What is a test cross?

A test cross is a genetic cross between an organism with an unknown dominant genotype and a homozygous recessive individual (e.g., aa or aabb). Because the recessive parent contributes only recessive alleles, any recessive allele in the unknown parent will show up in the offspring, revealing the unknown genotype.

How do I read the result?

If all offspring show the dominant phenotype (100%), the unknown parent is homozygous dominant (AA). If 50% of offspring show the recessive phenotype, the unknown parent is heterozygous (Aa). For a dihybrid cross, each gene pair is evaluated independently, producing ratios like 1:1:1:1 for AaBb × aabb.

What notation should I use?

Use uppercase letters for dominant alleles and lowercase for recessive alleles. For a monohybrid cross enter two characters (AA, Aa, or aa). For a dihybrid cross enter four characters (AABB, AaBb, AAbb, etc.). Both gene pairs must use letters that match their recessive counterparts (A/a and B/b).

Why is the recessive parent always homozygous recessive?

The defining characteristic of a test cross is a homozygous recessive tester parent. This ensures that any recessive offspring phenotype must have received a recessive allele from the unknown parent, directly revealing its genotype.

Does this tool support incomplete dominance or codominance?

No. This tool assumes complete dominance: any genotype with at least one dominant allele shows the dominant phenotype. For codominance or incomplete dominance the phenotype classification differs and you would need a separate model.

What is the expected ratio for a dihybrid test cross with a heterozygous parent?

An AaBb × aabb dihybrid test cross produces four equally likely offspring genotypes (AaBb, Aabb, aaBb, aabb), each at 25%, giving a 1:1:1:1 phenotype ratio. If the unknown parent is homozygous for one gene (AABb × aabb), you get a 1:1 ratio for that gene and all dominant for the other.

Test Cross Calculator

Name: Test Cross Calculator
Availability: InStock
Author: Nham Vu

Determine if a dominant-phenotype organism is homozygous or heterozygous by simulating a test cross against a homozygous recessive parent — for one or two genes.

Cross Setup

Mode:

Unknown Dominant Parent

Enter 2 characters, e.g. Aa, AA

Recessive Tester Parent (fixed)

Always homozygous recessive

Examples:

Enter the unknown parent genotype on the left to run the test cross.

Summary

Determine if a dominant-phenotype organism is homozygous or heterozygous by simulating a test cross against a homozygous recessive parent — for one or two genes.

How it works

Choose Monohybrid (one gene) or Dihybrid (two genes) cross mode.
Enter the genotype of the unknown dominant parent using standard notation (e.g., Aa or AaBb).
The recessive parent is always homozygous recessive (aa for monohybrid; aabb for dihybrid) — filled in automatically.
The tool expands each parent into gametes and pairs all combinations to build the Punnett square.
Genotype counts and phenotype counts are tallied and expressed as ratios and percentages.
An interpretation panel explains whether the unknown parent is homozygous dominant, heterozygous, or a mix across two genes.

Use cases

Determine if a dominant-phenotype organism is homozygous or heterozygous from a breeding experiment.
Predict the ratio of dominant to recessive offspring before running a test cross.
Verify understanding of Mendel's Law of Segregation and Independent Assortment.
Solve genetics homework problems involving unknown parental genotypes.
Model plant or animal breeding experiments requiring genotype determination.
Teach students how a 1:1 offspring ratio reveals heterozygosity.
Compare monohybrid and dihybrid test cross outcomes side by side.
Quickly check whether a 9:3:3:1, 1:1:1:1, or other ratio is expected for two genes.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu