What was the reason for Mendel's success?

Define the following terms: $Genes$ and $Alleles$.

What is the probability of a couple having four sons?

Given below are two statements:
Statement $I$: Back cross is the crossing of $F_1$ hybrid with one of the two parents from which they were derived.
Statement $II$: Test cross involves the crossing of $F_1$ hybrid with its homozygous recessive parent.
In light of the above statements,choose the correct answer from the options given below.

In $Pisum$ $sativum$,there are $14$ chromosomes. How many pairs with different chromosomal composition can be prepared?

Identify the contribution of the following scientists: Hugo de Vries,Carl Correns,and Erich von Tschermak.