Question

which is the genotype of a male with a recessive sex - linked trait carried on the x chromosome?
a. $x^{\text{d}}y$
b. $x^{\text{d}}x^{\text{d}}$
c. $x^{\text{d}}y^{\text{d}}$
d. $x^{\text{d}}y$
options: a, b, c, d

Explanation:

1. Males have sex chromosomes \( XY \), so options with \( XX \) (like B) or \( Y^a \) (like C) are invalid for males.
2. For a recessive sex - linked trait on the X - chromosome, a male (with one X and one Y) will express the recessive trait if his X chromosome has the recessive allele (\( X^a \)) since the Y chromosome doesn't carry a corresponding allele to mask it. Option A has a dominant allele (\( X^A \)) on the X - chromosome, so it represents a male with a dominant or normal allele. Option D (\( X^aY \)) has the recessive allele on the X - chromosome and the Y chromosome, so it is the genotype of a male with a recessive sex - linked trait.

Answer:

D. \( X^aY \)