# A couple are both heterozygous for the allele that causes cystic fibrosis. The allele for cystic fibrosis is recessive. Explain why there is a 75% probability that their child will not have cystic fibrosis.

> Edexcel GCSE Biology (1BI0) — 3.4 Inheritance · Explain · 4 marks

> Cystic fibrosis is an inherited disorder caused by a recessive allele. Use the letter F to represent the dominant allele and f to represent the recessive allele.

## Mark scheme (4 marks)

1. Both parents have the genotype Ff (heterozygous)
2. Correct identification of all four possible offspring genotypes from the cross: FF, Ff, Ff, ff
3. Cystic fibrosis only occurs in offspring with the genotype ff (homozygous recessive)
4. Three out of four possible genotypes contain at least one dominant allele (FF or Ff), so 75% of offspring will not have cystic fibrosis

## Key terms

- [heterozygous](https://www.gradenine.co.uk/glossary/heterozygous)
- [recessive](https://www.gradenine.co.uk/glossary/recessive)
- [allele](https://www.gradenine.co.uk/glossary/allele)

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-couple-are-both-heterozygous-for-90d9c3f9) · Published by Druglandscape Ltd.