Question

males are more often affected by sex - linked traits than females because...
○ sex - linked traits are located on autosomes that affect males more severely than females.
○ the presence of a y chromosome often worsens the effects of x - linked mutations.
○ x chromosomes in males generally have more mutations than x chromosomes in females.
○ males are hemizygous for the x chromosome.

Explanation:

1. Analyze the first option: Sex - linked traits are on sex chromosomes (X or Y), not autosomes, so this option is incorrect.
2. Analyze the second option: The Y chromosome does not worsen X - linked mutations. X - linked mutations' expression in males is due to hemizygosity, not Y chromosome effects, so this is wrong.
3. Analyze the third option: There is no evidence that X chromosomes in males have more mutations than in females. The difference in expression of sex - linked traits is due to chromosome dosage, not mutation rate, so this is incorrect.
4. Analyze the fourth option: Males have one X and one Y chromosome (hemizygous for X). So if there is a recessive allele on the X chromosome, males will express the trait as they have no corresponding dominant allele on the Y (unlike females who have two X chromosomes and can be carriers). This explains why males are more often affected by sex - linked (especially X - linked) traits.

Answer:

D. males are hemizygous for the X chromosome.