IB Biology SL 3.2.5 Dominant and recessive alleles Question Bank
Practise IB Biology SL 3.2.5 by solving inheritance crosses and predicting dominant or recessive phenotypes.
- Syllabus
- First assessment 2025
- Course
- Biology SL
- Level
- SL
Exam points
- Use Punnett grids to predict genotype and phenotype ratios for dominant or recessive alleles.
- Explain how recessive alleles can be carried without expression and appear in offspring.
D3.2.5—Dominant and recessive alleles 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.
The probability of a child inheriting Stargardt's disease is 1/4, or 25%. The phenotypic outcome is 3 normal : 1 with disease.
Marking guidance: award for correct allele identification, Punnett grid using heterozygous parent gametes, and correct phenotypic ratio/outcome.
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