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

Which one of the following was the rediscoverer of Mendel's work?

To determine whether $F_1$ hybrid is homozygous or heterozygous for a particular trait,a . . . . . . cross is performed.

How many types of gametes can be produced by an organism with the genotype $AABBCC$?

How many pairs of contrasting characters in pea plants were chosen by Mendel?

$A$ disease is caused by a dominant gene,but it shows only $20\%$ penetrance. $A$ heterozygous man marries a homozygous normal woman. What proportion of the children in the population is expected to have the disease?