CBSE Class 10 Science 2022 · Paper 31/3/1 · Question 13

Mendel crossed pea plants that differed in two traits at once: a pure-breeding plant with round, yellow seeds (RRYY) and one with wrinkled, green seeds (rryy). ${\text{F}}_1$: all the plants had round, yellow seeds (RrYy), because round and yellow are dominant. ${\text{F}}_2$: on self-pollinating the ${\text{F}}_1$ plants he obtained four kinds of seeds in the ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green. Round green and wrinkled yellow are new combinations that were present in neither parent. They appear because the factors (genes) for seed shape and for seed colour separate independently of each other when the gametes are formed, each ${\text{F}}_1$ plant forming RY, Ry, rY and ry gametes in equal numbers. Had the two traits been inherited together, only round yellow and wrinkled green seeds would have appeared. Hence the traits are inherited independently. (Each trait on its own still follows $3:1$: round : wrinkled $=12:4$ and yellow : green $=12:4$.)