Chapter 2, Problem 40P

Alkaptonuria is an infrequent autosomal recessive condi-tion. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phe-nylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria. Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband Frank are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children. The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, “I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condi-tion, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.” The two couples each have babies and both babies have alkaptonuria.

a. What are the genotypes of the four adults?

b. What was incorrect about the information given to Sara and James? What is incorrect about the informa-tion given to Mary and Frank?

c. What is the probability that the second child of Mary and Frank will have alkaptonuria?

d. What is the chance that the third child of Sara and James will be free of the condition?

e. The couples are worried that one of their grandchildren will inherit alkaptonuria. How would you assess the risk that one of the offspring of a child with alkapton-uria will inherit the condition?