You are working with a family practice group and a couple (Mark and Susan), planning to start a family comes to you for information. Mark was married once before, and he and his first wife had a daughter with cystic fibrosis who died of the age of 6. Susan had a teenage sister with cystic fibrosis who is doing fairly well with intensive medical treatment of her inherited disorder. Susan has no previous children. Both Susan and Mark are healthy and neither one of them has cystic fibrosis. All the members of their family are also healthy. Note that cystic fibrosis is a serious genetic disease caused by an autosomal recessive allele. Heterozygotes have a normal phenotype and show no health problems related to this allele.
If Mark had a daughter with cystic fibosis but he is healthy, his genotype is Aa. Susan's sister is affected, so their parents are both heterozygous. In this case Susan can be either homozygous dominant (AA) or heterozygous (Aa). If Susan is AA and Mark is Aa, they will not have the affected children. If Susan is Aa and Mark is Aa, then the probability of the affected child is 1/4.
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