Count gene copies, not individuals
- The idea
- Predict
- See it
- Why
- Check
The idea
“To get an allele's frequency, I divide by the number of individuals.”
Populations are counted in individuals, so it's natural to divide by the head count. Each diploid organism carries two copies of the gene, and allele frequency counts copies.
The short version
The mistake: To get an allele's frequency, I divide by the number of individuals.
What's actually true: Allele frequency is copies of that allele over all copies of the gene. N diploid individuals carry 2N copies.
- Mice and humans are diploid. Each one has two copies of every gene on its non-sex chromosomes, so N individuals carry 2N copies.
- To get an allele's frequency, divide its copies by 2N. A homozygote adds two copies of its allele, and a heterozygote adds one of each.
- The share of individuals that are Aa is a genotype frequency, a different number from q. To get q, count the a copies, one in each Aa and two in each aa.
On the exam: Diploid means two copies each. Divide allele copies by 2N, then check that p + q = 1.