Since the homozygous dominant genotype and the heterozygous genotype both express the same phenotype, we cannot be certain of the genotype of a plant that produces yellow seeds. We can determine its genotype by crossing it with a homozygous recessive plant, a true breeding plant that produces green seeds (genotype yy). This is called a test cross. The following two Punnett squares illustrate what progeny we would obtain from a test cross involving a homozygous dominant (left) or a heterozygote (right):
If the yellow seed strain is homozygous dominant, all the progeny from the test cross will have the yellow phenotype. However, if the yellow seed strain is heterozygous, half of the progeny from the test cross will have yellow seeds and half will have green seeds. In this manner, test crosses can be very valuable in determining the exact genotype of individuals who express the dominant phenotype.