Why do all the gametes formed in human females have an $X$ chromosome?

What are the basic features of the mechanism of inheritance?

How are chromosomes transmitted to the offspring?

Why is it that asexual reproduction produces exact copies, but sometimes minor variations are also seen in the next progeny?

$(a)$ 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).

$A$ guinea pig having black color,when crossed with a guinea pig having the same color,produced $80$ offspring,out of which $60$ were black and $20$ were white. Now,find out:
$(i)$ What is the possible genotype of the guinea pig?
$(ii)$ Which trait is dominant and which trait is recessive?
$(iii)$ What is this cross called,and what is its phenotypic ratio?