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Solution: John and Sue are expecting a child, but are concerned about a rare autosomal recessive disease that is present in both of their families. In the pedigree below, John is represented as individual III-11 and Sue is represented as individual III-12. John's sister, III-10, and Sue's brother, III-13, both do not show evidence of the disease, but John's paternal grandmother and Sue's maternal grandfather both had the disease.What is the probability that John is a carrier?What is the probability that both John and Sue are carriers?

Problem

John and Sue are expecting a child, but are concerned about a rare autosomal recessive disease that is present in both of their families. In the pedigree below, John is represented as individual III-11 and Sue is represented as individual III-12. John's sister, III-10, and Sue's brother, III-13, both do not show evidence of the disease, but John's paternal grandmother and Sue's maternal grandfather both had the disease.

What is the probability that John is a carrier?

What is the probability that both John and Sue are carriers?

Solution

Before the computation of the probability of the genotypes of John and Sue, the pedigree should be completed first as much as possible. It should be noted that John's father and Sue's mother are both carriers of the disease as they are definite recipients of the recessive allele from their affected parents. Additionally, John's first sister (III-1) is also known to have been affected. And since this disease is autosomal recessive, at least both parents should be carriers for it to be expressed in their children. The pedigree can be seen below.

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