Punnett squares

Science

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

  1. Determine the genotypes of both parents for the trait being studied.
  2. List each parent's possible gamete alleles along one axis of the grid.
  3. Fill in each box by combining the allele from the column with the allele from the row.
  4. Count the resulting genotypes and phenotypes to determine the predicted ratios.

Examples

  • Crossing Tt × Tt gives 1 TT : 2 Tt : 1 tt genotypic ratio (3:1 phenotypic ratio)
  • A test cross (Tt × tt) produces 50% heterozygous and 50% homozygous recessive offspring
  • 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).

Related Terms

Mendelian genetics Dominant and recessive traits Non-mendelian genetics Meiosis

Study This Concept

Practice Punnett squares with free review games in these units:

Biology Unit 3: Genetics and Heredity