In snapdragon, a true-breeding red-flowered plant (RR) is crossed with a true-breeding white-flowered plant (rr). The pink-flowered F1 plants are self-pollinated. Which of the following correctly describes the F2 generation?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Red, pink and white-flowered plants appear in the ratio 1 : 2 : 1, the same as the genotypic ratio.
In snapdragon, R (red) is not completely dominant over r (white), so the heterozygote Rr is pink. When the pink F1 (Rr) plants are self-pollinated, the genotypes come out as in any monohybrid cross: 1 RR : 2 Rr : 1 rr. Because each genotype has its own colour, the phenotypes follow the same ratio.
| R | r | |
|---|---|---|
| R | RR (red) | Rr (pink) |
| r | Rr (pink) | rr (white) |
Option (1) The F2 plants are pink-flowered, like the F1 plants. — Incorrect. Only the F1 is all pink. In the F2 the alleles segregate again, and red (RR) and white (rr) plants reappear.
Option (2) Red-flowered and white-flowered plants appear in the ratio 3 : 1, as in a mendelian monohybrid cross. — Incorrect. A 3 : 1 ratio appears only with complete dominance, where Rr would look like RR. Here Rr is pink, so it cannot be counted with the red plants.
Option (3) Red, pink and white-flowered plants appear in the ratio 1 : 2 : 1, the same as the genotypic ratio. — Correct. Red, pink and white plants appear in the ratio 1 : 2 : 1, the same as the genotypic ratio RR : Rr : rr.
Option (4) Pink-flowered and white-flowered plants appear in the ratio 3 : 1, as R is not completely dominant over r. — Incorrect. R is indeed not completely dominant over r, but that is why red, pink and white are all seen. Red plants (RR) also appear, so the ratio is not pink : white = 3 : 1.
NCERT Extract:
Incomplete Dominance – “… In a cross between true-breeding red-flowered (RR) and true-breeding 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. What happened was that R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).”
Figure 4.6 Results of monohybrid cross in the plant Snapdragon, where one allele is incompletely dominant over the other allele