test cross

A test cross is a way to explore the genotpye of an organism. Early use of the test cross was as an experimental mating test used to determine what alleles are present in the genotype. An organism's genetic makeup is called its genotype, and it reflects all of the alleles, or forms of the gene, that are carried by the organism. Consequently, a test cross can help determine whether a dominant phenotype is homozygous or heterozygous for a specific allele.

Diploid organisms, like humans, have two alleles at each genetic locus, or position, and one allele is inherited from each parent. Different alleles do not always produce equal outward effects or phenotypes. One allele can be dominant and mask the effect of a second recessive allele in a heterozygous organism that carries two different alleles at a specific locus. Recessive alleles only express their phenotype if an organism carries two identical copies of the recessive allele, meaning it is homozygous for the recessive allele. This means that the genotype of an organism with a dominant phenotype may be either homozygous or heterozygous for the dominant allele. Therefore, it is impossible to identify the genotype of an organism with a dominant trait by visually examining its phenotype.

To identify whether an organism exhibiting a dominant trait is homozygous or heterozygous for a specific allele, a scientist can perform a test cross. The organism in question is crossed with an organism that is homozygous for the recessive trait, and the offspring of the test cross are examined. If the test cross results in any recessive offspring, then the parent organism is heterozygous for the allele in question. If the test cross results in only phenotypically dominant offspring, then the parent organism is homozygous dominant for the allele in question.