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Explain the Mendel law of inheritence mono and dihybrid cross.

Mendel gave three laws of inheritance which are as follows :

Law of Dominance : According to this law, characters are controlled by discrete units called factors, which occur in pairs with one member of the pair dominating over the other in a dissimilar pair. This law explains expression of only one of the parental character in F1 generation and expression of both in F2 generation.

Law of Segregation : This law states that the two alleles of a pair segregate or separate during gamete formation such that a gamete receives only one of the two factors. In homozygous parents, all gametes produced are similar; while in heterozygous parents, two kinds of gametes are produced in equal proportions.

Law of independent Assortment : When two pairs of traits are combined in a hybrid, one pair of character segregates independent of the other pair of character. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes (RY, Ry, rY, ry) are produced. Each of these segregate independent of each other, each having a frequency of 25% of the total gametes produced.

Monohybrid cross : 

 

Dihybrid Cross

  • In dihybrid cross, we consider two characters. (e.g., seed colour and seed shape)

  • Yellow colour and round shape is dominant over green colour and wrinkled shape.

  • Phenotypic ratio − 9:3:3:1
    Round yellow − 9
    Round green − 3
    Wrinkled yellow − 3
    Wrinkled green −1

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Rules for the Inheritance of Traits –Mendel’s Contributions:

John Gregor Mendel (1856-63) conducted hybridization experiments on pea plants for many generations and studying how certain characters get transferred from one generation to the next generation. He also studied how certain characters become dormant or prominent in a particular generation. One of the experiments involved making hybrids of wrinkled seeds and smooth seeds. The results can be depicted by following diagram:


Interpretation of Results: In the first generation all the seeds were wrinkled. The 'wrinkled' character was dominant, while the 'smooth character was dormant. Both characters were present in the genotype or genetic makeup. In the next generation 25% of seeds were pure wrinkled with 'wrinkled' genotype, 25% seeds were pure smooth with 'smooth' genotype and 50% of seeds were wrinkled with 'wrinkled+smooth' genotype. This shows how sometimes children of tall father can be of average height as the 'tall' character becomes dormant in that generation.

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