Discuss the seven pairs of contrasting traits in pea plants studied by Mendel.

(A) Mendel conducted experiments on pea plants using true-breeding lines.
True-breeding lines are those that,having undergone continuous self-pollination,show the stable trait inheritance and expression for several generations.
Mendel selected $14$ true-breeding pea plant varieties,as pairs which were similar except for one character with contrasting traits.
The seven pairs of contrasting traits studied by Mendel are:
$1$. Stem height: Tall $(T)$ / Dwarf $(t)$
$2$. Flower colour: Violet $(V)$ / White $(v)$
$3$. Flower position: Axial $(A)$ / Terminal $(a)$
$4$. Pod shape: Inflated $(I)$ / Constricted $(i)$
$5$. Pod colour: Green $(G)$ / Yellow $(g)$
$6$. Seed shape: Round $(R)$ / Wrinkled $(r)$
$7$. Seed colour: Yellow $(Y)$ / Green $(y)$