4. Test Cross

With reference to Mendel's experiment in Section 1, you are given a pea plant that has purple flowers. How would you determine if the pea plant is homozygous dominant, ie PP or heterozygous dominant, Pp?

For a test cross, always use a homozygous recessive organism to do the cross, ie use plant with white flowers as the test organism.

Case 1: The pea plant has PP genotype. When crossed with the test organism with pp genotype, all the offspring would have Pp genotypes (phenotype: purple flowers).

Case 2: The pea plant has Pp genotype. When crossed with the test organism with pp genotype, half the offsprings would have Pp genotype (phenotype: purple flowers) and the other half of the offsprings would have pp genotype (phenotype: white flowers). A phenotypic ratio of Purple flower : White Flowers of 1:1 is expected.