Punnett Square Ratio Calculator
A Punnett square predicts the make-up of offspring from a genetic cross. For a single gene with two alleles, each parent passes on one allele at random, and the four combinations are equally likely. This calculator crosses two parent genotypes (choose homozygous dominant, heterozygous, or homozygous recessive for each), then reports the expected genotype counts, the genotype probabilities, and the dominant-to-recessive phenotype ratio. It is built for genetics study and the classic Mendelian monohybrid cross.
Single-gene cross method
Each parent passes one of its two alleles with equal chance
Offspring allele pair = (one allele from each parent)
4 equally likely combinations form the Punnett square
Dominant phenotype = P(AA) + P(Aa); recessive = P(aa)
Aa x Aa gives 25% AA, 50% Aa, 25% aa (3:1 phenotype)
Probabilities are exact for a single gene with simple dominance. Each offspring is an independent draw from these odds.
Genetics context
- Uppercase A is the dominant allele; lowercase a is recessive.
- The recessive phenotype appears only in aa offspring.
- Aa by Aa is the classic monohybrid cross with a 3 to 1 phenotype ratio.
- Ratios are expected averages; real offspring counts vary by chance.
- For two genes, run each gene separately and multiply the probabilities.
Punnett square: frequently asked questions
What is a Punnett square?
A Punnett square is a grid that predicts the genotypes of offspring from a genetic cross. For a single gene, each parent contributes one of its two alleles, and the square combines them to show all four equally likely allele pairings in the offspring.
What do dominant and recessive mean here?
A dominant allele (shown uppercase) masks a recessive allele (lowercase) in the phenotype. An offspring shows the recessive trait only when it inherits two recessive alleles. So genotypes AA and Aa both show the dominant phenotype, while aa shows the recessive phenotype.
What is the classic 3 to 1 ratio?
Crossing two heterozygous parents (Aa by Aa) gives offspring genotypes in a 1 AA : 2 Aa : 1 aa ratio, which is a 3 to 1 dominant-to-recessive phenotype ratio. This is Mendel's monohybrid cross result.
Does this handle two genes (dihybrid crosses)?
This calculator covers a single gene (monohybrid cross) with two alleles. Dihybrid crosses combine two independent single-gene crosses; you can run each gene separately and multiply the probabilities.
Are the ratios guaranteed in real offspring?
No. The ratios are expected probabilities for each offspring independently. Actual litters or families vary by chance, especially with small numbers. The predicted ratios describe the long-run average, not a guarantee for any single birth.
Official sources
- National Human Genome Research Institute: Genetics glossary.
- National Center for Biotechnology Information: NCBI Bookshelf.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.