Punnett crosses (every paper)

Grid, genotypes, phenotypes, RATIO — all four parts. Exam-style Two heterozygous brown mice (Bb × Bb) Genotype ratio, phenotype ratio, chance of a homozygous recessive pup. Punnett: BB, Bb, Bb, bb Genotype 1 BB : 2 Bb : 1 bb Phenotype 3 brown : 1 black; bb chance = 25% ✓ 1:2:1 and 3:1, 25%. State which letters are dominant and why. ℹ

Dihybrid grids

BbRr × BbRr: write the four gamete types (BR, Br, bR, br) on each side; the 16-cell grid gives 9:3:3:1. Papers often ask "explain why the ratio is not 9:3:3:1" when genes are LINKED on one chromosome — linked genes inherit together, and crossing over during meiosis is what occasionally separates them, producing the "unlinked-looking" recombinant offspring in a minority. ✕

Multiple alleles ≠ every individual carries them all

A gene can have MULTIPLE alleles across the whole population (blood type has three: I^A, I^B, i), but any single individual still only carries TWO — one from each parent. Do not confuse "multiple alleles exist in the population" with "one person has more than two alleles".

Practice for this standard covers the whole topic — flashcards and quiz.