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Matings of the homozygous recessive with individuals of unknown genotype such that genotypte information is inferred from progeny phenotype information.
Given a one-locus, two-allele system, if one allele is dominant to the other then it will be impossible to distinguish the homozygous dominant from the heterozygote solely in terms of phenotype. What to do? The classical approach, and one still very much valid so long as the genotypic basis of phenotype is not known, is to perform a test cross.
If A is dominant and a is recessive, then the dominant phenotype will consist of either AA or instead the Aa genotype. If you cross individuals with the dominant phenotype to a homozygous recessive individual then the possible matings are (1) AA × aa or, instead, (2) Aa × aa.
The key to the test cross is that each of these mating are expected to have different phenotypic outcomes. These are, respectively, (1) all progeny have the dominant phenotype versus (2) half of the progeny possess the dominant phenotype (with the other half the recessive phenotype). One thus performs the test cross and then observes the progeny to infer the previously unknown genotype. Note that this is a fairly definitive means of determining whether the unknown genotype also may be true breeding.
One way to conceptualize the test cross is that it represents a mating of an individual with unknown genotype with another individual with a genotype that basically is a "blank slate". That is, what phenotype the progeny end up with will be entirely a consequence of what allele or alleles they receive from the unknown genotype, i.e., either receiving a dominant allele or, instead, a recessive allele.
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