Monohybrid cross and the Punnet square

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Monohybrid cross :-

Monohybrid is the cross involves one character and single gene pair.It leads to monohybrid offspring in F₁ generation.

Mendel proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt respectively. TT and tt are called genotype of the plant while descriptive terms tall and dwarf are the phenotype.

  • Mendel found the phenotype of the F₁ heterozygous Tt to be exactly like the TT parent in appearance.

He proposed that in pair of dissimilar factors, one dominates the other and hence is called the dominant factor while other factor is recessive. In this case T is dominant over t.

  • A Punnet square used to undestand a typical monohybrid cross conducted by Mendel between true breeding tall plants and true breeding dwarf plants.
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Fig. 1. Punnet square for monohybrid cross.

Punnet square :-

The production of gametes by the parents, the formation of the zygotes, the F₁ and F₂ plants can be understood from a diagram called Punnet square.

  • It was developed by a british geneticist R. C. Punnet. It is the graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross.
  • The possible gametes are written on two sides usually the top row and left columns. All possible combinations are represented in boxes bellow in the squares, which generates a square output form.
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Fig. 2. Punnet Square.

Test cross :-

A test cross is the back cross between a hybrid and the recessive plant.

Example –

Tt x tt

Monohybrid test cross

TtSs x ttss

Dihybrid test cross

  • In monohybrid cross, to determine the genotype of a tall plant at F₂, Mendel crossed the tall plant from F₂ with a dwarf plant. This he called a test cross.

Back cross :-

Back cross is cross between a F₁ hybrid and a parent.

Example –

Tt x TT

Tt x tt

Monohybrid back cross.


TtSs x ttss

Dihybrid back cross.

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