Equilibrium doesn't mean equal alleles
- The idea
- Predict
- See it
- Why
- Check
The idea
“At Hardy-Weinberg equilibrium, the alleles should be 50:50, or the genotypes 1:2:1.”
Most examples use p = q = 0.5, and two heterozygotes give a neat 1:2:1. So equilibrium starts to sound like even alleles.
The short version
The mistake: At Hardy-Weinberg equilibrium, the alleles should be 50:50, or the genotypes 1:2:1.
What's actually true: Equilibrium works at any allele frequency. It means p and q stay the same each generation, with genotypes at p², 2pq and q².
- Equilibrium means allele and genotype frequencies stay the same each generation. It holds whether p is 0.5, 0.9 or 0.03.
- With random mating, the genotypes follow from p and q. Expect p² for AA, 2pq for Aa and q² for aa.
- 1:2:1 comes from an Aa × Aa cross in one family. A whole population shows it only when p = q = 0.5.
- Equal alleles don't prove equilibrium, and unequal ones don't mean evolution. Look for p changing over time, or genotypes that don't fit p², 2pq and q².
On the exam: Find p and q from the counts first. Equilibrium works at any p, and only p = 0.5 gives 1:2:1.