Inherited Disorders
Revise Inherited Disorders for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Polydactyly is caused by a dominant allele and cystic fibrosis by a recessive allele; practise working out the probability of inheriting each.
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Topic overview
Some disorders are inherited because they are caused by the inheritance of certain alleles. Two must be known.
Polydactyly, which causes extra fingers or toes, is caused by a dominant allele. Because it is dominant, it can be inherited from just one parent, and an affected person has at least one affected parent.
Cystic fibrosis is a disorder of cell membranes caused by a recessive allele. Because it is recessive, a child must inherit the allele from both parents. Parents may each be carriers — heterozygous, showing no symptoms themselves — which is why the disorder can appear unexpectedly.
Revision notes
Polydactyly
Caused by a dominant allele, so only one copy is needed for the condition to be shown.
It can therefore be inherited from just one parent, and an affected child will normally have at least one affected parent. It causes extra fingers or toes.
Cystic fibrosis
A disorder of cell membranes caused by a recessive allele.
Both parents must pass on the allele for a child to be affected. Parents who are heterozygous carry the allele without showing symptoms — they are carriers.
Embryo screening
Embryos can be screened for faulty alleles during IVF, and only unaffected embryos implanted.
Arguments for: it reduces suffering and treating disorders is expensive. Arguments against: it may lead to selecting other characteristics, screening is expensive, and some object that the embryos have a right to life.
Key points
- Polydactyly is caused by a dominant allele.
- Polydactyly causes extra fingers or toes.
- Cystic fibrosis is caused by a recessive allele.
- Cystic fibrosis affects cell membranes.
- A carrier is heterozygous and shows no symptoms.
- Embryo screening raises ethical concerns.
Worked examples
Example 1
Explain why a child can have cystic fibrosis when neither parent has the disorder. [3 marks]
Model answer
Cystic fibrosis is caused by a recessive allele (1 mark). Both parents can be carriers, meaning they are heterozygous and have one recessive allele each without showing symptoms (1 mark). If the child inherits the recessive allele from both parents it will have two recessive alleles and therefore the disorder (1 mark).
Example 2
Explain why polydactyly can be inherited from only one parent. [2 marks]
Model answer
Polydactyly is caused by a dominant allele (1 mark), so only one copy is needed for the condition to be expressed (1 mark).
Example 3
Give one argument for and one against embryo screening. [2 marks]
Model answer
For: it reduces the number of people suffering from inherited disorders, and treating them is expensive (1 mark). Against: it could lead to embryos being selected for other characteristics, or people may object that the embryos have a right to life (1 mark).
Common mistakes
Saying a carrier has the disorder.
A carrier is heterozygous and shows no symptoms, but can pass the allele on.
Mixing up which disorder is dominant.
Polydactyly is dominant; cystic fibrosis is recessive.
Giving only one side in an ethics question.
Both an argument for and against are usually required.
Saying both parents must have polydactyly.
Only one is needed, because the allele is dominant.
Exam tips
- Learn which disorder is dominant and which is recessive.
- Use the word carrier precisely — heterozygous with no symptoms.
- Give both sides when discussing embryo screening.
- Draw a Punnett square if a question asks about probabilities.
Key terms
- Polydactyly
- An inherited disorder causing extra fingers or toes, caused by a dominant allele.
- Cystic fibrosis
- A disorder of cell membranes caused by a recessive allele.
- Carrier
- A heterozygous individual who has a recessive allele but no symptoms.
- Embryo screening
- Testing embryos for faulty alleles before implantation.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.