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 healthy child for their fourth. What are the chances that the fourth child will be a healthy male?
A) 0
B) 1/8
C) 1/4
D) 3/8
E) 1/2

Question

Quizplus · Accepted Answer

The probability of having a healthy child for two heterozygous parents is 3/4, since one out of four children is expected to have cystic fibrosis (25%) and three out of four are expected to be healthy (75%). The probability of having a male child is 1/2, assuming equal probability of having a male or female child. Therefore, the probability of having a healthy male child is the product of these two probabilities: 3/4 (chance of being healthy) * 1/2 (chance of being male) = 3/8. Previous children do not affect the probability of the traits of future children due to independent assortment.

A Couple Who Does Not Have Cystic Fibrosis but Are