Science
Punnett squares.
Definition
A Punnett square is a grid diagram used to predict the genotypic and phenotypic ratios of offspring from a genetic cross. Each parent's possible gametes are placed along the top and side, and the boxes show all possible offspring combinations.
How it works · 4 phases
Step by step.
- Determine the genotypes of both parents for the trait being studied.
- List each parent's possible gamete alleles along one axis of the grid.
- Fill in each box by combining the allele from the column with the allele from the row.
- Count the resulting genotypes and phenotypes to determine the predicted ratios.
Examples
Real-world.
- 1 Crossing Tt × Tt gives 1 TT : 2 Tt : 1 tt genotypic ratio (3:1 phenotypic ratio)
- 2 A test cross (Tt × tt) produces 50% heterozygous and 50% homozygous recessive offspring
- 3 A dihybrid Punnett square (4×4 grid) predicts outcomes for two traits simultaneously
Key Fact
A monohybrid cross uses a 2×2 grid (4 boxes); a dihybrid cross uses a 4×4 grid (16 boxes).