Inherited disordersSpec 4.6.1.7
In short
Inherited disorders are caused by the inheritance of certain alleles. Polydactyly, having extra fingers or toes, is caused by a dominant allele, so one copy is enough to have it. Cystic fibrosis, a disorder of cell membranes, is caused by a recessive allele, so two carrier parents have a 1 in 4 chance of a child with the disorder.
Some disorders are inherited. They are caused by the inheritance of certain alleles.
- Polydactyly (having extra fingers or toes) is caused by a dominant allele.
- Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
Polydactyly
Polydactyly is caused by a dominant allele (D). One parent has polydactyly and is Dd. The other parent does not have polydactyly. What is the chance that a child has polydactyly?
- A person without polydactyly must be dd (a dominant allele is always expressed).
- Gametes: D and d from one parent, d and d from the other.
- Offspring: Dd, Dd, dd, dd.
- Two of the four have a D allele, so two of the four have polydactyly.
Answer: The ratio of polydactyly : no polydactyly is 1 : 1, so the chance of a child having polydactyly is 1 in 2 (50%).
Cystic fibrosis
Cystic fibrosis is caused by a recessive allele (f). Two parents do not have cystic fibrosis but both are carriers (Ff). What is the chance that a child has cystic fibrosis?
- Gametes from each parent: F and f.
- Offspring: FF, Ff, Ff, ff.
- Only ff has two recessive alleles, so only ff has cystic fibrosis.
Answer: 1 in 4 (25%). The ratio of unaffected to affected is 3 : 1.
Embryo screening and gene therapy
Embryos can be screened for faulty alleles before they are implanted, so that only embryos without the disorder are used. Modern medical research is also exploring gene therapy, which is genetic modification to overcome some inherited disorders. Both may alleviate suffering, but they raise ethical issues.
You may be asked to make an informed judgement about the economic, social and ethical issues, using the information given.
| For | Against | |
|---|---|---|
| Ethical | Prevents suffering for the child and family | Embryos that would have the disorder are destroyed, which some people believe is wrong |
| Social | Fewer children born with serious disorders | May lead to pressure to choose other characteristics; people living with the disorder may feel devalued |
| Economic | May reduce the long-term cost of treatment and care | The screening itself is expensive and is not available to everyone |
In 'evaluate' or 'discuss' questions, give points for both sides and finish with a judgement that uses the information given in the question.
Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026