Unit – II B – 02

Mendel’s Experiment (Monohybrid Cross) Cont.

Test Cross or Back Cross:

  • This is the method devised by Mendel to test the genotype of F1 Hybrids.
  • In F1 generation 25% plants are dwarf and we can definitely say that their genotype is ‘tt’ (Homozygous). But in a case of a tall plant, there are 25 % pure tall plants and 50% hybrid tall plants. Hence in case of tall plants genotype can be ‘TT’ (Homozygous) or ‘Tt’ (Heterozygous). Thus we are not sure of the genotype of tall plant in F1 generation.
  • In test cross, F1 hybrid is crossed with the homozygous recessive parent. Thus the offspring is crossed back with the parent, hence the test cross is also called as a back cross.



  • The F2 generation obtained will be 50 % tall and 50 % dwarfs.It can be explained as follows.

  • The recessive parent can produce only one type of gamete ‘t’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus half the progeny (F2 generation) will have genotype ‘Tt’ (tall) and remaining half ‘tt’ (dwarf).

A test cross is a back cross but a back cross is no necessarily a test cross:

  • Case – 1: When the F1 generation is crossed with Recessive Parent:

    The recessive parent can produce only one type of gamete ‘t’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus half the progeny (F2 generation) will have genotype ‘Tt’ (tall) and remaining half ‘tt’ (dwarf).

  • Case – 2: When the F1 generation is crossed with Dominant Parent:

    The dominant parent can produce only one type of gamete ‘T’, while the hybrid of the first generation can produce two types of gametes ‘T’ and ‘t’. Thus 100 % progeny is tall. half the progeny will have genotype ‘TT’ (Pure tall) and remaining half ‘Tt’ (Hybrid tall).

  • A test cross is a cross used to find the genotype of F1 generation. The test cross is a cross between individual with unknown genotype for a particular trait with a recessive plant for their trait, While back cross is a cross between individual with unknown genotype for a particular trait with a recessive or dominant plant for their trait.  Back cross can not indicate the genotype of F1 generation. Hence a test cross is a back cross but a back cross is not a test cross.
  • Test cross method can be used to introduce useful recessive trait. Which is important in rapid crop improvement programmes.


Diagrammatic Representation of Test Cross

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