Polydactyly is a genetic disorder in which a person is born with extra fingers or toes. It is caused by a dominant allele. Two parents, each with one dominant allele for polydactyly and one recessive allele for the normal number of digits, are expecting a child. Explain why a child born to these parents may or may not have polydactyly, and describe what a gene mutation is.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Polydactyly is inherited through a dominant allele. A person only needs to inherit one copy of the dominant allele to be affected. The parents in this case each carry one dominant allele and one recessive allele for the number of digits.
Model answer (5 marks)
1. A gene mutation is a change in the sequence of a gene.
2. Each parent can pass on either the dominant allele or the recessive allele to the child.
3. There is a 75 % (three in four) chance the child inherits at least one dominant allele and will have polydactyly.
4. There is a 25 % (one in four) chance the child inherits two recessive alleles and will not have polydactyly.
5. Because polydactyly is dominant, only one copy of the dominant allele is needed for the condition to be expressed.
2. Each parent can pass on either the dominant allele or the recessive allele to the child.
3. There is a 75 % (three in four) chance the child inherits at least one dominant allele and will have polydactyly.
4. There is a 25 % (one in four) chance the child inherits two recessive alleles and will not have polydactyly.
5. Because polydactyly is dominant, only one copy of the dominant allele is needed for the condition to be expressed.
Examiner tips
- Use the exact terms ‘gene mutation’, ‘dominant allele’, ‘recessive allele’, ‘75 %’ and ‘25 %’.
- Show the 3:1 ratio clearly – this is what the mark scheme rewards.
- Explain that the child may or may not have the condition – the key is the probability.
- Keep the answer concise and in student voice.
Mark scheme (5 marks)
- A gene mutation is a change in the sequence (base/DNA sequence) of a gene
- Each parent can pass on either the dominant allele or the recessive allele to the child (gametes carry one allele each)
- There is a 75% / three in four chance the child inherits at least one dominant allele and will have polydactyly
- There is a 25% / one in four chance the child inherits two recessive alleles and will not have polydactyly
- Because polydactyly is dominant, only one copy of the dominant allele is needed for the condition to be expressed / shown
Key terms in this question
gene mutation · dominant allele · recessive allele · polydactyly
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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