Answer to Question #113862 in Genetics for laura

Question

Jim was married once before and had a daughter with cystic fibrosis. A sister of his current wife katie died of cystic fibrosis Neither Jim nor any of their parents have cystic fibrosis If Jim and katie have three babies what is the probability that one or more will have cystic fibrosis?

Answer 1

Cystic fibrosis is inherited in an autosomal recessive manner. So person with genotype rr will have cystic fibrosis.

If Jim haven't cystic fibrosis, but he had a daughter with cystic fibrosis, Jim's genotype - Rr.

If Katie haven't cystic fibrosis, but her sister died of cystic fibrosis, most likely she has genotype - Rr. (If Katie has genotype - RR, probability that one child will have cystic fibrosis - 0%)

P: Rr x Rr

F₁: RR : 2 Rr : rr (25% : 50% : 25%)

So, probability that one child will have cystic fibrosis - 25%.