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CUET · BIOLOGY · PYQ PAPER 2025

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, F₁ (Rr) was pink. When F₁ was self-pollinated, the F₂ 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 F₁ (Rr) was self-pollinated in this cross?

  1. A 1 : 1
  2. B 1 : 2 : 1
  3. C 3 : 1
  4. D 1 : 3
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(C) 3 : 1

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3 : 1
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