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Monohybrid Cross Calculator

Cross two parent genotypes for a single gene and get the genotype ratio, phenotype ratio, trait probabilities and the expected number of each offspring type in a chosen brood size.

Parent 1 genotype

Parent 2 genotype

Number of offspring

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How it works

Each parent passes one allele at random → 4 equally likely squares · Genotype probability = squares of that genotype ÷ 4 · Expected count = probability × brood size

A monohybrid cross tracks a single gene with two alleles. Crossing the two alleles of one parent against the two of the other gives four equally likely offspring squares; grouping them by genotype gives the genotype ratio, and grouping every square carrying at least one dominant allele against the purely recessive ones gives the phenotype ratio. Multiplying each probability by the brood size turns the ratio into the expected number of each offspring type — the numbers you would compare a real cross against.

Worked example

Aa × Aa yields AA, Aa, Aa and aa: a 1 AA : 2 Aa : 1 aa genotype ratio and a 3 dominant : 1 recessive phenotype ratio. In a brood of 100, expect roughly 75 dominant and 25 recessive offspring — though real broods vary around these averages by chance.

Sources: Monohybrid cross — Wikipedia

Frequently asked questions

How is a monohybrid cross different from a Punnett square?

A monohybrid cross is the single-gene case that a 2×2 Punnett square solves. This tool builds that square, then adds the expected offspring counts for a brood size so you can compare it against real data.

Why do real broods not match 3:1 exactly?

The 3:1 ratio is an expectation, not a guarantee. Each offspring is an independent event with a 75% dominant chance, so small broods scatter around the average — much like coin flips rarely land exactly half heads.

Can I cross a homozygote with a heterozygote?

Yes. Aa × AA gives all dominant offspring (1 AA : 1 Aa, 100% dominant), while Aa × aa gives a 1:1 dominant-to-recessive ratio. Enter any two two-letter genotypes.

Related calculators

Punnett Square Calculator

Cross two parent genotypes for a single gene and get the offspring genotype ratio, phenotype ratio and the odds of dominant versus recessive traits.

Hardy-Weinberg Equilibrium Calculator

Work out allele and genotype frequencies under Hardy-Weinberg equilibrium from a dominant allele frequency p or from the frequency of the recessive phenotype q².

Allele Frequency Calculator

Count homozygous and heterozygous individuals to get the dominant and recessive allele frequencies and the genotype frequencies of a population.

Probability Calculator

Work out a probability with the steps shown: favorable ÷ total outcomes with odds, two events (and, or, neither, conditional), repeated trials with the binomial distribution, and the area under a normal curve.

Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific protocol.