Hardy-Weinberg and genetic drift
About this app
A population as a swarm, its alleles as colours. 20 populations run in parallel as a fan of lines: without evolutionary factors p stays constant, while drift, bottleneck, founder effect, selection, migration and mutation shift it. Work out the carrier share from the disease frequency step by step.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
When allele frequencies stay constant
The Hardy-Weinberg law says that in a very large population with random mating and without selection, mutation or migration, the allele frequencies p and q stay constant. Genotypes then follow p² + 2pq + q² = 1. Any deviation shows that an evolutionary factor is at work. A common mistake is to think dominant alleles become more common by themselves. Dominance only describes the phenotype of heterozygotes and does not change the allele frequency.
What the app shows
On the left the population swims as a swarm in a pond. Each creature carries two alleles, visible as its front and rear half. Below are the allele pool and genotype bars with ticks for p², 2pq and q². On the right 20 populations run in parallel as a fan of lines, using the Wright-Fisher model with a fixed seed. A dashed line shows the expectation without drift, the edge shows the distribution of populations, and a track below shows population size.
Try it yourself
Drag the edge of the pond inwards to create a bottleneck. Choose equilibrium, drift, founder effect, selection, migration or mutation and set N, s, m and μ. Tap a line to see that population in the pond, and switch to phenotype: Aa then looks like AA. In Calculate you work out the carrier share from the disease frequency. Six exam questions give reasoned feedback.
Frequently asked questions
- How do you calculate the share of heterozygotes with Hardy-Weinberg?
- For an autosomal recessive disease the disease frequency is q². From 1 : 2500 follow q² = 0.0004, q = 0.02 and p = 0.98. The share of heterozygotes is 2pq = 0.0392, which is 3.92% or about 1 in 25.5.
- What is genetic drift?
- Genetic drift is the random change of allele frequencies, because each generation is only a sample of the gametes of the one before. It is stronger the smaller the population is. Without selection an allele becomes fixed with a probability equal to its frequency.
- Do dominant alleles become more common by themselves?
- No. A parent passes on either allele with the same probability. Without evolutionary factors a dominant allele with p = 0.1 stays at 0.1. The 3 : 1 ratio applies to the cross Aa × Aa and does not describe a population.
- What can I do in the app myself?
- You set population size, selection, migration and mutation, run generations, trigger a bottleneck, compare 20 populations, calculate carrier shares and answer six exam questions.
Subject: Biology (Gymnasium)
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