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.
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).
Dihybrid Cross
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In dihybrid cross, we consider two characters. (e.g., seed colour and seed shape)
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Yellow colour and round shape is dominant over green colour and wrinkled shape.
