Punnett Squares Step by Step (with Practice Problems)

A Punnett square is a grid that shows every way the alleles from two parents can combine. It predicts the probability of each outcome — it doesn't guarantee what any single offspring will be.

Key words first

• Allele — a version of a gene. We use a capital letter for the dominant allele (T = tall) and the lower-case letter for the recessive one (t = short).
• Genotype — the pair of alleles (TT, Tt or tt).
• Phenotype — the trait you can see (tall or short).
• Homozygous — two identical alleles (TT or tt). Heterozygous — two different alleles (Tt).

Five steps

• Write each parent's genotype. Example: Tt × Tt.
• Split each parent's alleles: one parent across the top (T, t), the other down the side (T, t).
• Fill in each box with one allele from the top and one from the side: TT, Tt, Tt, tt.
• Count the genotypes: 1 TT : 2 Tt : 1 tt.
• Count the phenotypes: anything with a capital T is tall, so 3 tall : 1 short — a 75% chance of tall.

The four crosses worth memorizing

• TT × tt → all Tt → 100% dominant phenotype.
• Tt × Tt → 1 TT : 2 Tt : 1 tt → 3 : 1 dominant to recessive.
• Tt × tt → 2 Tt : 2 tt → 1 : 1 (50% each).
• TT × Tt → 2 TT : 2 Tt → 100% dominant.

Working backwards: the test cross

If a plant shows the dominant trait, is it TT or Tt? Cross it with a recessive (tt) plant. If any offspring show the recessive trait, the unknown parent must be Tt — a TT parent could never pass on a t. If half the offspring are recessive, that's the 1 : 1 pattern of Tt × tt.

Common mistake: writing the ratio of genotypes when the question asks for phenotypes (or the reverse). Underline which one the question wants before you start.