In a cross, when genotypic ratio and phenotypic ratio in F_2 generati…

The correct answer is Option (3) → Incomplete dominance

Option (1) Dominance — Incorrect. With dominance the F₂ phenotypic ratio is 3 : 1 but the genotypic ratio is 1 : 2 : 1.

Option (2) Co-dominance — Incorrect. Co-dominance also gives equal ratios, but the classic example in NCERT of equal ratios in the F₂ is incomplete dominance.

Option (3) Incomplete dominance — Correct. Red : pink : white = 1 : 2 : 1, the same as RR : Rr : rr.

Option (4) Polygenic inheritance — Incorrect. In polygenic inheritance a trait is controlled by three or more genes, as in human skin colour, and the F₂ shows a whole range of phenotypes. It does not give equal genotypic and phenotypic ratios for a single gene.

NCERT Extract:
Incomplete Dominance – “… In a cross between true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr), the F1 (Rr) was pink (Figure 4.6). When the F1 was self-pollinated the F2 resulted in the following ratio 1 (RR) Red : 2 (Rr) Pink : 1 (rr) White.”