Punnett square calculator
Enter two parent genotypes to see the square and the offspring ratios.
Write each gene as two letters, uppercase for dominant and lowercase for recessive. Use one gene (Aa) or two (AaBb). Results assume complete dominance and independent assortment.
Reading the square
Each parent passes on one allele per gene. The calculator lists every possible gamete, pairs them in a grid, and counts the resulting genotypes. With two genes it assumes they sort independently, which gives the classic 4 × 4 square.
Phenotypes assume complete dominance: an uppercase letter shows the dominant trait, and two lowercase letters show the recessive one. Linked genes, incomplete dominance, and codominance follow different rules.
Worked examples
One gene (monohybrid cross). Two heterozygous parents, Aa × Aa, each pass on A or a with equal chance. The square gives AA, Aa, Aa and aa: a genotype ratio of 1 AA : 2 Aa : 1 aa. If A is dominant, three of the four show the dominant trait, a phenotype ratio of 3 : 1.
Two genes (dihybrid cross). AaBb × AaBb for two genes on different chromosomes gives the classic phenotype ratio of 9 : 3 : 3 : 1 across 16 boxes: 9 show both dominant traits, 3 and 3 show one of them, and 1 shows both recessive traits.
Frequently asked questions
What is the difference between genotype and phenotype?
The genotype is the pair of alleles (AA, Aa or aa). The phenotype is the trait you can observe. With complete dominance, AA and Aa have the same phenotype.
What is a test cross?
Crossing an individual that shows the dominant trait with a homozygous recessive (aa) individual. If any offspring show the recessive trait, the unknown parent must be Aa.
Do the ratios mean exactly 3 out of 4 offspring?
No. They are probabilities for each offspring. A family of four children could easily have no children with the recessive trait, just as four coin flips don’t always give two heads.
What about incomplete dominance or codominance?
With incomplete dominance (red × white snapdragons give pink), the heterozygote has its own phenotype, so the phenotype ratio becomes 1 : 2 : 1, the same as the genotype ratio.