Gregor Mendel cultivated and tested about twenty nine thousand pea plants between eighteen fifty six and eighteen sixty three. One such test was the crossing of pea plants with yellow and round seeds and those with green and wrinkled seeds. As you know, such a cross between plants that differ in two traits or characters is known as a dihybrid cross. This experiment conducted by Mendel resulted in a pea plant that produced yellow and round seeds in the F one generation.
Obviously, the round shape is dominant over the wrinkled shape and the yellow colour dominant over green.
Mendel found that the phenotypic ratio in the F two hybrid was nine is to three is to three is to one, that is, there were nine round yellow seeds, three round green seeds, three wrinkled yellow seeds and one wrinkled green seed. Mendel had got the same phenotypic ratio in several dihybrid experiments that he had conducted. Mendel proposed a new set of generalisations, which was later called the Law of Independent Assortment. According to this law, “When two pairs of traits or characters are united in a hybridisation experiment, the segregation of one pair of characters is independent of the other pair of characters”. As you already saw, there are four different types of phenotypes formed and the phenotypic ratio is nine is to three is to three is to one. However, nine different genotypes result from the F two hybrid and the genotypic ratio is one is to two is to two is to one is to four is to two is to one is to two is to one. Thus, from the dihybrid cross experiment, Mendel found that the gene combination of the progeny differs from the parental gene combination. In a dihybrid cross genes assort independently.