Question

Tay-Sachs disease is an autosomal recessive disorder caused by a mutation in the HEXA gene that results in progressive deterioration of the nervous system. Affected indivisuals usually die by age 4. Let D represent the wildtype allele and d the disease-causing allele.

Because the Tay-Sachs disease allele has been observed at relatively high frequencies in several human populations, it has ocassionally been suggested that this locus may be subject to balancing selection due to heterozygote advantage. Here we will explore some of the consequences of this hypothesis. To this end, suppose that the relative fitnesses of the genotypes DD, Dd and dd are 1.0, 1.1, annd 0.0.

a.) Suppose that the frequency of the heterozygous genotype Dd in a population of adults is 0.1 in the present generation. Calculate the frequencies of the alleles in the population.

Frequency D:

Frequency d:

b.) Assuming that this population is randomly mating, calculate the frequencies of the three genotypes and two alleles in the zygotes produced by these matings. For simplicity, assume that the disease allele has no impact on fertility or the initial survival of the zygotes.

Frequency of D:

Frequency of d:

Frequency of DD:

Frequency of Dd:

Frequency of dd:

c.) Calculate the frequncies of the three genotypes and alles in the adults of the next generation.

Frequency of DD:

Frequency of Dd:

Frequency of dd:

Frequency of D:

Frequency of d:

e.) What was the percentage change in the frequency of the disease allele?

f.) Compare the percentage change when the locus is overdominant with the percentage change when directional selection is operating. Why do these differ?

Answer 1

Since we are entitled to answer only the first four subparts of a question. So, I answered the first four parts of this question.