Karnataka Board PUCPUC Science 2nd PUC Class 12

Punnett Square

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Notes

Punnett Square:

  • In a genetic cross the genotypes and phenotypes of offspring, resulting from combining gametes during fertilization can be easily understood with the help of a diagram called Punnett’s Square.
  • It was developed by a British geneticist, Reginald C. Punnett.
  • It is a 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 below in the squares, which generates a square output form.
  • The Punnett Square shows the parental tall TT (male) and dwarf tt (female) plants, the gametes produced by them, and the F1 Tt progeny. The F1 plants of genotype Tt are self-pollinated.
  • The symbols ♀ and are used to denote the female (eggs) and male (pollen) of the F1 generation, respectively.
  • The F1 plant of the genotype Tt when self-pollinated, produces gametes of the genotype T and t in equal proportion.

    A Punnett square used to understand a typical monohybrid cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants

  • From Punnett Square, it can be observed that:
    a) 1/4th of the random fertilisations lead to TT,
    b) 1/2 of the random fertilisations lead to Tt, and
    3) 1/4th of the random fertilisations lead to tt.
  • Though the F1 has a genotype of Tt, the phenotypic character seen is ‘tall’.
  • At F2, 3/4th of the plants are tall, where some of them are TT while others are Tt. Externally it is not possible to distinguish between the plants with the genotypes TT and Tt.
  • Hence, within the genopytic pair Tt, only one character ‘T’ tall is expressed. Hence the character T or ‘tall’ is said to dominate over the other allele t or ‘dwarf’ character which is a recessive character.
  • It is thus due to this dominance of one character over the other that all the F1 are tall (though the genotype is Tt) and in the F2 3/4th of the plants are tall (though genotypically 1/2 are Tt and only 1/4th are TT).
  • The `1/4: 1/2: 1/4` ratio of TT: Tt: tt is mathematically condensable to the form of the binomial expression (ax + by)2, that has the gametes bearing genes T or t in the equal frequency of `1/2`. The expression is expanded as given below:
    `(1/2 "T" + 1/2"t")^2 = (1/2"T" + 1/2"t") × (1/2"T" + 1/2"t") = 1/4 "TT" + 1/2 "Tt" + 1/4 "tt"`
  • Mendel self-pollinated F2 plants and he found that F2 plants continued to generate dwarf plants in F3 and f4 generations.
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