Cystic fibrosis is a genetic disorder caused by a gene mutation. Explain why two parents who do not have cystic fibrosis can have a child with the condition.

Edexcel A-Level Biology A: Salters-Nuffield (9BN0) — 1.5 Gene mutations and genetic disorders · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Cystic fibrosis is caused by a recessive allele. A person must inherit two copies of this allele to be affected by the disorder.

Model answer (5 marks)

Cystic fibrosis is caused by a recessive allele. Both parents are carriers, each having one recessive allele and one normal dominant allele. The normal allele masks the recessive allele, so the parents show no symptoms. During fertilisation each parent can pass either allele to the zygote. If the child inherits the recessive allele from both parents, it will have two copies of the mutant gene and will develop cystic fibrosis.

Examiner tips

Use the word ‘recessive’ and ‘carrier’ early to show understanding of genetics.
Explain the masking effect of the dominant allele to justify why parents are unaffected.
Show the 1/4 probability of a child inheriting two recessive alleles (1/2 from each parent).
Mention that the child will have cystic fibrosis if it receives two recessive copies.

Common mistakes

Confusing dominant with recessive or saying parents have the disease.
Failing to explain why carriers are symptom‑free.
Not recognising that each parent can pass either allele, leading to a 25% risk.

Mark scheme (5 marks)

The allele that causes cystic fibrosis is recessive
Each parent carries one copy of the recessive allele / parents are carriers
The parents also carry one dominant (normal) allele, which masks the recessive allele so they do not show symptoms
Each parent can pass on either their dominant or their recessive allele to their offspring / there is a chance the child inherits the recessive allele from each parent
If the child inherits two copies of the recessive allele (one from each parent), they will have cystic fibrosis

Key terms in this question

gene mutation · cystic fibrosis