Punnett squares

A Punnett square is a small grid for predicting the allele combinations offspring can receive. Along the top go the alleles one parent can pass, and down one side go the alleles the other can pass. Each box holds one pairing, and each pairing is equally likely when the alleles shuffle randomly. Like all probability tools, it predicts the chances for many offspring, not the fate of one particular child.

A key rule: each parent has two alleles for a trait, but passes on one of them per offspring. The parent's alleles separate during germ-cell formation, a separation called segregation. If a Bb parent can give B or b, and the other parent can also give B or b, the grid has 2 × 2 = 4 boxes: BB, Bb, bB and bb. Remember to count both orders, because each order is a different possible pairing.

B labels a dominant allele and b a recessive one. BB and Bb both show the dominant trait, and bb shows the recessive one, even though bB looks different in the grid. Counting the boxes gives the odds: a Bb × Bb cross produces about 3 dominant-looking offspring for every 1 recessive-looking one. The square works cleanly for one trait; two traits at once need a bigger grid.

How can two brown-eyed parents have a blue-eyed child? The parents showing the dominant trait can each carry the recessive allele hidden. The Punnett square shows that hidden allele sitting quietly in a box: Bb by Bb can make bb. A child cannot show a trait without the right alleles, but a hidden recessive can be passed for generations without ever being seen.