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3.3Mendel’s Law of Independent Assortment

Fig. 3-2 Mendel’s Dihybrid Cross

By focusing on two allelotypes, Mendel also discovered that several alleles are transmitted in an unrelated manner through cross-breeding (Fig. 3-2). Mendel crossbred peas with different seed colors (yellow, green) and shapes (round, wrinkled). In P, combinations of yellow/round and green/wrinkled peas both gave yellow/round peas in F1. F2 produced by self-fertilization of F1 gave a ratio of yellow/round:yellow/wrinkled:green/round:green/wrinkled of 315:101:108:32.

When calculating the segregation ratio of F2, the abovementioned ratio can be substituted with 9:3:3:1. When the yellow/green and round/wrinkled genes are A/a and B/b, respectively, then P is AABB or aabb and F1 is AaBb, i.e., yellow and round peas. Then, the 9:3:3:1 ratio in F2 as shown in the figure could be perfectly explained. Thus, the fact that gametes obtained from AaBb are segregated in a ratio of AB:Ab:aB:ab = 1:1:1:1 is called the law of segregation, and the fact that different alleles are inherited without having an overlapping effect on other traits is called the law of independent assortment.

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