Allele frequencies aren't genotype frequencies
- The idea
- Predict
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- Why
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The idea
“If 40% of the alleles are a, then 40% of the plants are aa.”
Both numbers describe the a allele, so they feel interchangeable. One counts gene copies, and the other counts whole plants.
The short version
The mistake: If 40% of the alleles are a, then 40% of the plants are aa.
What's actually true: Allele frequencies count gene copies, and genotype and phenotype frequencies count individuals. At equilibrium, aa is q², not q.
- An allele frequency counts gene copies. A diploid plant has two, so count 2 for each homozygote and 1 for each heterozygote. That works in any population.
- A genotype frequency counts plants. An offspring is aa only if sperm and egg both carry a, so at equilibrium aa is q², Aa is 2pq and AA is p².
- If A is fully dominant, AA and Aa look alike. At equilibrium the recessive phenotype is q², so its square root gives q.
On the exam: Alleles count gene copies. Genotypes and phenotypes count individuals. At equilibrium, aa is q², not q.