What explanations could you offer to explain the frequencies in those populations that are not in equilibrium?

6. You are consulted by a couple, Abby and Andrew, who tell you that Abby’s sister Anna has Hurler syndrome (a mucopolysaccharidosis) and that they are concerned that they themselves might have a child with the same disorder. Hurler syndrome is an autosomal recessive condition with a population incidence of approximately 1 in 90,000 in your community.

a. If Abby and Andrew are not consanguineous, what is the risk that Abby and Andrew’s first child will have Hurler syndrome?

b. If they are first cousins, what is the risk?

c. How would your answers to these questions differ if the disease in question were cystic fibrosis instead of Hurler syndrome?

7. In a certain population, each of three serious neuromuscular disorders—autosomal dominant facioscapulohumeral muscular dystrophy, autosomal recessive Friedreich ataxia, and X-linked Duchenne muscular dystrophy—has a population frequency of approximately 1 in 25,000.

a. What are the gene frequency and the heterozygote frequency for each of these?

b. Suppose that each one could be treated, so that selection against it is substantially reduced and affected individuals can have children. What would be the effect on the gene frequencies in each case? Why?

8. As discussed in this chapter, the autosomal recessive condition tyrosinemia type I has an observed incidence of 1 in 685 individuals in one population in the province of Quebec, but an incidence of approximately 1 in 100,000 elsewhere. What is the frequency of the mutant tyrosinemia allele in these two groups? Suggest two possible explanations for the difference in allele frequencies between the population in Quebec and populations elsewhere.

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