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).
In the above passage, the inheritance is an example of:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Incomplete dominance
In incomplete dominance, the heterozygote shows a phenotype in between the two parents, because one allele does not fully mask the other. Red (RR) × white (rr) gives pink (Rr), and the F2 shows red, pink and white in a 1 : 2 : 1 ratio.
Option (1) Co-dominance — Incorrect. In co-dominance, both alleles show fully and side by side in the heterozygote; there is no blending. The AB blood group, where both the A and B sugars are made, is the NCERT example.
Option (2) Dominance — Incorrect. Under complete dominance, Rr would look the same as RR, i.e. red. Pink F1 plants show that R is not completely dominant.
Option (3) Incomplete dominance — Correct. The pink F1 lies between red and white, the mark of incomplete dominance.
Option (4) Recessive trait — Incorrect. A recessive trait shows only in the homozygote (rr) and stays hidden in Rr. It names one trait, not the kind of inheritance the passage describes.
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 –
