patterns of inheritance; there are very few genes on the Y chromosome.

Alfred H. Sturtevant, a student of Thomas Hunt Morgan, used assumptions from observations of crossovers to map genes. What is a linkage map?

A linkage map is a genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.

What is a map unit?

A map unit is a unit of measurement of the distance between genes. One map unit is equivalent to a 1% recombination frequency.

Mendel's first law: Principle of segregation

Alternative versions of genes account for variations in inherited characteristics.

What unusual result suggested that the eye-color trait is located on the X chromosome?

The white-eye trait showed up only in males.

What is the P generation?

Parent generation (true-breeding parents)

State the addition rule and give an original example

The addition rule states that the probability that any two or more mutually exclusive events will occur is calculated by adding their individual probabilities.

In humans, how has that term been historically modified?

Typically, the sex-linked traits are actually X-linked genes.

What is the F1 generation?

First filial generation (hybrid offspring of the P generation)

Mendel's second law: Principle of independent assortment

For each character, an organism inherits two copies of a gene, one from each parent.

What is the SRY gene? Where is it found, and what does it do?

SRY refers to the Sex determining Region of Y, a gene found on the Y chromosome that is required for the development of testes.

What is the F2 generation?

Second filial generation (offspring of F1 generation)

Explain the law of independent assortment.

The law of independent assortment states that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.

X inactivation maintains the proper gene dosage. How is the X chromosome inactivated?

The selection of which X chromosome will form the Barr body occurs randomly and independently in each embryonic cell present at the time of X inactivation.

Explain the three main steps in amniocentesis.

1. A sample of amniotic fluid can be taken starting at the fourteenth to sixteenth week of pregnancy. 2. Biochemical and genetic tests can be performed immediately on the amniotic fluid or later on the cultured cell. 3. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping.

Explain the two main steps of CVS.

1. A sample of chorionic villus tissue can be taken as early as the eight to tenth week of pregnancy. 2. Karyotyping and biochemical and genetic tests can be performed ont he fetal cells immediately, providing results within a day or so.

Name and describe 3 human sex-linked disorders

1. Duchenne muscular dystrophy: A human genetic disease caused by a sex-linked recessive allele; characterized by progressive weakening and a loss of muscle tissue 2. Hemophilia: A human genetic disease caused by a sex-linked recessive allele, resulting in the absence of one or more blood-clotting proteins; characterized by excessive bleeding following injury 3. Color Blindness: A mild disorder almost always inherited as an X-linked allele

Thomas Hunt Morgan selected Drosophila melanogaster as his experimental organism. List at least three reasons the fruit fly is an excellent subject for genetic studies.

1. Single mating will produce hundreds of offspring. 2. A new generation can be bred every two weeks. 3. The fruit fly has only four pairs of chromosomes, which are easily distinguishable with a light microscope.

What is a Barr body? Why do human females show a Barr body in their cells?

A Barr body is a dense object lyring along the inside of the nuclear envelope in cells of female mammals, representing a highly condensed, inactivated X chromosome. Female mammals, including humans, inherit two X chromosomes—twice the number inherited by males; females show a Barr body in their cells so that the cells of females and males have the same effective dose (one copy) of most X-linked genes.

Define aneuploidy

A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number. Trisomy 21 (Down syndrome) is an aneuploidy.

Define polyploidy

A chromosomal alteration in which the organism possesses more than two complete chromosome sets. It is the result of an accident of cell division.

What is a dihybrid cross?

A cross between two organisms that are each heterzygous for both of the characters being followed (or the self-pollination of a plant that is heterozygous for both characters)

What is a monohybrid cross?

A cross between two organisms that are heterozygous for the character being followed (or the self-pollination of a heterozygous plant)

In pea plants, T is the allele for tall plants, while 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.

A testcross always involves crossing the unknown phenotype with an individual that is homozygous recessive for the trait in question. In this case, a homozygous tall plant crossed with a homozygous recessive dwarf will yield all tall offspring. A heterozygous tall plant crossed with a dwarf will yield an offspring ratio of one tall plant to one dwarf plant. The presence of dwarf plants indicates that the previously unknown tall plant is heterozygous.

In probability, what is an independent event?

An event whose outcome is unaffected by what has happened on previous trials

Use the figure below, which is from Figure 15.10. It shows the results of a cross between a fruit fly that is heterozygous for a gray body with normal wings, and a fruit fly that has a black body with vestigial wings. Because these genes are linked, the results are not what might have been predicted. Show the phenotypes and number of each type of offspring. Indicate which offspring are the recombinants and which are the parental type. Finally, calculate the map distance between the two genes. Show all your work here.

Since 391 of the total 2,300 offspring do not show the parental phenotypes, they are recombinants. Since the frequency of recombination is therefore 17%, the two genes are 17 map units apart.

Explain the law of segregation.

The law of segregation states that the two alleles for each gene separate during gamete formation.

State the multiplication rule and give an original example

The multiplication rule states that to determine this probability, we multiply the probability of one event by the probability of the other event. For example, by the multiplication rule, the probability that both coins will land heads up is ½ × ½ = ¼.

Why can you say that all calico cats are females?

The tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype requires the presence of two different alleles, one for orange fur and one for black fur. Normally, only females can have both alleles, because only they have two X chromosomes.

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.

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 zygote expresses only one allele of an imprinted gene that is inherited from either the female or the male parent. The imprints are then transmitted to all body cells during development.

Blood groups are so important medically that you should be able to solve genetics problems based on blood types. The first step in accomplishing that is to understand the genotypes of each blood type. Before working any problems, complete this ABO blood type chart.

Genotype: a) I^A I^A or I^A i b) I^B I^B or I^B i c) I^A I^B d)ii Phenotype (blood group) a) A b) B c) AB d) O

You should have identified mitochondrial DNA as the correct response to question 29 above. What other organelle has its own genes? These are extranuclear genes.

Chloroplast or mitochondria

What causes Down syndrome? What are four characteristics of Down syndrome?

Down syndrome is usually the result of an extra chromosome 21, so that each cell has a total of 47 chromosomes. Because the cells are trisomic for chromosome 21, Down syndrome is often called trisomy 21. Four characteristics of Down syndrome: 1. Characteristic facial features 2. Short stature 3. Correctable heart defects 4. Developmental delays

Explain Mendel's law of independent assortment

Each pair of alleles segregates independently of each other pair of alleles during gamete formation.

In sexually reproducing organisms, why are there exactly two chromosomes in each homologue?

Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent.

Explain epistasis.

Epistatis is a type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene.

Which generation is completely heterozygous?

F1 generation

Which generation has both heterozygous and homozygous offspring?

F2 generation

Why is height a good example of polygenic inheritance?

For many characters, such as human skin color and height, an either-or classification is impossible because the characters vary in the population in gradients along a continuum.

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?

Geneticists call the offspring that show these new combinations recombinant types, or recombinants for short.

Compare and contrast codominance with incomplete dominance.

In codominance, the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways, such as in the human MN blood group, determined by the codominant alleles for two specific molecules located on the surface of red blood cells, the M and N molecules.

Explain how incomplete dominance is different from complete dominance, and give an example.

Incomplete dominance: The situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. Complete dominance: The situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable.

What are linked genes? Do linked genes sort independently?

Linked genes are genes located close enough together on a chromosome that they tend to be inherited together. These genes do not sort independently, but rather are transmitted as a unit. It is important to note that as Morgan's experiments illustrated, some mechanism (later discovered to be "crossing over") occasionally breaks the linkage between specific alleles of genes on the same chromosome.

One of the keys to success for Mendel was using pea plants. Explain how using pea plants allowed Mendel to control mating; that is, how did this approach let Mendel be positive about the exact characteristics of each parent?

Mendel could always be sure of the parentage of new seeds because he removed the immature stamens of a plant before they produced pollen. He then dusted pollen from another plant onto the altered flowers, which resulted in zygotes developed into plant embryos.

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?

Mitochondrial DNA

Explain what is meant when a gene is said to have multiple alleles.

Most genes exist in more than two allelic forms

Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.

Natural selection determines how common an allele is in the gene pool. For example, having six fingers (polydactyly) is dominant to five fingers, but the presence of six fingers is not common in the human gene pool.

What occurs in nondisjunction?

Nondisjunction is an error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.

What is the F2 phenotypic and genotypic ratio?

Phenotype 3:1, Genotype 1:2:1

What is pleiotropy? Explain why this is important in diseases like cystic fibrosis and sickle-cell disease.

Pleiotropy is the ability of a single gene to have multiple effects. In humans, pleiotropic alleles are responsible for multiple symptons associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.

Quantitative variation usually indicates _________________.

Polygenic inheritance

Define monosomy

Refers to a diploid cell that has only one copy of a particular chromosome instead of the normal two. Turner syndrome is a human monosomy; the female has only one X chromosome

Define trisomy

Refers to a diploid cell that has three copies of a particular chromosome instead of the normal two. Trisomy 21 is trisomic for chromosome 21

Explain how Mendel's simple cross of purple and white flowers did the following: -refuted blending -determined dominant and recessive characteristics -demonstrated the merit of experiments that covered multiple generations

Refuted blending: The reappearance of white-flowered plants in the F2 generation was evidence that the heritable factor causing white flowers had not been diluted or destroyed by coexisting with the purple-flower factor Determined dominant and recessive characteristics: Mendel reasoned that the heritable factor for white flowers did not disappear in the F1 plants, but was somehow hidden, or masked, when the purple-flower factor was present. In Mendel's terminology, purple flower color is a dominant trait, and white flower color is a recessive trait. Demonstrated the merit of experiments that covered multiple generations: Analyzing thousands of genetic crosses from F2 plants allowed him to deduce two fundamental principles of heredity: the law of segregation and the law of independent assortment

What are the strengths and weaknesses of each fetal test?

Strength of amniocentesis: In addition to fetal cells, amniotic fluid is also collected. Amniotic fluid can be used to detect additional enzymatic or developmental problems not detectable from the karyotype. Weakness of amniocentesis: Cells must be cultured for several weeks before karyotyping, and the test cannot be performed until the fourteenth to sixteenth week. Strength of CVS: These cells proliferate rapidly enough to allow karyotyping to be carried out immediately, and CVS can be performed as early as the eighth to tenth week. Weakness of CVS: No amniotic fluid is collected with this technique.

Explain the concept of blending, and then describe how Mendel's particulate (gene) theory was different.

The blending concept is genetic material contributed by the two parents mixes in a manner like mixing blue and yellow paint to make green. It predicted that over many generations, a freely mating population would produce a uniform population of individuals. But the gene hypothesis said parents pass on discreet heritable genes that retain their separate identities in offspring.

Using the terms norm of reaction and multifactorial, explain the potential influence of the environment on phenotypic expression.

The outcome of a genotype lies within its norm of reaction, a phenotype range that depends on the environment in which the genotype of expressed. For some characters, such as the ABO blood group system, the norm of reaction has no breadth whatsoever. Other characteristics, such as a person's blood count of red and white cells, varies quite a bit, depending on such factors as the altitude, the customary level of physical activity, and the presence of infectious agents. Genetics refers to such characters as multifactorial, meaning that many factors, both genetic and environmental, collectively influence phenotype.

Explain why you know the genotype of one female in the third generation, but are unsure of the other.

The presence of a free earlobe could indicate either an FF or Ff genotype, as F is the dominant allele, resulting in free earlobes. The female with the recessive trait can only have one genotype. The female with the dominant trait could be homozygous or heterozygous.

What is the probability that a couple will have a girl, a boy, a girl, and a boy in this specific order?

The probability is 1/16.

Explain the symptoms of phenylketonuria, and describe how newborn screening is used to identify children with this disorder.

The symptoms of phenylketonuria include an inability to metabolize the amino acid phenylalanine, causing severe mental intellectual disability. Some genetic disorders, including phenylketonuria, can be detected at birth by simple biochemical tests that are now routinely performed in most hospitals in the United States.

Describe what you think is medically important to know about the behavior of recessive alleles.

Thousands of genetic disorders are known to be inherited as simple recessive traits. These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancers and vision problems) to life-threatening such as cystic fibrosis.

What is the chromosome theory of inheritance.

a basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.

Students are expected to have a general knowledge of the pattern of inheritance and the common symptoms of a number of genetic disorders. Provide this information for the disorders listed below. a) cystic fibrosis b) sickle-cell disease c) achondroplasia d) Huntington's disease

a) A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated. b) A recessively inherited human blood disorder in which a single nucleotide change in the β-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals. c) A form of dwarfism that occurs in one of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Like the presence of extra fingers or toes, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele. d) : A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.

For each of the following human aneuploidies, give the sex of the individual as well as any physical manifestation of the syndrome. a) XXY b) XXX c) XO d) XYY

a) Male - Klinefelter syndrome: male sex organs, but abnormally small/ sterile testes; some breast enlargement and other female characteristics b) Female - Trisomy X: no unusual physical features other than being slightly taller than average; at risk for learning disabilities; fertile c) Female - Turner syndrome: phenotypically female, but sterile due to lack of maturation in sex organs; secondary sex characteristics developed with estrogen replacement; normal intelligence d) Male - Normal sexual development; taller than average stature

The notation for wild type and mutant traits follows some accepted conventions. Notate the following genotypes for a female fruit fly: a) a fly homozygous for red eyes b) a fly heterozygous for red eyes c) a fly homozygous for white eyes

a) X^W+ X^W+ b) X^W+ X^W c) X^W X^W

When Thomas Hunt Morgan mated a white-eyed male fly with a red-eyed female, he came to the startling conclusion that the trait for eye color was located on the chromosome that determines sex. Show this cross. Begin with the parental generation, and go through the F2. a) parental generation b) F1 generation c) F2 generation

a) a red-eyed female was crossed with a white-eyed male b) all the offspring of the P generation had red eyes c) the offspring showed a ratio of three red-eyed flies to one white-eyed fly. However, there were no white-eyed females.

Alternate versions of the same gene, like purple and white flower color, are termed _______.

alleles

Chromosome strucutre can be altered in several ways. Label each type of alteration shown in this figure, and explain what occurs.

deletion: A deficiency in a chromosome resulting from the loss of a fragment through breakage duplication: An aberration in chromosome structure due to fusion with a fragment from a homologous chromosome, such that a portion of a chromosome is duplicated inversion: An aberration in chromosome structure resulting from reattachment of a chromosomal fragment in a reverse orientation to the chromosome from which it originated translocation: An aberration in chromosome structure resulting from attachment of a chromosomal fragment to a nonhomologous chromosome