CBSE Class 10 Science 2017 · Paper 31/1/1/C · Question 22

(i) Mendel crossed pure-breeding tall pea plants ($TT$) with pure-breeding short plants ($tt$). All the $F_1$ plants were tall ($Tt$); none was short or of medium height, so only tallness was expressed although each $F_1$ plant had also inherited shortness. When the $F_1$ plants were self-pollinated, the $F_2$ had tall and short plants in the ratio $3:1$ ($TT:Tt:tt=1:2:1$): shortness reappeared. So a single copy of $T$ makes a plant tall, while a plant is short only when both copies are $t$. The trait expressed in $F_1$ (tallness) is dominant, and the trait hidden in $F_1$ that reappears in $F_2$ (shortness) is recessive. (ii) Mendel crossed plants that differed in two traits, for example round yellow seeds ($RRYY$) with wrinkled green seeds ($rryy$). The $F_1$ were all round and yellow ($RrYy$). On self-pollination the $F_2$ had round yellow, round green, wrinkled yellow and wrinkled green seeds in the ratio $9:3:3:1$. Round green and wrinkled yellow are new combinations found in neither parent, and each trait by itself still shows $3:1$ (round : wrinkled $=12:4$, yellow : green $=12:4$). So seed shape and seed colour are inherited independently of each other.