A plant or animal that shows a dominant trait could be homozygous (AA) or heterozygous (Aa), and no amount of looking will tell you which. A test cross settles it: mate it with a homozygous recessive partner and read the offspring. This calculator opens on the dihybrid version, AaBb × aabb, and lets you swap in any genotype you want to test.
The tester contributes only recessive alleles, so it cannot add any dominant trait of its own. Every offspring therefore shows exactly the allele it received from the parent under test. If that parent is Aa, half its gametes carry a, half the offspring are aa and show the recessive trait, and the square splits 1:1. If it is AA, every gamete carries A, every offspring is Aa and the recessive trait never appears. The offspring ratio is a direct readout of the unknown parent’s gametes, which is why geneticists also use test crosses to measure how often linked genes recombine.
With two genes the logic is the same, only wider. The aabb tester makes a single kind of gamete, ab, while an AaBb parent makes AB, Ab, aB and ab in equal numbers. The square shows the four offspring genotypes AaBb, Aabb, aaBb and aabb, each with probability 1/4, and the four phenotypes appear in a 1:1:1:1 ratio, one of the classic ratios the Analysis panel recognises. A parent that is homozygous for one gene changes the pattern: AABb × aabb gives only two phenotypes, 1:1, because every offspring inherits A. Try AABB × aabb and the square turns uniform, every offspring dominant for both traits.
The terms overlap but are not the same. A backcross is any mating of an F1 individual with one of its parents or with an organism of a parental genotype. Backcrossing to the dominant parent (Aa × AA) produces all dominant offspring and tells you nothing about the F1’s genotype; backcrossing to the recessive parent is the test cross. Change the second parent to AABB here to compare the two outcomes.
Suppose a purple-flowered pea plant is crossed with a white-flowered one and produces 48 purple and 52 white offspring. Enter Pp × pp with the slider on one gene: the square predicts 1:1, so the purple parent was heterozygous. Had all 100 been purple, PP × pp would be the matching cross. Open Analysis to see the gametes of each parent and tap any cell to highlight all offspring with the same result. To follow the same idea with two heterozygous parents instead of a tester, switch to the dihybrid cross preset.
To find out the genotype of an organism that shows a dominant trait. Because AA and Aa look the same, crossing it with a homozygous recessive partner (aa) lets the recessive allele, if present, show up in the offspring.
1:1 if the tested parent is heterozygous (Aa × aa) and all dominant if it is homozygous (AA × aa). For two genes, AaBb × aabb gives 1:1:1:1.
A backcross mates an offspring with one of its parents or with an organism of the same genotype as a parent. A test cross is the special backcross where that partner is homozygous recessive, which is the only version that reveals the unknown genotype.
Each dominant offspring from an Aa parent has a 1/2 chance, so ten dominant offspring in a row from a heterozygote would happen only about once in 1,000 trials. Ten or more all-dominant offspring make AA very likely; a single recessive offspring proves Aa.
Because it contributes only recessive alleles, it hides nothing: every offspring's phenotype is decided by the allele it got from the parent being tested, so the offspring ratio reads out that parent's gametes directly.