Test Cross

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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:

Diagrammatic representation of a test cross

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