Question 1

In mammals, dosage compensation of X-linked genes is achieved by which of the following mechanisms?

Accepted Answer

Inactivation of one X chromosome in females, forming a Barr body

Question 2

A female cat is heterozygous for a gene on the X chromosome that controls fur color (one allele for black, one for orange). What is the most likely explanation for the observed patchy coloration in her fur?

Accepted Answer

Random X-inactivation in each cell during development

Question 3

In Drosophila, sex is determined by the ratio of X chromosomes to sets of autosomes. Which of the following genotypes would result in a female fly?

Accepted Answer

$$\mathrm{XX}; 2A$$

Question 4

A human with Turner syndrome has which of the following karyotypes?

Accepted Answer

$$45, X$$

Question 5

Which of the following best describes a reciprocal cross?

Accepted Answer

A cross in which the phenotypes of the male and female parents are reversed

Question 6

A male with Klinefelter syndrome has which of the following chromosomal compositions?

Accepted Answer

$$47, XXY$$

Question 7

In birds, which of the following is true regarding sex determination?

Accepted Answer

Females are heterogametic ($$ZW$$), and males are homogametic ($$ZZ)$$

Question 8

A woman who is a carrier for an X-linked recessive disorder has children with a man who does not have the disorder. What is the probability that their son will have the disorder?

Accepted Answer

$$50\%$$

Question 9

Which of the following best explains why X-linked recessive traits are more commonly expressed in males than in females?

Accepted Answer

Males have only one X chromosome, so a single recessive allele will be expressed

Question 10

A geneticist observes a family in which a trait appears only in males and is passed from mother to son, but never from father to son. What is the most likely mode of inheritance?

Accepted Answer

X-linked recessive

Question 11

In a cross between a female Drosophila with normal wings (homozygous dominant) and a male with miniature wings (X-linked recessive), what is the expected phenotype of the F1 offspring?

Accepted Answer

All females and all males will have normal wings

Question 12

Which of the following is a real-world example of X-inactivation in humans?

Accepted Answer

The presence of Barr bodies in the nuclei of female somatic cells

Question 13

A genetic disorder is found to be caused by a deletion on the X chromosome. A female with one normal and one deleted X chromosome is phenotypically normal, but her son is affected. What is the most likely explanation?

Accepted Answer

The disorder is X-linked recessive, and males are hemizygous for X-linked genes

Question 14

Which of the following best describes the genetic mechanism underlying Turner syndrome?

Accepted Answer

Monosomy of the X chromosome

Question 15

A male patient is found to have two X chromosomes and one Y chromosome. Which syndrome does he most likely have, and what is a common symptom?

Accepted Answer

Klinefelter syndrome; reduced fertility

Question 16

In a population of cats, the gene for orange fur is X-linked and recessive. If a female cat with genotype $$X^O X^o ($$heterozygous) mates with a male $$X^o Y$$, what proportion of their male offspring will have orange fur?

Accepted Answer

$$50\%$$

Question 17

Which of the following statements about dosage compensation is correct?

Accepted Answer

It ensures that males and females have similar levels of X-linked gene expression

Question 18

A geneticist is studying a rare condition in which individuals with two X chromosomes develop male physical characteristics. Which of the following best explains this phenomenon?

Accepted Answer

The presence of a translocated SRY gene on one X chromosome

Question 19

In Drosophila, what would be the expected sex of an individual with the genotype $$XXY$$?

Accepted Answer

Female

Question 20

A researcher performs a reciprocal cross between a female with an X-linked dominant trait and a male without the trait. Which of the following is true about the F1 offspring?

Accepted Answer

All daughters will have the trait, but only half of the sons will have the trait

Sex Linkage and Sex Chromosome Inheritance

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