A pure pea plant having round (R), yellow (Y) seeds is crossed with another pure pea plant having wrinkled (r), green (y) seeds. Subsequently $F_1$ progeny is self-pollinated to obtain $F_2$ progeny.

(a) What do the seeds of $F_1$ generation look like?

(b) Give the possible combinations of traits in seeds of $F_2$ generation. Also give their ratio.

(c) State the reason of obtaining seeds of new combination of traits in $F_2$ generation.

(a) The $F_1$ generation seeds are round and yellow. Since round (R) and yellow (Y) are dominant traits (as established by Mendel's pea experiments), the cross RRYY (round, yellow) $\times$ rryy (wrinkled, green) produces $F_1$ progeny with genotype RrYy, and only the dominant traits — round shape and yellow colour — are expressed, even though both dominant and recessive factors are present.

(b) When the $F_1$ (RrYy) plants are self-pollinated, each parent produces four types of gametes — RY, Ry, rY, ry — in equal proportion, because the factors for seed shape and seed colour assort independently of each other. Combining these gametes gives the $F_2$ seeds in the following combinations and ratio:

So the $F_2$ seeds show four combinations of traits — Round Yellow : Round Green : Wrinkled Yellow : Wrinkled Green — in the ratio 9 : 3 : 3 : 1.

(c) The new combinations seen in $F_2$ (Round Green and Wrinkled Yellow, which were not present in either parent) arise because the factors controlling seed shape (R/r) and seed colour (Y/y) are inherited independently of each other. Each gene pair separates and assorts into the gametes independently of the other gene pair, so during self-pollination the R/r and Y/y factors can recombine in new ways in the zygote — as the NCERT chapter states, 'new combinations of traits are formed in $F_2$ offspring when factors controlling for seed shape and seed colour recombine to form zygote leading to form $F_2$ offspring.' This independent assortment of the two traits is why $F_2$ shows combinations different from both parental types.