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A cross between an individual of uncertain genetic constitution and a homozygous recessive. The latter is often a parent, in which case the test cross is a specific form of backcross. The purpose of a test cross is to ascertain the unknown genotype. For example, if Τ (tall peas) is dominant to t (short peas) then any tall peas (T_) could be either TT or Tt. If T_ were test crossed to tt and all the offspring were tall then the second allele could be assumed to be T. Alternatively, if there were any short (tt) peas among the offspring the second allele must be t.
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