In pea plants, flower color is controlled by a pair of alleles where purple (P) is dominant to white (p). Stem length is controlled by another pair of alleles where tall (T) is dominant to short (t). These two allele pairs segregate independently. Suppose you have a pea plant that has purple flowers and tall stems and you wish to carry out a testcross in order to determine its genotype. Complete the predictions using the correct terms.
In order to carry out the testcross, you should cross your original pea plant with another pea plant whose phenotype is ___________

A test cross is performed usually to find out the genotype of a dominant phenotype because it can be homozygous or heterozygous. To perform the test cross, the test organism is mated with the recessive phenotype organism. Here, plant having white flowers and short stem (pptt) will be used in test cross. Few examples of test cross results are as follows:

If the original dominant phenotype plant is homozygous for both traits, all the offspring in the next generation will have dominant phenotype:

PPTT X pptt =

pt

PT PpTt

If the plant is heterozygous for both traits, four types of offspring will be produced:

PpTt X pptt =

pt

PT PpTt

Pt Pptt

pT ppTt

pt pptt

Hence varying results after a test cross can help to determine the genotype of the original plant.