A recessive allele on the X chromosome is responsible for red-green colour blindness in humans. A woman with normal vision whose father is colour blind marries a colour-blind male. What is the probability that this couple's first son will be colour blind?
A) 1/4
B) 1/2
C) 2/3
D) 3/4

Question

Quizplus · Accepted Answer

The woman is a carrier of the colour blindness allele as she has one normal and one recessive allele on her X chromosome. Her father, who is colour blind, must have passed on the recessive allele to her. The male is colour blind because he only has one X chromosome and it carries the recessive allele. Therefore, the probability of the couple's first son being colour blind is 50%, or 1/2. Since the son will inherit one X chromosome from his mother and one Y chromosome from his father, if he inherits the recessive allele from his mother, he will be colour blind.

A Recessive Allele on the X Chromosome Is Responsible for Red-Green