Joan is phenotypically normal but had a child with the autosomal recessive disease cystic fibrosis (CF) from a previous marriage. Joan's father has hemophilia 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? (Hint: This problem requires that you utilize concepts from Chapter 3 as well as Chapter 4.)

Question

Quizplus · Accepted Answer

Joan is a carrier for CF (since she had a child with CF) and Bill is also a carrier (since his sister has CF). The probability that their child will have CF is 1/4. Joan is not a carrier for hemophilia A because she is phenotypically normal and her father has the condition, meaning she must have inherited the normal X chromosome from her mother. Therefore, the probability that their child will not have hemophilia A is 1. The combined probability is 1/4 * 1 = 1/4. However, the probability that the child will have CF but not hemophilia A is 1/4 * 1/2 = 1/8, considering the child could be male or female.

Joan Is Phenotypically Normal but Had a Child with the Autosomal