The frequency of certain genes in a population changes over generations. This is the essence of the idea of evolution.

There is a sub-population $A$ and another sub-population $B$. Both are separated by a natural barrier. What will happen in such a case?

$(a)$ State any two factors that could lead to the rise of a new species.
$(b)$ How do analogous organs provide evidence in favour of evolution?

What is the $23^{rd}$ pair of chromosomes in humans called?

In humans, there are two pairs of sex chromosomes.

Mendel crossed a pea plant having inflated green pod with a constricted yellow pod.
$(a)$ What type of a cross is it?
$(b)$ What type of plants are obtained in $F_{1}$ generation?
$(c)$ In $F_{2}$ generation the phenotype ratio is $9:3:3:1$. State the rule for the inheritance of traits observed by Mendel.