Q.Explain Mendel's third law of inheritance with the help of a dihybrid cross. OR Explain the primitive earth condition. Write any two theories related with the origin of life.
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Start your 14-day free trial to unlock the full solution →The Law of Independent Assortment is proven by the dihybrid cross's 9:3:3:1 F₂ ratio, showing seed shape and colour genes segregate independently of each other.
Mendel's Third Law — Law of Independent Assortment: When two pairs of contrasting traits (controlled by genes on different chromosome pairs) are considered together in a cross, the two genes segregate independently of one another during the formation of gametes; each possible combination of alleles has an equal chance of occurring together in a gamete.
Demonstration with a dihybrid cross (using seed shape — round R dominant over wrinkled r — and seed colour — yellow Y dominant over green y — in garden pea):
- P generation: RRYY (round, yellow) × rryy (wrinkled, green)
- F₁ generation: All offspring are RrYy — round and yellow (showing both dominant traits, as per the law of dominance).
- Gametes from F₁ (RrYy): Because R/r and Y/y assort independently, F₁ produces four types of gametes in equal proportion — RY, Ry, rY, and ry — including two new ("recombinant") combinations (Ry and rY) that were not present in either parent.
- F₂ generation (from selfing F₁, i.e., RrYy × RrYy): Using a Punnett square (4×4 = 16 combinations), the F₂ generation shows a phenotypic ratio of: 9 Round Yellow : 3 Round Green : 3 Wrinkled Yellow : 1 Wrinkled Green …
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