The law of segregation follows the inheritance of two alleles for a single gene. In a second set of experiments, mendel examined the inheritance of two different traits, each attributable to gene with two different alleles. Specifically, mendel looked at seed shape, which was either round or wrinkled, and seed color, which was either yellow or green. When he crossed plants with round, yellow seeds to plants with wrinkled, green seeds, all the progeny had round, yellow seeds. Therefore, round was completely dominant to wrinkled, and yellow was completely dominant to green.
Next, mendel took first generation plants and crossed them to each other in a dihybrid cross, so named because two individuals heterozygous for two genes are crossed. He found four types of seeds in the second generation: round, yellow; round, green; wrinkled, yellow; and wrinkled, green. This is an approximate ratio of 9:3:3:1.
Mendel then took each plant from the second generation and crossed it to wrinkled, green plants. These test crosses established whether each second generation plant was true-breeding for both genes, true-breeding for one but heterozygous for the other, or heterozygous for both genes. Based upon the results of the dihybrid cross, mendel concluded that a gene for one trait does not influence transmission of a gene for another trait. This is of a gene for another trait. This is mendel’s second law, the law of independent assortment. It is true only for genes on different chromosomes. The seed shape and seed color genes that mendel worked with fit this criterion.
With the idea of independent assortment, mendel had again inferred a principle of inheritance that has its physical basis in meiosis. Independent assortment occurs because chromosomes from each parent combine in a random fashion. In mendel’s dihybrid cross, each parent produces equal numbers of gametes of four different types: RY, Ry,rY, and ry. A punnett square for this cros shows that the four types of seeds-round, green; round, yellow; wrinkled, green; and wrinkled, yellow-are present in the ratio 9:3:3:1, just as mendel found.