Question

A husband and wife have normal vision, although both of the couple's fathers are red–green color-blind, an inherited X-linked recessive condition. What is the probability that the couple's first child will be a normal daughter? Enter your answer as a fraction (example: 3/16) or a whole number (example: 1).

Answer 1

1/2

Explanation:

Since the recessive gene is carried in the x chromosome and a father can only pass it to a daughter, the father doesn't have the disease nor carries it.

The mother carries the recessive gene because her father passed her his conditioned gene (only having one x chromosome), while her mother gave her a normal one so she can have normal vision.

The chance of having a boy or a girl is 1/2 and the chance of this baby of having both recessive gene is null because in order to have a girl that is color blind both her x chromosomes must have the recessive gene, but the father can't pass it since he doesn't carries it. Therefore the combined probability is 1/2*(1-0) = 1/2.