Question

Define test cross. Explain it with an example.

Answer 1

A test cross is defined as a cross between an individual of unknown genotype with a homozygous recessive.

In F₁ generation of Mendel’s monohybrid cross of tall and dwarf pea plants, he got only tall plants. In F₂ generation the tall and dwarf plants were in the ratio 3:1. He self-pollinated the dwarf F₂ plants and he got only dwarf plants in F₃ and F₄ generation so he concluded that the genotype of the dwarf plant was homozygous recessive (tt). But the genotype of tall plants TT or Tt cannot be predicted. Mendel crossed the tall plants of F₂ with the homozygous recessive dwarf plant. This is known as a test cross. From the progenies of the test cross, the genotype of tall plants can easily be identified. Thus a typical test cross is to cross an organism showing dominant phenotype is crossed with the recessive parent instead of self crossing. Therefore test cross can be used to identify whether an individual is homozygous or heterozygous for the dominant character.