$(a)$ When a pesticide is sprayed on a population of insects,all insects do not get killed but few of them survive. Give reason.
$(b)$ When is a recessive trait capable of expressing itself? Write its expression with respect to the height of a plant (genotype).

(N/A) The insects that survive have developed resistance against the pesticide by generating variations in their $DNA$ in comparison to other insects. The insects that did not have these variations in their $DNA$ could not develop resistance against the pesticide and were thus killed (eliminated).
$(b)$ $A$ recessive trait is capable of expressing itself only when it is in a homozygous recessive state,i.e.,both chromosomes have the same type of gene representing the trait.
For example,in the case of plant height,the recessive trait (dwarfness) is expressed only in the genotype $tt$.