A Punnett square is a small table that lists every way the parents' alleles can combine. It uses genotypes and turns a cross into something you can count.
Key idea
Write one parent's two alleles across the top and the other parent's two alleles down the side. Fill each box with one allele from the top and one from the side. The four boxes are the possible genotypes of an offspring.
Why it works
Each parent passes on one allele of the pair, and each of the two is equally likely. Listing the two from each parent and pairing them every possible way gives four equally likely combinations. The square is just a tidy way to make sure none is missed or counted twice.
Worked example: Tt × Tt
Cross two tall pea plants, Tt and Tt. List the boxes.
- Top: T, t. Side: T, t.
- Row T: TT, Tt. Row t: Tt, tt.
- Genotypes: 1 TT, 2 Tt, 1 tt.
- Phenotypes: TT and Tt are tall (3 boxes); tt is short (1 box).
Answer: 1 TT : 2 Tt : 1 tt, so 3 tall : 1 short.
Worked example: Bb × bb
A black guinea pig, Bb, is crossed with a white one, bb. What can the offspring be?
- Top: b, b. Side: B, b.
- Row B: Bb, Bb. Row b: bb, bb.
Answer: 2 Bb (black) : 2 bb (white), so half black and half white.
Common mistake
Two mistakes are common. Some students put both of a parent's alleles in one box, like "TTt". Others combine two letters from the same parent. Each box gets exactly two letters: one from the top, one from the side. Check that every box has one letter from each parent.