In a population with the following genotype frequencies: AA = 0.2, Aa = 0.2, aa = 0.6, what are p and q, the frequencies of the A and a alleles?

Sourav PanOctober 25, 2024

Answered

To calculate the frequencies of the A and a alleles (denoted as p for allele A and q for allele a) from the given genotype frequencies, follow these steps:

Given Genotype Frequencies:

• AA: 0.2
• Aa: 0.2
• aa: 0.6

Calculating Allele Frequencies:

1. Count the number of each allele in the population, considering each individual contributes two alleles:
   • From AA (0.2): 0.2 * 2 = 0.4 (since each AA individual contributes 2 A alleles)
   • From Aa (0.2): 0.2 * 1 = 0.2 (since each Aa individual contributes 1 A allele and 1 a allele)
   • From aa (0.6): 0.6 * 2 = 1.2 (since each aa individual contributes 2 a alleles, but we’re focusing on A alleles here, so this contributes 0 to A, but will be used for total allele count)
2. Total A alleles: 0.4 (from AA) + 0.2 (from Aa) = 0.6
3. Total alleles in the population (to calculate the frequency of each allele):
   • Since each individual has 2 alleles, and if we had the total number of individuals, we’d multiply that by 2. However, we’re working with frequencies, which inherently account for the total population size. Thus, the total frequency of all alleles must sum to 2 (considering the entire population’s allele pool).
   • Total Allele Frequency Pool: 2 (representing 100% of the alleles in the population)

Calculating p (A allele frequency) and q (a allele frequency):

• p (Frequency of A allele): Total A alleles / Total allele frequency pool = 0.6 / 2 = 0.3
• Alternatively, to find q (a allele frequency), you can either directly calculate from the aa and Aa genotypes or use the fact that p + q = 1:
  • Direct Calculation for q (not necessary given p + q = 1): (0.21 for Aa) + (0.62 for aa) = 0.2 + 1.2 = 1.4 “a” allele frequency contributions, but since we’re working with frequencies, we actually calculate q as the proportion of “a” alleles, which directly can be derived from q = 1 – p.
  • q (Frequency of a allele) using p + q = 1: q = 1 – p = 1 – 0.3 = 0.7

Results:

• p (A allele frequency): 0.3
• q (a allele frequency): 0.7