Chapter 7 Gentics

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is inherited as an X-linked recessive gene in humans. A woman whose father suffered from G6PD marries a normal man. (a) What proportion of their sons is expected to be G6PD? (b) If the husband were not normal, but were G6PD deficient, would you change your answer in part (a)?

(a) 1/2 (b) no

One form of hemophilia is caused by a sex-linked recessive gene. Assume that a man with hemophilia marries a phenotypically normal woman whose father had hemophilia. What is the probability that their first son will have hemophilia?

1/2

A cross is made between a female calico cat and a male cat having the gene for black fur on his X chromosome. What fraction of the offspring would one expect to be calico?

1/4

One form of hemophilia is caused by a sex-linked recessive gene. Assume that a man with hemophilia marries a phenotypically normal woman whose father had hemophilia. What is the probability that they will have a daughter with hemophilia? (Note: In this problem, you must include the probability of having a daughter in your computation of the final probability.

1/4

The sex of birds, some insects, and other organisms is determined by a ZW chromosomal arrangement in which the males have like sex chromosomes (ZZ) and females are ZW (similar to XY in humans). Assume that a recessive lethal allele on the Z chromosome causes death of an embryo in birds. What sex ratio would result in the offspring if a cross were made between a male heterozygous for the lethal allele and a normal female?

2:1 male to female

Klinefelter and Turner syndromes have how many chromosomes, respectively?

47, 45

Klinefelter syndrome in humans, which leads to underdeveloped testes and sterility, is caused by which chromosomal condition?

47, XXY

A recessive gene for red-green color blindness is located on the X chromosome in humans. Assume that a woman with normal vision (her father is color blind) marries a color-blind male. What is the likelihood that this couple's first son will be color blind?

50%

Dosage compensation leads to a variety of interesting coat color patterns in certain mammals. For instance, a female cat that is heterozygous for two coat color alleles, say black and orange, will usually have the "calico" or mosaic phenotype. Describe the chromosomal basis for the mosaicism (calico) in the female. Explain why chromosomally normal male cats do not show the mosaic phenotype, but XXY male cats can be calico.

Because of dosage compensation, one of the X chromosomes randomly "turns off" early in development. Once such a chromosome is inactivated, it remains so in daughter cells. Recessive alleles on the remaining active X chromosome are expressed because their normal allele (on the inactive X chromosome) is not capable of expression. Because males typically have only one X chromosome, X chromosome inactivation does not occur; however, in XXY males that are heterozygous for certain coat color genes, such inactivation and mosaicism is possible.

List three abnormalities involving numbers of X chromosomes.

Klinefelter syndrome, Turner syndrome, XXXX syndrome

Assume that you are told that a particular organism, Drosophila, has the XO chromosome complement. You are also told that the autosomal complement is a normal 2n. You know that in humans the XO complement is female determining. Would you be correct in assuming that the Drosophila sex for XO is also female? Choose the answer that includes the correct explanation.

No, the chromosomal basis for sex determination in Drosophila based on the balance between the number of X chromosomes and haploid sets of autosomes.

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's father had red eyes, and the mother had white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality.

Nondisjunction could have occurred either at meiosis I or meiosis II in the mother, thus giving the XwXwY complement in the offspring.

In Drosophila, an individual female fly was observed to be of the XXY chromosome complement (normal autosomal complement) and to have white eyes as contrasted with the normal red eye color of wild type. The female's mother and father had red eyes. The mother, however, was heterozygous for the gene for white eyes. Knowing that white eyes are X-linked and recessive, present an explanation for the genetic and chromosomal constitution of the XXY, white-eyed individual. It is important that you state in which parent and at what stage the chromosomal event occurred that caused the genetic and cytogenetic abnormality

Nondisjunction would have occurred at meiosis II in the mother, thus giving the XwXwY complement in the offspring. Section: 7.6

Describe three distinct genetic regions of the human Y chromosome.

PARs = pseudoautosomal regions, NRY = nonrecombining region of the Y, SRY = sex-determining region

A small part of the human Y chromosome contains the gene that is responsible for determining maleness. What is the name of this gene?

SRY (sex-determining region Y)

Give the sex of the following organisms assuming that the autosomes are present in the normal number. Sex Chromosome Organism Complement Humans Drosophila XX _____ _____ XY _____ _____ XO _____ _____ XXX _____ _____ XXY _____ _____

Sex Chromosome Organism Complement Humans Drosophila XX female female XY male male XO female male XXX female female XXY male female

A color-blind, chromatin-positive male child (one Barr body) has a maternal grandfather who was color blind. The boy's mother and father are phenotypically normal. Construct and support (using appropriately labeled diagrams) a rationale whereby the chromosomal and genetic attributes of the chromatin-positive male are fully explained.

The female (mother) must be heterozygous and undergo nondisjunction at meiosis II to produce the XrgXrgY boy.

A color-blind woman with Turner syndrome (XO) has a father who is color blind. Given that the gene for the color-blind condition is recessive and X-linked, provide a likely explanation for the origin of the color-blind and cytogenetic conditions in the woman.

The woman inherited an Xrg chromosome from the father. Nondisjunction in the female (either at meiosis I or II) produced an egg with no X chromosome, which, when fertilizedby the Xrg-bearing sperm, produced the Turner syndrome condition.

Studies done in the 1960s suggested that individuals with the XYY condition were prone to criminal behavior. What conclusions presently seem appropriate concerning this chromosomal condition?

There is a high, but not constant, correlation between the extra Y chromosome and the predisposition of males to have behavioral problems.

Under what condition might a human female have the XY sex chromosome complement?

This female would have one complete X chromosome and a Y chromosome that lacks SRY.

Describe an experiment in which transgenic mice were used to identify the male-determining region of the Y chromosome.

When DNA containing only the mouse SRY is injected into normal XX mouse eggs, most of the offspring develop into males.

In Drosophila, sex is determined by a balance between the number of haploid sets of autosomes and the number of ________.

X chromosomes

What is the composition of a Barr body?

X chromosome—with associated proteins

Give the sex-chromosome constitution (X and Y chromosomes) and possible genotypes of offspring resulting from a cross between a white-eyed female (Xw XwY) and a wild-type male (normal chromosome complement) in Drosophila melanogaster. Include all zygotic combinations whether viable or inviable.

X+XwXw = inviable (dies at third instar stage) XwXwY = white-eyed female X+Y = wild-type male YY = inviable (dies at egg stage) X+XwY = wild-type female XwYY = white-eyed male X+Xw = wild-type female XwY = white-eyed male

The Protenor mode of sex determination is the ________.

XX/XO scheme

The Lygaeus mode of sex determination is the ________.

XY/XX scheme

What particular karyotype was at one time considered to be related to criminal predisposition?

XYY

Data produced by C. Bridges in the early part of this century indicate that sex in Drosophila is determined by ________.

a balance between the number of X chromosomes and the number of haploid sets of autosomes

In humans, the genetic basis for determining the sex "male" is accomplished by the presence of ________.

a portion of the Y chromosome

Assume that a man who carries an X-linked gene has children. Assuming normal meiosis and random combination of gametes, the man would pass this gene to ________.

all of his daughters

Dosage compensation in mammals typically involves the random inactivation of one of the two X chromosomes relatively early in development. Such X-chromosome inactivation often leads to phenotypic mosaicism. Assume that black fur in cats is due to the X-linked recessive gene b, whereas its dominant allele B produces yellow fur. A Bb heterozygote is a mosaic called "tortoise shell" or "calico." Using appropriate gene symbols, diagram a mating between a black male and a calico female. Give the phenotypes and genotypes of all the offspring.

bY × Bb = Bb (calico female), bb (black female), BY (yellow male), bY (black male)

What can cause phenotypic mosaicism for X-linked genes in female mammals?

dosage compensation involving the X chromosome

In Drosophila, the sex of a fly with the karyotype XO:2A is ________.

male

Assuming a normal number of autosomes, what would be the sex of the following: XXY mouse, XXY Drosophila?

male and female, respectively

Individuals have been identified who have two different karyotypes, such as 45,X/46,XY or 45,X/46,XX. Such individuals are called ________.

mosaics

For an individual with the XXY chromosomal composition, the expected number of Barr bodies in interphase cells is ________.

one

How many Barr bodies would one expect to see in cells of Turner syndrome females and Klinefelter syndrome males?

zero and one, respectively