What is a test cross?
A test cross is a technique used by geneticists in which an individual whose genotype is unknown for a dominant phenotype (it could be homozygous dominant or heterozygous dominant) is crossed with an individual that is homozygous recessive at the locus in question.
The ratio of phenotypes in the offspring reveals the unknown genotype.
Illustrating a test cross: Drosophila melanogaster
Drosophila melanogaster is the scientific name for the fruit fly and is an excellent example of where test crosses are used as they have a large number of variants.
The long-winged condition is dominant (V) to the vestigial, recessive (non-useful) wing (v).
If a pure-breeding long-winged fly (VV) is mated with a vestigial-winged fly (vv), then the F1 generation (first filial) will all be heterozygous (Vv) and have long wings.
However, how can it be decided whether a given F2 long-winged fly will be homozygous dominant (VV) or heterozygous (Vv)? This is determined by crossing the unknown genotype with a vestigial-winged fly.
As a vestigial-winged fly must be homozygous recessive (vv), if the long-winged fly whose genotype is unknown is VV, crossing with the homozygous recessive fly will produce all long-winged flies. However, if the unknown fly is heterozygous (Vv), there will be a mix of long and vestigial-winged flies in equal numbers (approximately).