Test Cross Calculator

Cross an organism of unknown genotype with a homozygous recessive tester to reveal whether it is homozygous or heterozygous. Supports monohybrid and dihybrid test crosses and backcrosses.

Cross type
Organism being tested (dominant phenotype, unknown genotype)
Cross with (tester)
Tester genotype: aa
Punnett Square
Phenotype Ratio
Genotype Ratio

What is a test cross?

An organism showing a dominant trait could be homozygous (AA) or heterozygous (Aa) — you cannot tell by looking. A test cross mates it with a homozygous recessive (aa) partner, and the offspring give the answer.

  • All dominant offspring → tested parent is AA
  • 1:1 dominant : recessive → tested parent is Aa

Test cross vs backcross

A backcross mates an individual with one of its parental genotypes. A test cross is the special backcross to the homozygous recessive parent, used to deduce an unknown genotype.

Switch the tester to homozygous dominant to model a backcross to the dominant parent instead.

Dihybrid test cross

For two genes, a double heterozygote crossed with aabb gives a telling ratio:

  • AaBb × aabb → 1:1:1:1 (heterozygous for both)
  • AABb × aabb → 1:1 (homozygous for gene 1)
  • AABB × aabb → all dominant (homozygous for both)

How to use it

  • Choose monohybrid or dihybrid mode.
  • Set the genotype you want to test (or are hypothesizing).
  • Keep the tester as homozygous recessive for a true test cross.
  • Read the verdict, Punnett square, and ratio tables.

Why it works

A homozygous recessive tester only ever donates a recessive allele. That means each offspring's phenotype is decided entirely by the allele it receives from the tested parent — making hidden recessive alleles visible.

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