An X-linked recessive gene (c)produces red-green color blindness.A normal woman whose father was color blind marries a color-blind man.What are the chances that the first son will be color blind?
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%

Question

An X-linked recessive gene (c)produces red-green color blindness.A normal woman whose father was color blind marries a color-blind man.What are the chances that the first son will be color blind?  
A) 0%
B) 25%
C) 50%
D) 75%
E) 100%

Quizplus · Accepted Answer

Since the woman is normal, she has two X chromosomes that are both carrying a normal allele (C). However, since her father was color blind, he had at least one affected X chromosome, which he could have passed to his daughter. Thus, there is a 50% chance that the woman is a carrier (heterozygous: XcX or XCX). The father is color blind and must carry an affected X chromosome (XcY). When the couple has a son, he has a 50% chance of inheriting the mother's X chromosome with the normal allele (XC) and a 50% chance of inheriting her X chromosome with the recessive, color-blindness allele (Xc). If he inherits the latter, he will be color blind since he doesn't have a normal allele to mask the effect of the recessive allele. Therefore, the chance that their first son will be color blind is 50%.

An X-Linked Recessive Gene (C)produces Red-Green Color Blindness