What would be the genotypic ratio of red, pink and white flowers in F2 generation involving monohybrid cross between red and white flower coloured plant snapdragon?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 1 Red: 2 Pink: 1 White
Flower colour in snapdragon shows incomplete dominance. The allele for red (R) is not completely dominant over the allele for white (r). So the heterozygote Rr is pink, a colour in between the two parents.
Parents: RR (red) × rr (white) → F1: Rr (pink)
F1 selfed: Rr × Rr
| R | r | |
|---|---|---|
| R | RR (red) | Rr (pink) |
| r | Rr (pink) | rr (white) |
F2 genotypes: 1 RR : 2 Rr : 1 rr, which appear as 1 red : 2 pink : 1 white. The genotypic ratio is the usual 1:2:1 of a monohybrid cross. Here the phenotypic ratio is also 1:2:1, instead of 3:1, because each genotype has its own colour.
(1) 1 Red: 2 Pink: 1 White — Correct. This is the ratio worked out above.
(2) 1 Pink: 2 Red: 1 White — Incorrect. Pink (Rr) is the class that appears twice, not red.
(3) 1 White: 2 Red: 1 Pink — Incorrect. Red and white are one part each; pink is two parts.
(4) 1 Pink: 2 Red: 1 blue — Incorrect. No blue flower comes from this cross.
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. Here the genotype ratios were exactly as we would expect in any mendelian monohybrid cross, but the phenotype ratios had changed from the 3:1 dominant : recessive ratio.”
