Punnett Square Ratio Calculator
A Punnett square predicts the possible genotype outcomes of a genetic cross. For a monohybrid cross (one gene locus, two alleles), enter one allele character per parent gamete slot. The calculator fills the 2x2 grid and tallies the genotype frequencies (e.g., 1 AA : 2 Aa : 1 aa) and phenotype frequencies (e.g., 3 dominant : 1 recessive, assuming complete dominance). Use uppercase letters for dominant alleles and lowercase for recessive alleles. Common crosses include Aa x Aa (heterozygous), AA x Aa, and Aa x aa (test cross). This tool is fundamental to introductory genetics, plant breeding, and medical genetics counselling.
Punnett square method
Each parent contributes one allele per gamete.
Cross the gametes in a 2x2 grid to get 4 offspring combinations.
Count each genotype class (AA, Aa, aa) to get the ratio.
Dominant phenotype = AA + Aa; Recessive = aa.
For a heterozygous x heterozygous cross (Aa x Aa): 1/4 AA, 2/4 Aa, 1/4 aa = 3:1 phenotype ratio with complete dominance.
Common cross results
- AA x AA: all offspring AA (100% dominant phenotype).
- AA x Aa: 1 AA : 1 Aa (100% dominant phenotype).
- Aa x Aa: 1 AA : 2 Aa : 1 aa (75% dominant, 25% recessive).
- AA x aa: all offspring Aa (100% dominant phenotype, all carriers).
- Aa x aa: 1 Aa : 1 aa (50% dominant, 50% recessive, test cross).
- aa x aa: all offspring aa (100% recessive phenotype).
Frequently asked questions
What is a Punnett square?
A Punnett square is a grid used to predict the genotype combinations possible in offspring from a genetic cross. It was developed by British geneticist Reginald Crundall Punnett in the early 20th century.
What is the 3:1 phenotype ratio?
When two heterozygous parents (Aa x Aa) cross, the offspring genotype ratio is 1 AA : 2 Aa : 1 aa, and the phenotype ratio is 3 dominant (A_) to 1 recessive (aa). This is the classical Mendelian ratio for a monohybrid cross.
How does this calculator handle incomplete dominance?
This calculator reports genotype ratios, which apply regardless of dominance type. For incomplete dominance (e.g., red x white = pink), interpret each genotype class according to the specific trait's expression pattern rather than dominant/recessive phenotypes.
What is the difference between genotype and phenotype ratio?
The genotype ratio counts distinct allele combinations (AA, Aa, aa). The phenotype ratio groups genotypes by observable trait: AA and Aa both show the dominant phenotype when dominance is complete, so phenotype ratio may be simpler than genotype ratio.
Can this calculator handle test crosses?
Yes. A test cross is a dominant phenotype individual (unknown genotype) crossed with a homozygous recessive (aa). Enter AA x aa for a 100% Aa result, or Aa x aa for a 1 Aa : 1 aa (1:1) ratio, which reveals whether the dominant parent is AA or Aa.
Official sources
- National Human Genome Research Institute (NHGRI): Punnett Square glossary entry.
- NCBI Bookshelf: Mendelian genetics principles.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.