Genotype proportions don't carry over
- The idea
- Predict
- See it
- Why
- Check
The idea
“If 20% of the parents are Aa, then 20% of their offspring will be Aa too.”
It sounds fair that 20% Aa parents would have 20% Aa offspring. Each parent hands down only one allele, though, and alleles pair up fresh.
The short version
The mistake: If 20% of the parents are Aa, then 20% of their offspring will be Aa too.
What's actually true: One round of random mating sets genotypes to p squared, 2pq and q squared, from the allele frequencies alone.
- The next generation is built from the parents' allele frequencies, p and q. How those alleles were grouped into genotypes doesn't matter.
- Random mating pairs gametes by chance. AA needs two A's, so it's p squared. aa is q squared. Aa forms two ways, A from either parent, so it's 2pq.
- So a population out of Hardy-Weinberg proportions reaches them after one round of random mating, if allele frequencies hold steady.
On the exam: Count the alleles first. One round of random mating turns them into p squared, 2pq and q squared.