Law of Independent Assortment

3. Law of Independent Assortment :

The Law of Independent Assortment is built on the evidence of the dihybrid cross, where two gene pairs are tracked simultaneously. Mendel found that when a hybrid (e.g., RrYy) forms gametes, the alleles of one gene pair (seed shape) sort into gametes independently of the alleles of the other gene pair (seed colour) — the two characters do not travel together as a block, provided their genes lie on separate chromosomes. As a result, each dihybrid F1 produces four kinds of gametes in equal proportion (RY, Ry, rY, ry), and combining these across a 4×4 Punnett square recovers an F2 phenotypic ratio of 9:3:3:1 — direct numerical proof that the two traits assorted independently rather than staying linked to their original parental combination.

Formally: 'When a hybrid possessing two (or more) pairs of contrasting factors (alleles) forms gametes, the factors in each pair segregate independently of the other pair.' The 9:3:3:1 ratio breaks down as 9 showing both dominant traits, 3 showing the first dominant with the second recessive, 3 showing the reverse combination, and 1 showing both recessive traits — proof that all four dominant/recessive combinations occur, and in the expected proportions, once assortment is truly independent.