Punnett Squares

The Punnett square is a simple diagram named after its inventor, Reginald Punnett. It is a graphical tool used to predict the probability of offspring inheriting specific genotypes and phenotypes for a single-gene trait from their parents. One example is lactose intolerance, a trait linked to the body's ability to produce the enzyme lactase, which digests lactose.

In this example, adult lactose digestion is linked to alleles, which help control lactase production.

The dominant T allele is linked to lactase persistence, so a person can digest lactose as an adult. The recessive C allele is linked to reduced lactase production, which can lead to lactose intolerance in adults.

Here, both parents are heterozygous. Each parent has one T allele and one C allele, and both can digest lactose. Their alleles are placed along the top and left sides of the grid.

Each box combines one allele from each parent, giving three possible genotypes: TT, TC and CC. There is a 25% chance of TT, a 50% chance of TC, and a 25% chance of CC.

These genotypes translate into physical traits, known as phenotypes. Because the T allele is dominant, both TT and TC genotypes result in lactase persistence. This means there is a 75% chance that the child will be able to digest lactose as an adult and a 25% chance of reduced lactase production.