How many pairs of contrasting traits were selected by Mendel for his study on garden peas?

What will a hybrid plant resulting from two alternative alleles be called?

State the reasons for Mendel's success.

Assertion: The genetic complement of an organism is called genotype.
Reason: Genotype is the type of hereditary properties of an organism.

The cause of variation is hidden in the process of $.....P.....$ - this knowledge was acquired by humans in $.....Q.....$.
$\quad \quad P \quad\quad\quad Q$

How many pairs of true-breeding pea plant varieties were selected by Mendel,which were identical except for one character with contrasting traits?