Dihybrid problems, herbarium and gene interaction

In a dihybrid problem first write the gametes of each organism: every gamete carries one gene of each allele pair (AaBb — AB, Ab, aB, ab; AABb — AB, Ab; aabb — only ab). The number of gamete types is 2ⁿ, where n is the number of heterozygous pairs. Without building a big Punnett square you can work out each trait separately and multiply the ratios, because the traits are inherited independently: Aa × Aa gives 3:1 and Bb × bb gives 1:1, so AaBb × Aabb gives (3:1)(1:1) = 3:3:1:1. To find the parents’ genotypes, split the ratio in F by trait: 3:1 — both parents heterozygous (Aa × Aa), 1:1 — Aa × aa, all dominant and uniform — at least one parent is homozygous dominant (AA). For counting, divide by the sum of the ratio parts and multiply by each class.