How a monohybrid Punnett square works
A Punnett square lays out one parent's alleles along the top and the other parent's down the side, then fills each cell with one allele from each. The four cells represent the four equally likely allele combinations, which you tally into genotype ratios such as the famous 1:2:1 from a heterozygous cross.
The four cells are equally likely because of Mendel's law of segregation: each parent passes on exactly one of its two alleles, chosen at random, and the two parents contribute independently. Each cell of the square is an equally likely combination, so the probability calculator covers the reasoning behind the ratios.
How to use this calculator
Type each parent's genotype as two letters, for example Aa, AA or aa. The tool crosses every allele pairing, groups identical genotypes, and reports each as a percentage of offspring. For two-gene crosses, use the dihybrid cross calculator.
Both parents must use the same letter for the gene — pairing Aa with Bb is not a monohybrid cross and will not give meaningful results.
Worked example
Cross Aa × Aa. The four cells are AA, Aa, Aa and aa, giving 25% AA, 50% Aa and 25% aa. If A is dominant, three of four offspring (75%) show the dominant phenotype.
This is why two carriers of a recessive condition can have an affected child even though neither parent shows the trait. It also shows why genotype and phenotype ratios differ: the 1:2:1 genotype split collapses to a 3:1 phenotype split because AA and Aa look identical when A is fully dominant. Scaled up from one couple to a whole population, the same allele maths becomes the Hardy-Weinberg calculator.
Frequently asked questions
- What is a monohybrid cross?
- A cross that tracks a single gene with two alleles, such as Aa × Aa, to predict the genotype and phenotype ratios of the offspring.
- How do I write a genotype?
- Use two letters for one gene — a capital for the dominant allele and lowercase for recessive, for example Aa, AA or aa.
- What ratio comes from Aa × Aa?
- A 1:2:1 genotype ratio — 25% AA, 50% Aa, 25% aa — which gives a 3:1 dominant-to-recessive phenotype ratio.
- Does case matter?
- The calculator treats the same letter as the same gene regardless of case, so it pairs A with a correctly even if you mix capitals and lowercase.
- Are the percentages guaranteed in a real litter?
- No — they are probabilities per offspring, not counts. A cross of Aa × Aa gives each individual a 25% chance of being aa, so a litter of four can easily contain no recessive offspring. The predicted ratios only emerge reliably across large numbers.