AQA · GCSE · Biology · Higher

Recessive allele population probability

Q06.4 4 marks

Cystic fibrosis (CF) is another inherited disorder caused by a mutation. The mutation occurs in a gene called CFTR. For the CFTR gene, one allele in every 50 in the UK population is the cystic fibrosis allele. Explain why only one person in 2500 in the UK population has cystic fibrosis.

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Marking points

1 States that the cystic fibrosis allele is recessive.
2 Explains that two CF alleles are required for the condition.
3 Uses 1/50 as the chance of inheriting each CF allele.
4 Multiplies the independent probabilities to obtain 1/2500.

Full-mark answer

The cystic fibrosis allele is recessive, so a person must inherit two copies to have CF. The chance of each CF allele is 1/50, so the chance of two is $\frac{1}{50} \times \frac{1}{50} = \frac{1}{2500}$ .

Why this answer loses marks

Multiply the population figures because that gives the disorder frequency.

Link the recessive phenotype to needing two copies, then multiply the independent allele probabilities.

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