Principles of Genetics

D. Peter Snustad, Michael J. Simmons

Chapter 6

Variation in Chromosome Number and Structure - all with Video Answers

Educators

Chapter Questions

Problem 1

In the human karyotype, the X chromosome is approximately the same size as seven of the autosomes (the socalled $C$ group of chromosomes). What procedure could be used to distinguish the $\mathrm{X}$ chromosome from the other members of this group?

Hailey Ames

Hailey Ames

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Problem 2

In humans, a cytologically abnormal chromosome 22,who had 46 chromosomes in his somatic cells, including one normal 22 and one Philadelphia chromosome? called the "Philadelphia" chromosome because of the city in which it was discovered, is associated with chronic leukemia. This chromosome is missing part of its long arm. How would you denote the karyotype of an individual

Hailey Ames

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Problem 3

During meiosis, why do some tetraploids behave more regularly than triploids?

Hailey Ames

Numerade Educator

Problem 4

The following table presents chromosome data on four species of plants and their $\mathrm{F}_{1}$ hybrids:
(a) Deduce the chromosomal origin of species $A$.
(b) How many bivalents and univalents would you expect to observe at meiotic metaphase I in a hybrid between species $C$ and species B?
(c) How many bivalents and univalents would you expect to observe at meiotic metaphase I in a hybrid between species D and species $\mathrm{B}$ ?

Danielle Ashley

Danielle Ashley

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Problem 5

A plant species $A,$ which has seven chromosomes in its gametes, was crossed with a related species $\mathrm{B}$, which has nine. The hybrids were sterile, and microscopic observation of their pollen mother cells showed no chromosome pairing. A section from one of the hybrids that grew vigorously was propagated vegetatively, producing a plant with 32 chromosomes in its somatic cells. This plant was fertile. Explain.

Hailey Ames

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Problem 6

A plant species $X$ with $n=5$ was crossed with a related species $Y$ with $n=7 .$ The $F_{1}$ hybrid produced only a few pollen grains, which were used to fertilize the ovules of species Y. A few plants were produced from this cross, and all had 19 chromosomes. Following self-fertilization, the $F_{1}$ hybrids produced a few $F_{2}$ plants, each with 24 chromosomes. These plants were phenotypically different from either of the original species and were highly fertile. Explain the sequence of events that produced these fertile $\mathrm{F}_{2}$, hybrids.

Hailey Ames

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Problem 7

Identify the sexual phenotypes of the following genotypes in humans: $\mathrm{XX}, \mathrm{XY}, \mathrm{XO}, \mathrm{XXX}, \mathrm{XXY}, \mathrm{XYY}$

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Problem 8

If nondisjunction of chromosome 21 occurs in the division of a secondary oocyte in a human female, what is the chance that a mature egg derived from this division will receive two number 21 chromosomes?

Hailey Ames

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Problem 9

A Drosopbila female homozygous for a recessive X-linked mutation causing yellow body was crossed to a wild-type male. Among the progeny, one fly had sectors of yellow pigment in an otherwise gray body. These yellow sectors were distinctly male, whereas the gray areas were female. Explain the peculiar phenotype of this fly.

Hailey Ames

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Problem 10

The Drosopbila fourth chromosome is so small that flies monosomic or trisomic for it survive and are fertile. Several genes, including eyeless $(e y),$ have been located on this chromosome. If a cytologically normal fly homozygous for a recessive eyeless mutation is crossed to a fly monosomic for a wild-type fourth chromosome, what kinds of progeny will be produced, and in what proportions?

Hailey Ames

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Problem 11

A woman with X-linked color blindness and Turner syndrome had a color-blind father and a normal mother. In which of her parents did nondisjunction of the sex chromosomes occur?

Hailey Ames

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Problem 12

In humans, Hunter syndrome is known to be an X-linked trait with complete penetrance. In family A, two phenotypically normal parents have produced a normal son, a daugbter with Hunter and Turner syndromes, and a son with Hunter syndrome. In family $B$, two phenotypically normal parents have produced two phenotypically normal daughters and a son with Hunter and Klinefelter syndromes. In family $C$, two phenotypically normal parents have produced a phenotypically normal daughter, a daugbter with Hunter syndrome, and a son with Hunter syndrome. For each family, explain the origin of the child indicated in italics.

Hailey Ames

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Problem 13

Although XYY men are phenotypically normal, would they be expected to produce more children with sex chromosome abnormalities than XY men? Explain.

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Problem 14

In a Drosopbila salivary chromosome, the bands have a sequence of $12345678 .$ The homologue with which this chromosome is synapsed has a sequence of 12365478.What kind of chromosome change has occurred? Draw the synapsed chromosomes.

Hailey Ames

Numerade Educator

Problem 15

Other chromosomes have sequences as follows:
(a) $125678 ;(b) 123445678 ;(c) 12345876 .$ What
kind of chromosome change is present in each? Illustrate how these chromosomes would pair with a chromosome whose sequence is 12345678.

Hailey Ames

Numerade Educator

Problem 16

In plants translocation heterozygotes display about 50 percent pollen abortion. Why?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 17

One chromosome in a plant has the sequence $A B C D E F,$ and another has the sequence $M$ N $O$ P $Q$ R. A reciprocal translocation between these chromosomes produced the following arrangement: $A B C P Q R$ on one chromosome and $M N O D E F$ on the other. Illustrate how these translocated chromosomes would pair with their normal counterparts in a heterozygous individual during meiosis.

Hailey Ames

Numerade Educator

Problem 18

In Drosopbila , the genes $b w$ and $s t$ are located on chromosomes 2 and $3,$ respectively. Flies homozygous for $b w$ mutations have brown eyes, flies homozygous for st mutations have scarlet eyes, and flies homozygous for $b w$ and st mutations have white eyes. Doubly heterozygous males were mated individually to homozygous $b w ;$ st females. All but one of the matings produced four classes of progeny:
wild-type, and brown-, scarlet- and white-eyed. The single exception produced only wild-type and white-eyed progeny. Explain the nature of this exception.

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 19

A phenotypically normal boy has 45 chromosomes, but his sister, who has Down syndrome, has $46 .$ Suggest an explanation for this paradox.

Hailey Ames

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Problem 20

Distinguish between a compound chromosome and a Robertsonian translocation.

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Problem 21

A yellow-bodied Drosopbila female with attached-X chromosomes was crossed to a white-eyed male. Both of the parental phenotypes are caused by X-linked recessive mutations. Predict the phenotypes of the progeny.

Hailey Ames

Numerade Educator

Problem 22

A man has attached chromosomes $21 .$ If his wife is cytologically normal, what is the chance their first child will have Down syndrome?

Josee Pacheco

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Problem 23

Analysis of the polytene chromosomes of three populations of Drosophila has revealed three different banding sequences in a region of the second chromosome:
$$\begin{array}{cl}
\text { Population } & \text { Banding Sequence } \\
\hline \mathrm{P} 1 & 12345678910 \\
\mathrm{P} 2 & 12398765410 \\
\mathrm{P} 3 & 12398567410 \\
\hline
\end{array}$$
Explain the evolutionary relationships among these populations.

Josee Pacheco

Numerade Educator

Problem 24

Each of six populations of Drosopbila in different geographic regions had a specific arrangement of bands in one of the large autosomes:
(a) 12345678
(b) 12263478
(c) 15432678
(d) 14322678
(e) 16223478
(f) 154322678
Assume that arrangement (a) is the original one. In what order did the other arrangements most likely arise, and what type of chromosomal aberration is responsible for each change?

Hailey Ames

Numerade Educator

Problem 25

The following diagram shows two pairs of chromosomes in the karyotypes of a man, a woman, and their child. The man and the woman are phenotypically normal, but the child (a boy) suffers from a syndrome of abnormalities, including poor motor control and severe mental impairment. What is the genetic basis of the child's abnormal phenotype? Is the child hyperploid or hypoploid for a segment in one of his chromosomes?

Hailey Ames

Numerade Educator

Problem 26

A male mouse that is heterozygous for a reciprocal translocation between the $\mathrm{X}$ chromosome and an autosome is crossed to a female mouse with a normal karyotype. The autosome involved in the translocation carries a gene responsible for coloration of the fur. The allele on the male's translocated autosome is wild-type, and the allele on its nontranslocated autosome is mutant; however, because the wild-type allele is dominant to the mutant allele, the male's fur is wild-type (dark in color). The female mouse has light color in her fur because she is homozygous for the mutant allele of the color-determining gene. When the offspring of the cross are examined, all the males have light fur and all the females have patches of light and dark fur. Explain these peculiar results.

Hailey Ames

Numerade Educator

Problem 27

In Drosopbila , the autosomal genes cinnabar $(\mathrm{cn})$ and brown $(b w)$ control the production of brown and red eye pigments, respectively. Flies homozygous for cinnabar mutations have bright red eyes, flies homozygous for brown mutations have brown eyes, and flies homozygous for mutations in both of these genes have white eyes. A male homozygous for mutations in the $c n$ and $b w$ genes has bright red eyes because a small duplication that carries the wild-type allele of $b w\left(b w^{+}\right)$ is attached to the $Y$ chromosome. If this male is mated to a karyotypically normal female that is homozygous for the $c n$ and $b$ re mutations, what types of progeny will be produced?

Hailey Ames

Numerade Educator

Problem 28

In Drosophila, vestigial wing (vg), hairy body ( $b$ ), and eyeless $(e y)$ are recessive mutations on chromosomes $2,3,$ and
4, respectively. Wild-type males that had been irradiated with $\mathrm{X}$ rays were crossed to triply homozygous recessive females. The $\mathrm{F}_{1}$ males (all phenotypically wild-type) were then testcrossed to triply homozygous recessive females. Most of the $\mathrm{F}_{1}$ males produced eight classes of progeny in approximately equal proportions, as would be expected if the $v g, b,$ and $e y$ genes assort independently. However, one $\mathrm{F}_{1}$ male produced only four classes of offspring, each approximately one-fourth of the total:
(1) wild-type,
(2) eyeless,
(3) vestigial, hairy, and
(4) vestigial, hairy, eyeless. What kind of chromosome aberration did the exceptional $\mathrm{F}_{1}$ male carry, and which chromosomes were involved?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 29

Cytological examination of the sex chromosomes in a man has revealed that he carries an insertional translocation. A small segment has been deleted from the Y chromosome and inserted into the short arm of the X chromosome; this segment contains the gene responsible for male differentiation $(S R Y) .$ If this man marries a karyotypically normal woman, what types of progeny will the couple produce?

Hailey Ames

Numerade Educator