Mendel’s experimental study on the pattern of inheritance was a turning point in the history of genetics. Based on his investigation, Mendel proposed three laws of inheritance. Mendel’s Law of Inheritance is the fundamental principle of inheritance in genetics. Here we will discuss the Law of Independent Assortment.
Law of Inheritance
Law of Dominance, Law of Segregation and Law of Independent Assortment are collectively known as Mendel’s Laws of Inheritance. Mendel chose to perform monohybrid cross of a pair of contrasting traits. The observations of monohybrid cross led to the formulation of Law of Segregation and Law of Dominance. Followed by this, Mendel performed dihybrid cross taking two contradicting traits together for crossing.
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 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 color 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.
Law of Independent Assortment
Mendel’s experiment always portrayed that the combinations of traits of the progeny are always different from their parental traits. Based on this, he formulated the Law of Independent 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.
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 gene 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).
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