Law of segregation and assortment

The Law of Independent Assortment states that during a dihybrid cross (crossing of two pairs of traits), an assortment of each pair of traits is independent of the other. In other words, during gamete formation, one pair of trait segregates from another pair of traits independently. This gives each pair of characters a chance of expression.

In the dihybrid cross, he chose round-yellow seed and wrinkled green seed and crossed them. He obtained only round yellow seeds in the F₁ generation. Later, self-pollination of F₁ progeny gave four different combinations of seeds in the F₂ generation. He obtained round-yellow, wrinkled-yellow, round green and wrinkled green seeds in the phenotypic ratio 9:3:3:1.

The phenotypic ratio 3:1 of yellow: green colour and the ratio 3:1 of the round: wrinkled seed shape during monohybrid cross was retained in the dihybrid cross as well. Thus, he concluded that characters are distributed independently and inherited independently. Based on this observation, he developed his third law – Law of Independent Assortment.

The dihybrid crosses between the parental genotype RRYY (round yellow seeds) and rryy (green wrinkled seeds) explains the law. Here the chances of formation of gametes with the gene R and the <a href="https://byjus.com/biology/genes/">gene </a>r are 50:50. Also, the chances of formation of gametes with the gene Y and the gene y are 50:50. Thus, each gamete should have either R or r and Y or y.

The Law of Independent Assortment states that the segregation of R and r is independent of the segregation of Y and y. This results in four types of gametes RY, Ry, rY, and ry. These combinations of alleles are different from their parental combination (RR, YY, rr and yy).