Read the following passage carefully.
MONOHYBRID CROSS
The inheritance of flower color in the dog flower (Snapdragon or Antirrhinum sp.) can be understood by crossbreeding between true-breeding red-flowered (RR) and true-breeding white-flowered plants (rr). When they were crossed, \( F_1 \) (Rr) was pink. When \( F_1 \) was self-pollinated, the \( F_2 \) results were Red, Pink and White. Here the genotype ratios were exactly as expected, but the phenotype ratios have been changed. 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).
What will be the phenotypic ratio in \( F_2 \) generation when \( F_1 \) (Rr) was self-pollinated?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 1 : 2 : 1
In the dog flower, R is not completely dominant over r, so the heterozygote Rr shows its own colour, pink. When F1 (Rr) is self-pollinated, each parent gives two kinds of gametes, R and r, in equal numbers.
| Gametes | R | r |
|---|---|---|
| R | RR (red) | Rr (pink) |
| r | Rr (pink) | rr (white) |
So the F2 has 1 RR : 2 Rr : 1 rr. Since every genotype has its own colour, the phenotypic ratio is also 1 red : 2 pink : 1 white.
Option (1) 1 : 1 — Incorrect. A 1 : 1 ratio comes from a test cross (Rr × rr), not from selfing Rr. Here half the plants would be pink and half white.
Option (2) 1 : 2 : 1 — Correct. With incomplete dominance, red, pink and white appear in the ratio 1 : 2 : 1, the same as the genotypic ratio.
Option (3) 3 : 1 — Incorrect. 3 : 1 is the phenotypic ratio when one allele is fully dominant. Then RR and Rr would both look red, which does not happen in the dog flower.
Option (4) 1 : 3 — Incorrect. It turns the dominance ratio upside down. No monohybrid cross gives one dominant-looking plant for every three recessive ones.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Incomplete Dominance –
