CBSE Class 10 Science 2017 · Paper 31/1/3 · Question 11

Mendel crossed pure-breeding pea plants with round, yellow seeds ($\text{RRYY}$) with plants having wrinkled, green seeds ($\text{rryy}$). The ${\text{F}}_1$ plants all had round, yellow seeds ($\text{RrYy}$), showing that round and yellow are dominant. When the ${\text{F}}_1$ plants were self-pollinated, the ${\text{F}}_2$ generation had four kinds of seed: round-yellow, round-green, wrinkled-yellow and wrinkled-green, in the ratio $9:3:3:1$. Round-green and wrinkled-yellow are new combinations, found in neither parent. They appeared, about 3 in every 16 seeds each, as independent separation predicts, because the factors for seed shape and for seed colour separated from each other when the gametes formed, each gamete receiving one factor for shape and one for colour at random (RY, Ry, rY and ry in equal numbers). This showed that the two traits are inherited independently of each other. Each trait on its own still gave a $3:1$ ratio (round : wrinkled $=12:4$, yellow : green $=12:4$).