Test cross - Mendelian genetics

Is there a way to distinguish the genotype of an organism expressing the dominant phenotype? The answer is YES! Mendel devised a method called test cross, where an organism of dominant phenotype but unknown genotype is crossed with a homozygous recessive individual.

Case 1 If a purple-flowered plant (PP) is test-crossed to a white-flowered plant (pp), all offspring will produce purple flowers (Pp). All purple-flowered offspring show that the unknown parental genotype is homozygous dominant. Case 2 If a puple-flowered plant (Pp) is test-crossed to a white-flowered plant (pp), half of the offspring will produce purple flowers (Pp) and the other half will produce white flowers (pp). A 1:1 ratio of purple:white offspring shows that the unknown parental genotype is heterozygous dominant.

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