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IB Biology HL D3.2.10 Incomplete Dominance and Codominance Questions

Explain how both alleles are expressed in a codominant heterozygote, or how incomplete dominance produces an intermediate phenotype, then predict offspring phenotypes from crosses in IB Biology HL D3.2.10.

Syllabus
First assessment 2025
Course
Biology HL
Level
HL

Exam points

  • Explain codominance as the expression of both alleles in a heterozygote and incomplete dominance as an intermediate heterozygote phenotype.
  • Use an incomplete-dominance cross to predict offspring phenotypes and ratios, including the intermediate phenotype.
  • Predict phenotype ratios from a codominant cross in which the heterozygote expresses both allele-associated traits.

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

[Maximum number: 1]

A variety of Pelargonium has yellow leaves. When plants of this variety are crossed, the resulting seeds produce green, yellow and white seedlings in the ratio 1: 2: 1. If plants with yellow leaves are crossed with plants with green leaves, what would the expected ratio of phenotypes in the offspring be?

Green

Yellow

White

1

2

1

3

1

0

2

2

0

2

1

1

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