CBSE Class 10 Science 2020 · Paper 31/1/1 · Question 28

(a) Law of dominance: when a pair of contrasting forms of a trait is crossed, only one of the two parental traits is expressed in the ${\text{F}}_1$ generation; it is called the dominant trait, and the one that is not expressed is called the recessive trait. Example: Mendel crossed pure tall pea plants (TT) with pure dwarf pea plants (tt). The gametes were T and t, and every ${\text{F}}_1$ plant was Tt and tall. So the factor for tallness (T) is dominant over the factor for dwarfness (t), which is present in the ${\text{F}}_1$ but cannot express itself in the presence of T; the dwarf trait reappears in one-fourth of the ${\text{F}}_2$ plants. (b) Traits acquired during the lifetime of an individual (for example, a tail cut off or muscles built up by exercise) are changes in the somatic (body) cells only; they do not change the DNA of the germ cells (sperm and egg). Only the DNA of the germ cells is passed on to the next generation, so acquired traits are not inherited.