Mendel crossed pea plants having round green seeds $(RRyy)$ with pea plants having wrinkled yellow seeds $(rrYY)$. (Image $(i)$)

Since the $F_1$ plants are formed after crossing pea plants having green round seeds and pea plants having yellow wrinkled seeds, $F_1$ generation will have both these characters in them. However, as we know that yellow seed colour and round seeds are dominant characters, therefore, the $F_1$ plants will have yellow round seeds.

Then this $F_1$ progeny was self-pollinated and the $F_2$ progeny was found to have yellow round seeds, green round seeds, yellow wrinkled seeds, and green wrinkled seeds in the ratio of $9:3:3:1.$

In the above cross, more than two factors are involved, and these are independently inherited. (Image $(ii)$)