4.11 X-Linkage Describes Genes on the X Chromosome

Multiple Choice

Who first demonstrated X-linked inheritance in Drosophila through studies of white-eyed mutations?
a) Gregor Mendel
b) Thomas Hunt Morgan
c) Barbara McClintock
d) Rosalind Franklin

b

True or False

Morgan’s experiments with white-eyed Drosophila showed that the gene for eye color is located on the autosomes.

False — it is on the X chromosome.

Multiple Choice

What observation from Morgan’s reciprocal crosses demonstrated that eye color in Drosophila is X-linked?
a) The F1 generation always had white eyes.
b) The results differed depending on which parent carried the mutant allele.
c) Only females showed the mutation.
d) Both males and females showed equal inheritance patterns.

b

Short Answer

Why are males considered hemizygous for X-linked genes?

Because males have only one X chromosome, and the Y chromosome lacks homologous alleles for most X-linked genes, so any allele on the X is expressed.

Multiple Choice

In the crisscross pattern of inheritance, a recessive X-linked trait is passed from:
a) Father to son
b) Mother to daughter
c) Homozygous mother to all sons
d) Heterozygous mother to all daughters

c

Multiple Choice

Which of the following statements about X-linked recessive inheritance in humans is true?
a) Affected fathers will pass the condition to their sons.
b) The disorder occurs equally in males and females.
c) Affected mothers pass the condition to all their sons.
d) The condition skips generations only through male carriers.

c

Short Answer

Why are X-linked recessive disorders more common in males than females?

Males have only one X chromosome, so a single recessive allele is sufficient to express the disorder; females need two recessive alleles.

Multiple Choice

Which of the following disorders is a lethal X-linked recessive condition in humans?
a) Cystic fibrosis
b) Duchenne muscular dystrophy
c) Red-green color blindness
d) Huntington’s disease

b

True or False

Females can be carriers of X-linked recessive disorders without showing symptoms.

true

Multiple Choice

Which statement about Y-linked genes is correct?
a) They are inherited from mothers to sons.
b) They are passed only from fathers to sons.
c) They can recombine with the X chromosome.
d) They are found in both sexes.

b

Applied Question (Drosophila example)

Miniature wings (min) is an X-linked recessive mutation in fruit flies.
If a min-winged female (XaXa) is crossed with a wild-type male (XAY), what proportion of F₁ progeny will have miniature wings?

a) 0%
b) 25%
c) 50%
d) 100%

c — All F₁ males (XaY) will have miniature wings, and all F₁ females (XAXa) will have normal wings.

Multiple Choice

Which of the following is true for nonlethal X-linked recessive disorders in humans?
a) Only males can have the disorder.
b) Affected fathers pass the disorder to all sons.
c) The disorder occurs equally in both sexes.
d) The disorder occurs more frequently in males than in females.

d

Short Answer (Conceptual)

Explain why reciprocal crosses involving X-linked traits produce different results.

Because the sex of the parent carrying the X-linked allele determines which sex of offspring inherits the mutation — males inherit their X from their mother, females from both parents.