Back Cross and Test Cross

Back cross:

• The F1 individuals obtained in a cross are usually selfed to get the F2 progeny.
• They can also be crossed with one of the two parents from which they were derived (either recessive or dominant). Such a cross is known as a back cross.

Example:

Consider a cross between a tall pea plant (TT) and a dwarf pea plant (tt). The F1 offspring are all heterozygous tall plants (Tt). A back cross can then be performed by crossing these F1 heterozygous plants with either the homozygous dominant (TT) or homozygous recessive (tt) parent.

Case 1: Back Cross with Dominant Parent (TT)

1. Parents: F1 (Tt) x Parent 1 (TT)
2. Gametes Produced:
   • F1 (Tt) produces T and t gametes.
   • Parent 1 (TT) produces T gametes.
3. Offspring (F2 Generation):
   • The possible combinations of gametes are T from Parent 1 and T or t from F1 hybrid.
   • Thus, the offspring could be either TT or Tt, all of which are tall plants.

Case 2: Back Cross with Recessive Parent (tt)

1. Parents: F1 (Tt) x Parent 2 (tt)
2. Gametes Produced:
   • F1 (Tt) produces T and t gametes.
   • Parent 2 (tt) produces t gametes.
3. Offspring (F2 Generation):
   • The possible combinations of gametes are t from Parent 2 and T or t from F1 hybrid.
   • The offspring could be either Tt (tall) or tt (dwarf), resulting in a 1:1 ratio of tall and dwarf plants.

Explanation

• In Case 1, when the F1 hybrid is crossed with the dominant parent (TT), all offspring will show the dominant trait (Tall), as they inherit the dominant allele from one or both parents.
• In Case 2, when the F1 hybrid is crossed with the recessive parent (tt), half of the offspring will be tall (Tt) and the other half will be dwarf (tt), showing a 1:1 phenotypic ratio.

• 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: