Principles of Inheritance and Variation

Mendelian inheritance


  • Gregor johann mendel is called as “Father of Genetics”, as he was the first to demonstrate the mechanism of Inheritance.
  • He carried out experiments using Pisum sativum (Garden pea) for 7 years as it had 7 contrasting characters with 2n = 14.
  • Selection of preline parents, Hybridisotion, selfing of first filial generation and calculation using pannett square were the four steps involved in his experiments.
  • Cross pollination of flowers involves steps such as Emasculation, Bagging, dusting and tagging.
  • Mendel’s monohybrid cross (cross involving single pair of contrasting characters) yield a phenotypic ratio of 3 : 1 [Tall : dwarf] and a genotypic ratio of 1 : 2 : 1. [TT : Tt : tt]
  • By performing a cross experiment with tall (TT) and dwarf plant (tt)
  • From monohybrid cross, Mendel could frame 2 important laws of inheritance
    (i) Law of segregation [alleles get segregated into gametes individually]
    (ii) Law of Dominance [In order to express a dominant character, at least one of the alleles should be dominant; both the alleles should be recessive to express a recessive character]
  • Dihybrid cross (cross involving 2 pairs of contrasting characters – seed colour and seed shape) between round yellow(RRYY) and wrinkled(rryy) green plants yield a phenotypic ratio of 9 : 3 : 3 : 1 [Round – yellow : Round – green; wrinkled yellow; wrinkled green].
  • The above cross shows that the factors of 2 characters assort independent that leads to formation of independent assortment factors, by which he could frame the third law [law of independent assortment].

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