In Aa × Aa, Aa is twice as likely as AA or aa
- The idea
- Predict
- See it
- Why
- Check
The idea
“Two heterozygotes make AA, Aa and aa offspring, so each genotype is a third.”
Write out Aa × Aa and you find three genotypes, so a third each seems fair. Aa, though, can be built two different ways.
The short version
The mistake: Two heterozygotes make AA, Aa and aa offspring, so each genotype is a third.
What's actually true: In Aa × Aa, the heterozygote forms two ways, so the offspring are 1/4 AA, 1/2 Aa and 1/4 aa.
- Each Aa parent makes A and a gametes in equal numbers. Eggs and sperm meet at random, so the four Punnett boxes are equally likely.
- The three genotypes are not. Aa can form from an A egg and an a sperm, or from an a egg and an A sperm, so it fills two boxes.
- That gives AA, Aa and aa in a 1:2:1 ratio. With incomplete dominance or codominance, each genotype has its own phenotype, so phenotypes are 1:2:1 too.
On the exam: Count Punnett boxes, not genotype names. Aa fills two boxes, so it is twice as likely as AA or aa.