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IB DP Biology- D3.2 Inheritance- IB Style Questions For HL Paper 2 -FA 2025

IBDP Biology HL Paper 2 -FA 2025- All Chapters
Question

Bateson and Punnett performed a cross between two white-flowered sweet pea plants (Lathyrus odoratus). Although both parent varieties had white flowers, all F₁ offspring produced purple flowers. When these F₁ plants were self-fertilized, the resulting F₂ offspring showed a 9:7 ratio of purple to white flowers. From these results, Bateson and Punnett inferred that the genotypes of the two white-flowered parental strains were CCrr and ccRR.

a. Identify the genotype of the F₁ hybrids. [1]

b. Describe how the 9:7 phenotypic ratio is produced in the F₂ generation. [3]

c. Predict the phenotypic ratio in the offspring of a cross between Ccrr × ccRr plants. [1]

▶️ Answer/Explanation
Detailed Solution
a.
  • Answer: CcRr
  • Explanation:

    The parent plants CCrr and ccRR produce gametes Cr and cR. Their combination results in F₁ individuals with genotype CcRr. Both dominant alleles (C and R) are necessary for purple pigmentation, so the F₁ generation appears purple.

b.
  • Key Points:
    1. Interaction of genes: Two independent genes (C and R) must both carry a dominant allele for purple flowers to be produced (genotype C_R_).
    2. F₂ outcomes: A dihybrid CcRr × CcRr cross produces:
      • 9/16 C_R_ (purple)
      • 3/16 C_rr (white)
      • 3/16 ccR_ (white)
      • 1/16 ccrr (white)
    3. Why 9:7? All three genotypes lacking at least one dominant allele (3 + 3 + 1) result in white flowers, collapsing the usual 9:3:3:1 ratio into 9 purple : 7 white.
  • Punnett summary: Gametes CR, Cr, cR, cr combine to yield a 9 purple : 7 white ratio.
c.
  • Answer: 1 purple : 3 white
  • Explanation:

    Gametes produced:

    • Ccrr → Cr, cr
    • ccRr → cR, cr

    Possible offspring:

    1 CcRr (purple)
    1 Ccrr (white)
    1 ccRr (white)
    1 ccrr (white)

    GenotypePhenotypeRatio
    CcRrPurple1
    OthersWhite3
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