Question

The presence of spots in a ladybug species is controlled by a single locus. The allele S produces spots while the allele s produces no spots. A breeding study shows that S is completely dominant to s. In a random sample of 758 individuals, 69 completely lack spots.

a) What are the expected frequencies of the three genotypes in this population and what assumption(s) did you make to solve this problem?

Answer 1

given, S is the allele that produces spots on ladybug species, which is dominant. Then s is the recessive allele.

No. of individuals in the population = 758

No. of individuals that completely lack spots = 69

as the S is fully dominant over 's' the SS/ Ss individuals will be spotted, so, only ss individuals can be those which lacks spots.

Hence,

No. of individuals with genotype ss = 69

Genotype frequency of ss genotype = 69 / 758 = 0.09

Allele frequency of s allele , q = root of 0.09

= 0.3

Allele frequency of S allele , p = 1 - allele frequency of s allele ( Let us assume that the population is in HWE )

= 1 - 0.3

= 0.7

Expected genotype frequency of SS = p^2 = 0.7 x 0.7 = 0.49

Expected genotype frequency of Ss = 2 x p x q = 2 x 0.7 x 0.3 = 0.42