$(a)$ What are monohybrid and dihybrid crosses?
$(b)$ How did Mendel prove that tallness is the dominant trait and dwarfness is the recessive trait in a pea plant? Explain with the help of a monohybrid cross.

(N/A) monohybrid cross is a cross between two parents that differ in only one pair of contrasting traits. It results in a phenotypic ratio of $3:1$ in the $F_{2}$ generation. $A$ dihybrid cross is a cross between two parents that differ in two pairs of contrasting traits. It results in a phenotypic ratio of $9:3:3:1$ in the $F_{2}$ generation.
$(b)$ Mendel performed a monohybrid cross by taking a pure-breeding tall pea plant $(TT)$ and a pure-breeding dwarf pea plant $(tt)$.
When these plants were cross-pollinated,all the offspring in the $F_{1}$ generation were found to be tall $(Tt)$.
Since the $F_{1}$ generation only expressed the tall trait despite carrying the gene for dwarfness,Mendel concluded that tallness is the dominant trait and dwarfness is the recessive trait.