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 was the expected phenotypic ratio in any Mendelian monohybrid cross that had changed when \( F_1 \) (Rr) was self-pollinated in this cross?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 3 : 1
In a Mendelian monohybrid cross, one allele is dominant over the other. Selfing F1 (Rr) gives 1 RR : 2 Rr : 1 rr, but RR and Rr look the same. So the expected phenotypic ratio is 3 dominant : 1 recessive. In the dog flower this changed to 1 red : 2 pink : 1 white, because Rr shows a colour of its own.
Option (1) 1 : 1 — Incorrect. It is the ratio of a test cross (Rr × rr), not of a self-pollinated F1.
Option (2) 1 : 2 : 1 — Incorrect. This is the ratio that the dog flower actually showed, and it is also the genotypic ratio of every monohybrid cross. It is not the expected Mendelian phenotypic ratio.
Option (3) 3 : 1 — Correct. Under complete dominance, RR and Rr both show the dominant trait, giving three dominant-looking plants to one recessive.
Option (4) 1 : 3 — Incorrect. It reverses the dominant and recessive classes. The recessive type is always the smaller class in an F2.
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 –
