A couple who does not have cystic fibrosis but are both heterozygous has already had three girls with cystic fibrosis, and are hoping to have a normal child for their fourth. What are the chances that the fourth child will be a normal male?
A) 0
B) 1/8
C) 1/4
D) 3/8
E) 1/2

Question

Quizplus · Accepted Answer

The parents are both heterozygous for cystic fibrosis, so each has a 50% chance of passing on the defective gene. The child will be normal if it inherits two normal genes, one from each parent. The probability of inheriting a normal gene from the mother is 1/2, and the probability of inheriting a normal gene from the father is also 1/2. The probability of inheriting two normal genes is therefore (1/2) * (1/2) = 1/4. The child will be male if it inherits a Y chromosome from the father. The probability of inheriting a Y chromosome is 1/2. The probability of the child being a normal male is therefore (1/4) * (1/2) = 1/8.

A Couple Who Does Not Have Cystic Fibrosis but Are