How many true-breeding pea plant varieties did Mendel select as pairs,which were similar except in one character with contrasting traits?

What is represented by the following cross?
$Tt \times tt$

Mendel studied several traits in pea plants. Which of the following combinations represents the traits he studied?

$A$ gene that expresses itself in a heterozygous condition and shows its characteristic is called a...

The synthesis of starch in pea seeds is controlled by a single gene. This gene has two alleles,$B$ and $b$. If the shape of the pea seed is considered as the phenotype,what type of inheritance does this represent?

What is the cross between the progeny of $F_{1}$ and the homozygous recessive parent called? How is it useful?