The parents set the ratio
- The idea
- Predict
- See it
- Why
- Check
The idea
“Any cross with one dominant allele gives 3:1, and any two-gene cross gives 9:3:3:1.”
3:1 and 9:3:3:1 come to mind first because you practiced them most. Each fits only one kind of cross.
The short version
The mistake: Any cross with one dominant allele gives 3:1, and any two-gene cross gives 9:3:3:1.
What's actually true: The parents' gametes set the ratio. With complete dominance, 3:1 fits Aa × Aa, and 9:3:3:1 fits AaBb × AaBb for unlinked genes.
- Each parent passes on one allele per gene. Aa gives A or a equally often, and AA or aa gives only one kind.
- Pair the gametes in a Punnett square. With A dominant, Aa × Aa gives 3:1, and Aa × aa gives 1:1.
- For unlinked genes, work each gene alone and multiply. In AaBb × AaBb, 3/4 × 3/4 gives the 9/16 in 9:3:3:1.
- With incomplete dominance, Aa × Aa gives 1:2:1, since the heterozygote has its own phenotype.
On the exam: Write each parent's gametes first and let them set the ratio.