In an experiment to study independent inheritance of two separate traits: shape and colour of seeds, the ratio of the different combinations in $F_2$ progeny would be

In a dihybrid cross studying two separate traits (such as seed shape: round/wrinkled and seed colour: yellow/green), the $F_2$ generation exhibits a phenotypic ratio of $9:3:3:1$. This represents 9 round-yellow (both dominant), 3 round-green (one dominant, one recessive), 3 wrinkled-yellow (one recessive, one dominant), and 1 wrinkled-green (both recessive) combinations. This ratio is a direct consequence of the independent assortment of the genes controlling these two traits.

• Option A (1:3 / 3:1) is the phenotypic ratio of a monohybrid cross in the $F_2$ generation, not a dihybrid cross.
• Option B (1:2:1) is the genotypic ratio of a monohybrid cross in the $F_2$ generation ($TT:Tt:tt$).
• Option D (9:1:1:3) is an incorrect ratio that does not represent Mendelian independent assortment.