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 percentage of progeny in \( F_1 \) generation when RR (red flowered plant) is crossed with rr (white flowered plant) in the above passage?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 100% Rr
Both parents are true-breeding. The red plant RR makes only R gametes, and the white plant rr makes only r gametes. Every F1 plant therefore gets one R and one r, so all the F1 plants are Rr. The passage says the same: “F1 (Rr) was pink”.
Option (2) 75% RR and 25% rr — Incorrect. RR and rr plants can arise only when both parents carry both alleles. A cross of two pure lines cannot give back the parents in the F1.
Option (3) 25% RR, 50% Rr and 25% rr — Incorrect. This is the F2 genotype ratio, obtained when the F1 (Rr) is self-pollinated.
Option (4) 25% RR and 75% rr — Incorrect. No monohybrid cross gives this split. The F1 of RR × rr has no homozygotes at all.
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.