Count only the boxes that fit the phenotype
- The idea
- Predict
- See it
- Why
- Check
The idea
“Half of Rr x Rr offspring are Rr, so a round one I pick is Rr half the time.”
The Punnett square's 1/2 Rr is true for all the offspring. This question already told you the seed is round, and that takes a box away.
The short version
The mistake: Half of Rr x Rr offspring are Rr, so a round one I pick is Rr half the time.
What's actually true: If you're told the phenotype, count only the genotypes that can make it. A round offspring of Rr x Rr is Rr 2/3 of the time.
- Rr x Rr gives 1/4 RR, 1/2 Rr and 1/4 rr, so 3/4 of all the offspring are round.
- When a question gives the phenotype, drop every genotype that can't make it. That's conditional probability.
- Then ask what share of the rest has the genotype you want. Rr is 1/2 of all offspring, so that's 1/2 divided by 3/4, or 2/3.
- Hardy-Weinberg works the same way. Among people with the dominant phenotype, carriers are 2pq out of p^2 + 2pq.
On the exam: If you know the phenotype, drop the boxes that can't make it and divide by what's left.