CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 21

(a) Mendel crossed pure tall pea plants ($TT$) with pure short plants ($tt$). All the $F_1$ plants were tall ($Tt$); none was short. When the $F_1$ plants were self-pollinated, the $F_2$ generation had tall and short plants in the ratio $3:1$ ($TT:Tt:tt=1:2:1$). The short trait, which did not appear in $F_1$, reappeared in $F_2$, so it had been carried by the $F_1$ plants without being expressed. The trait expressed in $F_1$ (tallness) is dominant, and the trait that remained hidden in $F_1$ (shortness) is recessive. (b) Mendel crossed plants with round, yellow seeds ($RRYY$) with plants with wrinkled, green seeds ($rryy$). All the $F_1$ plants had round, yellow seeds ($RrYy$). On self-pollination of $F_1$, the $F_2$ plants had four kinds of seeds: round-yellow, round-green, wrinkled-yellow and wrinkled-green, in the ratio $9:3:3:1$. Round-green and wrinkled-yellow are new combinations not present in either parent. This shows that the factors for seed shape and seed colour are inherited independently of each other.