Cross two genes at once. Pick each parent's genotype and instantly generate the 4×4 Punnett square with full genotype ratios and the classic 9:3:3:1 phenotype ratio.
A dihybrid cross tracks two genes at the same time. Each parent carries two alleles per gene (a genotype such as AaBb), so the offspring outcomes are mapped on a 4×4 Punnett square with 16 boxes.
Gregor Mendel ran dihybrid crosses on pea plants — seed shape (round vs wrinkled) and seed colour (yellow vs green) — and discovered the law of independent assortment.
Crossing two double-heterozygotes (AaBb × AaBb) gives the famous 9:3:3:1 phenotype ratio:
The genotype ratio for the same cross is 1:2:1:2:4:2:1:2:1 across nine genotypes.
Tip: any pair of traits works — map A/a and B/b onto fur colour, height, or any Mendelian gene.
AaBb × AaBb (round/wrinkled × yellow/green):
That is the textbook 9:3:3:1 ratio — load the example above to see all 16 boxes filled in.
How many gametes does AaBb make? Four — AB, Ab, aB and ab — because the genes assort independently.
Why 16 boxes? 4 gametes from each parent → 4 × 4 = 16 equally-likely combinations.
What if the genes are linked? Linked genes on the same chromosome do not assort independently, so the ratio drifts away from 9:3:3:1.