What is a test cross (Genetics)?

In genetics, sometimes it is not easy to tell if an organism is heterozygous (having two different alleles of a gene) for a certain trait or if it is a homozygous (having the same identical alleles of a gene). Therefore, we do not know if the phenotype (the characteristic) is due to homozygosity or heterozygosity. This is where the test cross comes in. In a test cross, we test a suspected heterozygote (A_ (the gap showing that it is a suspected heterozygote)) with a known homozygous recessive (aa) by crossing them together.

When we perform a test cross, we construct a punnet square, crossing the A_ with the aa. If after the cross, we get a result of 2 Aa and 2 aa pairs, then we know that the organism was in fact a heterozygote. However, if all of the results turn out to be heterozygote Aa, this means that the suspected heterozygote is a homozygous dominant.