Explain the significance of test cross.

Explanation

(i) Test cross is crossing an individual of unknown genotype with a homoztgous recessive.
(ii) In Mendel's monohybrid cross all the plants are tall in `F_(1)` generation. In `F_(2)` tall and dwarf plants were in the ratio of `3:1`. Mendel self pollinated dwarf `F_(2)` plants and got dwarf plants in `F_(3)` and `F_(4)` generations. So he concluded that the genotype of dwarf was homozygous (tt). The genotypes of tall plants TT or Tt from `F_(1)` and `F_(2)` cannot be predicted.
(iii) To determine the genotype of a tall plant Mendel crossed the plants from `F_(2)` with the homozygous recessive dwarf plant. This he called a test cross.
(iv) The progenies of the test cross can be easily analysed to predict the genotype of the plant or the test organism.
(v) Thus in a typical test cross an organism (pea plants) showing dominant phenotype (whose genotype is to be determined) is crossed with the recessive parent instead of self crossing. Test cross is used to identify whether an individual is homozygous or heterozygous for dominant character.