Punnett Square Calculator

What a Punnett square shows

A Punnett square lists every gamete one parent can make along the top and every gamete of the other parent down the side; each cell is one equally likely fertilisation. For a single gene, a cross of two heterozygotes, Aa × Aa, gives four cells: AA, Aa, aA and aa. The genotype ratio is 1 AA : 2 Aa : 1 aa, and if A is completely dominant, the phenotype ratio is 3 dominant : 1 recessive. Each offspring independently has these probabilities, so a family of four children can easily have no recessive child, or two; the ratio describes probabilities, not exact counts.

Dihybrid and trihybrid crosses

With two genes on different chromosomes, each parent makes four gamete types in equal proportions, by Mendel’s law of independent assortment. AaBb makes AB, Ab, aB and ab, and AaBb × AaBb gives a 4 × 4 square of 16 cells. Nine show both dominant traits, three show only the first, three only the second and one neither: the famous 9 : 3 : 3 : 1. There are nine distinct genotypes, in the ratio 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1, with AaBb the most common at 4/16. A trihybrid AaBbCc × AaBbCc has an 8 × 8 square of 64 cells and a phenotype ratio of 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. A test cross with a recessive parent, AaBb × aabb, gives 1 : 1 : 1 : 1, which is how geneticists reveal an unknown genotype.

When the square is wrong

The classic ratios rest on assumptions. Genes close together on the same chromosome are linked and do not assort independently, so the parental combinations appear more often than the square predicts; this is how the first genetic maps were made. With incomplete dominance, as in snapdragon flower colour, heterozygotes look intermediate, and the 1 : 2 : 1 genotype ratio is also the phenotype ratio. With epistasis, one gene masks another, turning 9 : 3 : 3 : 1 into ratios such as 9 : 3 : 4 or 9 : 7. A common misconception is that dominant alleles are more common or “stronger”; dominance only describes which phenotype a heterozygote shows. Human traits like eye colour involve many genes and do not follow a simple square.

Notation

Use one letter per gene, uppercase for the dominant allele and lowercase for the recessive, and list the genes in the same order for both parents. The calculator writes each genotype with the dominant allele first and summarises phenotypes with a blank: A_ means AA or Aa. To check a result, count the cells: every row and column of a dihybrid square must contain each allele equally often.

How to use it

1. Enter each parent’s genotype as letter pairs, one pair per gene, such as AaBb.
2. Use the same letters in the same order for both parents.
3. Read the gametes and the Punnett square.
4. Read the phenotype and genotype ratios with their probabilities.