Test Cross

Test cross:

• Test cross is crossing an individual of unknown genotype with a homozygous recessive.
• In Mendel’s monohybrid cross all the plants are tall in F₁ generation.
• In F₂ tall and dwarf plants were in the ratio of 3:1.
• Mendel self-pollinated dwarf F₂ plants and got dwarf plants in F₃ and F₄ generations. So he concluded that the genotype of dwarf was homozygous (tt). The genotypes of tall plants TT or Tt from F₁ and F₂ cannot be predicted. To determine the genotype of a tall plant Mendel crossed the plants from F₂ with the homozygous recessive dwarf plant. This he called a test cross.
• The progenies of the test cross can be easily analyzed to predict the genotype of the plant or the test organism.
• Thus in a typical test cross, an organism (pea plants) showing a dominant phenotype (whose genotype is to be determined) is crossed with the recessive parent instead of self crossing.
• A test cross is used to identify whether an individual is homozygous or heterozygous for the dominant character.

• Consider a typical test cross where violet coloured flower (W) is dominant over the white coloured flower (w).
  Thus, genotypically a violet flower can be: WW or Ww
  While a white flower is recessive hence, its genotype will be: ww
  There can be two possibilities: