How to Calculate Allele Frequencies from Genotype Data: Step-by-Step Hardy-Weinberg Example

Calculating allele frequencies from observed genotype frequencies is a fundamental skill in population genetics. The Hardy-Weinberg principle provides a simple framework for these calculations, allowing researchers to assess genetic diversity and evolutionary stability in a population.

Given Genotype Frequencies

• $f(A1A1)=0.59$

• $f(A1A2)=0.16$

• $f(A2A2)=0.25$

Step 1: Understanding the Formula

For two alleles (A1 and A2), the frequency of each allele can be calculated as:

• Frequency of A1 ($p$):

  p=f(A1A1)+12f(A1A2)p=f(A1A1)+21f(A1A2)

• Frequency of A2 ($q$):

  q=f(A2A2)+12f(A1A2)q=f(A2A2)+21f(A1A2)

These formulas work because each homozygote contributes two copies of its allele, and each heterozygote contributes one copy of each allele.

Step 2: Plug in the Values

• Frequency of A1:

  p=0.59+12(0.16)=0.59+0.08=0.67p=0.59+21(0.16)=0.59+0.08=0.67

• Frequency of A2:

  q=0.25+12(0.16)=0.25+0.08=0.33q=0.25+21(0.16)=0.25+0.08=0.33

Step 3: Check the Sum

$$p+q=0.67+0.33=1.00$$

This confirms the calculations are correct, as allele frequencies should always sum to 1.

Step 4: Match with the Options

• (1) A1=0.59 A2=41

• (2) A1=0.75 A2=25

• (3) A1=0.67 A2=33

• (4) A1=0.55 A2=44

The correct answer is (3) A1=0.67 A2=33.

Why This Matters

• Population Health: Knowing allele frequencies helps predict genetic disease risks.

• Evolutionary Studies: Detects shifts in genetic structure over time.

• Conservation: Informs strategies to maintain genetic diversity.

Conclusion

Given the genotype frequencies, the allele frequencies at this locus are A1 = 0.67 and A2 = 0.33.

Correct answer: (3) A1=0.67 A2=33