Deck 3: Sex Determination and Sex-Linked Characteristics

Question 1

In sexual reproduction, parents contribute genes to produce an offspring that is genetically distinct from both parents. In most eukaryotes, sexual reproduction consists of meiosis, which produces haploid gametes (or spores), and fertilization, which produces a diploid zygote.
-What process causes the genetic variation seen in offspring produced by sexual reproduction?

Answer 1

Meiosis

Question 2

The discovery that the presence or absence of particular chromosomes determines sex in insects provided evidence that genes are located on chromosomes. In XX-XO sex determination, the male is XO and heterogametic, and the female is XX and homogametic. In XX-XY sex determination, the male is XY and the female is XX; in this system, the male is heterogametic. In ZZ-ZW sex determination, the female is ZW and the male is ZZ; in this system, the female is the heterogametic sex.

-How does the heterogametic sex differ from the homogametic sex?

A) The heterogametic sex is male; the homogametic sex is female.
B) Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.
C) Gametes of the heterogametic sex all contain a Y chromosome; gametes of the homogametic sex all contain an X chromosome.

Answer 2

Gametes of the heterogametic sex have different sex chromosomes; gametes of homogametic sex have the same sex chromosome.

Question 3

In genic sex determination, sex is determined by genes at one or more loci, but there are no obvious differences in the chromosomes of males and females. In environmental sex determination, sex is determined fully or in part by environmental factors.
-How do chromosomal, genic, and environmental sex determination differ?

Answer 3

In chromosomal sex determination, males and females have chromosomes that are distinguishable. In genic sex determination, sex is determined by genes, but the chromosomes of males and females are indistinguishable. In environmental sex determination, sex is determined fully or in part by environmental effects.

Question 4

The sexual phenotype of a fruit fly is determined by the ratio of the number of X chromosomes to the number of haploid sets of autosomal chromosomes (the X : A ratio)

-What is the sexual phenotype of a fruit fly that has XXYYY sex chromosomes and two sets of autosomes?

A) Male
B) Female
C) Intersex
D) Metamale

Question 5

The presence of the SRY gene on the Y chromosome causes a human embryo to develop as a male. In the absence of this gene, a human embryo develops as a female.

-What is the phenotype of a person who has XXXY sex chromosomes?

A) Klinefelter syndrome
B) Turner syndrome
C) Poly-X female

Question 6

By showing that the appearance of rare phenotypes is associated with the inheritance of particular chromosomes, Bridges proved that sex-linked genes are located on the X chromosome and that the chromosome theory of inheritance is correct.

-What was the genotype of the few F₁ red-eyed males obtained by Bridges when he crossed a white-eyed female with a red-eyed male?

A) X⁺
B) X^W X⁺ Y
C) X⁺ Y
D) X⁺ X⁺ Y

Question 7

Characteristics determined by genes on the sex chromosomes are called sex-linked characteristics. Diploid females have two alleles at each X-linked locus, whereas diploid males possess a single allele at each X-linked locus. Females inherit X-linked alleles from both parents, but males inherit a single X-linked allele from their mothers.
-Hemophilia (reduced blood clotting) is an X-linked recessive disease in humans. A woman with hemophilia mates with a man who exhibits normal blood clotting. What is the probability that their child will have hemophilia?

Question 8

Y-linked characteristics exhibit a distinct pattern of inheritance: they are present only in males, and all male offspring of a male with a Y-linked trait inherit the trait. Palindromic sequences within the Y chromosome can undergo internal recombination, but such recombination may lead to chromosome anomalies.
-What unusual feature of the Y chromosome allows some recombination among the genes found on it?

Question 9

In mammals, dosage compensation ensures that the same amount of X-linked gene product will be produced in the cells of both males and females. All but one X chromosome are inactivated in each cell; which of the X chromosomes is inactivated is random and varies from cell to cell.
-How many Barr bodies will a male with XXXYY chromosomes have in each of his cells? What are these Barr bodies?