Question

In humans, the condition of color blindness is caused by a recessive allele located on the X chromosome. A color-blind woman marries a man with normal vision. What is the probability that their son will be color-blind?

• A. \( 0\% \)
• B. \( 25\% \)
• C. \( 50\% \)
• D. \( 100\% \)

Hint:

For sex-linked traits, remember that males express the phenotype of the single X chromosome they inherit. If a mother is homozygous recessive for a sex-linked recessive trait, all her sons will express the trait, as they inherit her recessive X chromosome.

Answer 1

The Correct Option is D

Color blindness is a sex-linked recessive trait caused by a recessive allele (\( X^c \)) on the X chromosome. Normal vision is due to the dominant allele (\( X^C \)). Since males have one X and one Y chromosome, their phenotype depends on the single X chromosome they carry. - The woman is color-blind, so her genotype is \( X^c X^c \) (homozygous recessive, as she expresses the recessive trait). - The man has normal vision, so his genotype is \( X^C Y \) (hemizygous for the dominant allele). To determine the probability of their son being color-blind, we analyze the possible genotypes of their offspring using a Punnett square. The woman (\( X^c X^c \)) can only contribute an \( X^c \) gamete. The man (\( X^C Y \)) can contribute either an \( X^C \) or a \( Y \) gamete. The possible offspring genotypes are: \[ \begin{array}{c|cc} & X^c & X^c
\hline X^C & X^C X^c & X^C X^c
Y & X^c Y & X^c Y
\end{array} \] - Daughters (\( X^C X^c \)): They inherit one \( X^C \) from the father and one \( X^c \) from the mother. They are carriers (normal vision, as the dominant allele \( X^C \) masks the recessive \( X^c \)). - Sons (\( X^c Y \)): They inherit the \( X^c \) from the mother and the \( Y \) from the father. Since the \( X^c \) carries the recessive allele and there is no second X chromosome to mask it, they are color-blind. Since all sons will have the genotype \( X^c Y \), they will all be color-blind. Thus, the probability that their son will be color-blind is \( 100\% \).