Heterozygotes are 2pq, not pq
- The idea
- Predict
- See it
- Why
- Check
The idea
“Heterozygotes are p times q. With p = 0.6 and q = 0.4, that makes Bb 0.6 × 0.4 = 0.24.”
Bb has one of each allele, so p times q looks like the whole answer. That product counts only one of the two ways to make Bb.
The short version
The mistake: Heterozygotes are p times q. With p = 0.6 and q = 0.4, that makes Bb 0.6 × 0.4 = 0.24.
What's actually true: With random mating, B can come from the egg or from the sperm. Bb forms two ways, so it's pq + pq = 2pq.
- Each offspring gets one allele from the egg and one from the sperm. With random mating, these are independent draws.
- Bb forms two ways, B from the egg or B from the sperm. Each way has chance pq, so Bb is pq + pq = 2pq.
- BB forms only one way, with B from both parents, so it's p². Likewise bb is q². Only the middle term gets a 2.
On the exam: Bb forms two ways, so it gets a 2. Check that p² + 2pq + q² adds up to 1.