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IB Biology SL D3.2.5 Dominant and Recessive Alleles Questions

Apply dominant and recessive allele rules to explain heterozygous expression, interpret Punnett grids and infer offspring outcomes in IB Biology SL D3.2.5.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam points

  • Explain that a dominant allele is expressed in a heterozygote, while a recessive allele is expressed when no dominant allele masks it.
  • Construct or interpret an autosomal Punnett grid to calculate genotype and recessive-condition probabilities from two heterozygous parents.
  • Calculate or identify dominant/recessive offspring phenotypes and ratios, including crosses with lethal dominant homozygotes.
  • Infer an unknown genotype or dominance relationship from a test-cross or observed offspring phenotype result.

IB Biology SL D3.2.5 Dominant and Recessive Alleles Questions question 1

[Maximum number: 3]

The most common form of Stargardt's disease is known to be autosomal recessive. Using a Punnett grid, deduce the probability of a child inheriting Stargardt's disease, if both of the parents are carriers of the disease but do not have the disease themselves.

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