A phenotypically normal man has phenotypically normal parents but he has a sister who has cystic fibrosis caused by a recessive mutant allele. What is the probability that the man is heterozygous for the cystic fibrosis allele?
A) 1/4
B) 1/2
C) 3/4
D) 2/3
E) 1/3

Question

Quizplus · Accepted Answer

The man's sister has cystic fibrosis, which means both parents are carriers (heterozygous). The man, being phenotypically normal, could either be homozygous normal or heterozygous. The probability of him being heterozygous (carrier) is 2/3, given that he is not affected.

A Phenotypically Normal Man Has Phenotypically Normal Parents but He