Calculating Allele Frequency

Question

 In the plant species Grandi floria, most individuals have large flowers. However, plants that are homozygous recessive at the G allele have small flowers. If there are 75 plants with large flowers (G_), and 25 plants with small flowers (gg), calculate the following (assume Hardy-Weinberg equilibrium):

a) The frequency of alleles G and g

b) The genotype frequencies

Answer 1

a) The frequency of alleles G and g

From the question, we know that there 75 plants with large flowers. However, these include some individuals with genotype Gg and others with genotype GG, therefore, we can't estimate the frequencies of G and g from the frequency of plants with large flowers. In contrast, because small flowers are homozygous recessive, we know that all individuals with small flowers have the genotype gg.

First, we calculate the frequency of gg.

gg = 25 / (25 + 75) = .25

Recall that gg = q^2. Squaring both sides gives us the frequency of q (i.e, g)

Therefore, g = Sqrt(.25) = .5

b) The heterozygosity (H)

Using our estimate of g above, we can calculate the frequency of G using the formula p = 1 - q

p = 1 - .5 = .5

Recall that H = 2 * p * q = 2 * .5 * .5 = .5