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IB Biology SL D3.2.10 Incomplete Dominance and Codominance Questions

Distinguish codominant expression from an intermediate incomplete-dominance phenotype, then predict offspring ratios in IB Biology SL D3.2.10.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam points

  • Explain codominance as expression of both alleles in a heterozygote, as in AB blood type, and distinguish it from the intermediate heterozygote phenotype in incomplete dominance.
  • Predict offspring phenotypes and ratios in incomplete-dominance crosses, including the intermediate heterozygote, as in red, pink and white flowers.
  • Use codominant crosses, such as speckled chicken feathers, to identify heterozygote expression and predict phenotypic ratios.

IB Biology SL D3.2.10 Incomplete Dominance and Codominance Questions question 1

[Maximum number: 1]

A Mirabilis jalapa plant with red flowers was crossed with one with white flowers. All plants in the F1 generation had pink flowers. What phenotype ratio would be expected in the F2 generation?

A

100 % pink

B

50 % red and 50 % white

C

25 % white, 50 % pink and 25 % red

D

75 % red and 25 % white

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