Question

Joan is phenotypically normal but had a child with the autosomal recessive disease cystic fibrosis (CF) from a previous marriage. Joan's father has hemophila A, a sex-linked recessive condition where the blood fails to clot properly. Her father has survived due to recent treatment advances. Joan now intends to marry Bill, who is also phenotypically normal but who has a sister, Jill, with CF. Bill's parents are phenotypically normal, and there is no history of hemophilia A in his family. Assume that Joan and Bill do marry and have a child. What is the probability that this child will have CF, but will not have hemophilia A?

a) 1/16
b) 1/8
c) 1/4
d) 1/2

Answer 1

The probability that the child will have CF but will not have hemophilia A is 1/4 or 25%.

Step-by-step explanation:

According to the information given, Joan and Bill are both phenotypically normal, but they are carriers of the cystic fibrosis gene. There is a 25% chance that a child born to them will have cystic fibrosis (CF) and a 75% chance that the child will be normal.

However, there is no history of hemophilia A in Bill's family, so the child will not have hemophilia A. Hemophilia A is a sex-linked recessive condition and in this case, it is not passed down from either parent.

So, the probability that the child will have CF but will not have hemophilia A is 1/4 or 25% (option c).