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Test Cross Ratio Calculator

Cross an organism of unknown genotype against a fully recessive tester and get the expected phenotype ratio, number of phenotype classes and offspring counts that reveal how many genes are heterozygous.

Tested parent genotype

Number of offspring

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How it works

Tester is homozygous recessive · Each heterozygous gene in the tested parent segregates 1:1 · Phenotype classes = 2^(heterozygous genes) · Each class = 1 / 2^k of offspring

A test cross reveals an unknown genotype by crossing it with a fully recessive tester. Because the tester contributes only recessive alleles, the offspring phenotype at each gene mirrors exactly which allele the unknown parent passed. A homozygous-dominant gene yields all dominant offspring, while a heterozygous gene splits 1:1 — so counting phenotype classes tells you how many genes were heterozygous (k), giving 2^k equally sized classes.

Worked example

A double-heterozygote AaBb tested against aabb produces four equal phenotype classes (A_B_, A_bb, aaB_, aabb) in a 1:1:1:1 ratio. Over 100 offspring, expect about 25 of each — the signature that both genes were heterozygous.

Sources: Test cross — Wikipedia

Frequently asked questions

What is a test cross used for?

It determines whether an organism showing a dominant trait is homozygous (AA) or heterozygous (Aa). Crossing it with a recessive tester (aa) exposes the hidden allele: any recessive offspring proves the parent carried a recessive allele.

Why does a heterozygous gene give a 1:1 ratio?

The heterozygous parent passes its dominant and recessive alleles with equal 50% probability, and the tester always adds a recessive allele. So half the offspring are dominant and half recessive for that gene.

What if the tested parent is homozygous dominant?

Then it can only pass a dominant allele, so every offspring shows the dominant phenotype — one class, no 1:1 split. Seeing no recessive offspring is strong evidence of a homozygous-dominant parent.

Related calculators

Dihybrid Cross Calculator

Cross two parents across two independent genes and get the phenotype ratio, the probability of each of the four trait combinations and the expected offspring counts for a chosen brood size.

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.

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.

Allele Frequency Calculator

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

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