A test cross is a method to expel the genotype of an organism. The use of testcross as an experimental mating test allows geneticists to determine what alleles are present in the genotype and whether a dominant phenotype is homozygous or heterozygous for a specific allele.  To identify this, the organism in question is crossed with an organism that is homozygous for a recessive trait and the offspring of this cross are examined.  If the test cross results in phenotypically dominant offspring then the parent organism is homozygous dominant for the allele in question. If the cross results in any recessive offspring then the parent organism is heterozygous for the allele in question.