CBSE Class 10 Science 2019 · Paper 31/4/1 · Question 21

(a) Two visible traits: height of the plant (tall or dwarf) and shape of the seed (round or wrinkled); others are seed colour (yellow or green) and flower colour (violet or white). Mendel crossed pure tall plants (TT) with pure dwarf plants (tt). All the ${\text{F}}_1$ plants were tall (Tt), although each carried a factor for dwarfness; on self-pollination, the ${\text{F}}_2$ had tall and dwarf plants in the ratio $3:1$. The trait expressed in ${\text{F}}_1$ (tallness) is dominant; the trait that remained hidden in ${\text{F}}_1$ and reappeared in ${\text{F}}_2$ (dwarfness) is recessive. A single copy of T makes a plant tall (TT and Tt are tall), while only tt plants are dwarf. (b) Flow diagram: Father XY $\times$ Mother XX $\to$ gametes: sperms X or Y (in equal numbers), eggs X only $\to$ egg X + sperm X gives XX (girl); egg X + sperm Y gives XY (boy). The mother gives only X, so she cannot decide the sex; whether an X-sperm or a Y-sperm fuses with the egg is purely a matter of chance, so a girl and a boy are equally likely ($1:1$) and neither parent is responsible for the sex of the newborn.