A couple is both heterozygous for the autosomal recessive disease cystic fibrosis (CF).What is the probability that their first child will either be a boy or have CF?
A)6/8
B)5/8
C)3/8
D)2/8
E)1/8

Question

Quizplus · Accepted Answer

If both parents are heterozygous carriers for CF (Cc), then there is a 25% chance that their offspring will inherit two copies of the recessive allele and thus have CF (cc). 
Similarly, there is a 50% chance that their offspring will inherit one copy of the recessive allele and be a carrier (Cc). 
Finally, there is also a 50% chance that their offspring will be male (and 50% chance to be female). 
Therefore, the probability that their first child will either be a boy or have CF is the sum of the probabilities of the two mutually exclusive events, which is: 
P(boy) + P(CF) = 0.5 + 0.25 = 0.75 or 5/8.

A Couple Is Both Heterozygous for the Autosomal Recessive Disease