phenotypes
Consider the following example: Suppose you have a purple and white flower and purple color P is dominant to white p . The white flower must be homozygous for the recessive allele, but the genotype of the purple flower is unknown. It could be either PP or Pp . A testcross will determine the organism's genotype. The unknown genotype can be determined by observing the phenotypes of the resulting offspring. If crossing the unknown dominant phenotype PP or Pp genotype individual with the recessive phenotype individual produces only dominant phenotypes no recessive , then the unknown individual is homozygous dominant. If any recessive phenotypic individuals result from the cross, then the unknown individual must carry the recessive allele, and have the heterozygous genotype.