Adding up to 1 doesn't prove Hardy-Weinberg
- The idea
- Predict
- See it
- Why
- Check
The idea
“The counts add up, p + q = 1 and p holds steady, so the population is in equilibrium.”
Both Hardy-Weinberg equations equal 1, so checking that things add up feels like testing the model. Every population passes that check.
The short version
The mistake: The counts add up, p + q = 1 and p holds steady, so the population is in equilibrium.
What's actually true: A population fits Hardy-Weinberg only if its genotype counts match those that p², 2pq and q² predict from its allele frequencies.
- Counts always add up to the sample, and p + q always equals 1. Neither can show equilibrium.
- To test it, find p and q, then multiply p², 2pq and q² by N, the number of individuals.
- If observed counts are close to expected, the data fit. A chi-square test judges how close. Too few heterozygotes points to nonrandom mating, selection or another cause.
- A steady p isn't enough. Inbreeding makes homozygotes more common without changing p, so heterozygotes fall below 2pq.
On the exam: Compare each genotype count with p², 2pq or q² times N.