Genetic problem?

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.

(a) Draw a pedigree illustrating the relationships of this family (described above) and showing the incidence of cystic fibrosis. Be sure to include Susan’s parents and Susan’s sister in the pedigree, as well as Mark and his first wife and daughter. Use standard symbols for the pedigree and indicate all heterozygotes that you can deduce from the information given above.

(b) What is the probability that Susan's mom and dad are carriers of cystic fibrosis allele?

(c) What is the probability that Susan is a carrier of the cystic fibrosis allele? Remember that she- we know is healthy (but does not have cystic fibrosis) and this affects the calculation of this probability.

(d) What is the probability that Mark is a carrier of the cystic fibrosis allele?

(e) If Mark and Susan have a baby, what is the probability that their child will have cystic fibrosis? Hint: Remember that the probability of two (or more) events occurring together is the product of the probability of each event occurring separately. You need to multiply the probability that Susan is a carrier times the probability.

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