Sickle cell disease is a genetic disorder caused by a recessive gene mutation. A person with sickle cell disease has red blood cells that are an abnormal shape. Explain how a gene mutation can lead to the production of abnormal red blood cells, and explain why two parents who are carriers of sickle cell disease have a 25% chance of having a child with sickle cell disease.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sickle cell disease is caused by a mutation in the gene that codes for haemoglobin, the protein found in red blood cells. Carriers of sickle cell disease have one copy of the mutant allele and one copy of the normal allele.
Model answer (5 marks)
A gene mutation is a change in the DNA base sequence of a gene.
The altered gene codes for a different haemoglobin protein, which causes the red blood cells to take on a sickle shape.
Carriers have one recessive mutant allele and one dominant normal allele, so they do not show the disease.
Each parent can pass either the normal or the mutant allele to the child.
There is a 1 in 4 (25 %) chance that the child inherits the mutant allele from both parents, giving two copies of the mutant allele and sickle cell disease.
The altered gene codes for a different haemoglobin protein, which causes the red blood cells to take on a sickle shape.
Carriers have one recessive mutant allele and one dominant normal allele, so they do not show the disease.
Each parent can pass either the normal or the mutant allele to the child.
There is a 1 in 4 (25 %) chance that the child inherits the mutant allele from both parents, giving two copies of the mutant allele and sickle cell disease.
Examiner tips
- Use the exact terms ‘gene mutation’, ‘haemoglobin’, ‘recessive allele’, ‘dominant allele’, ‘25 % chance’
- Show the 1 in 4 probability calculation or state the Punnett square logic
- Explain both the protein change and the inheritance pattern
Common mistakes
- Confusing dominant with recessive terminology
- Failing to mention that carriers are asymptomatic
- Not linking the mutation to the abnormal haemoglobin shape
Mark scheme (5 marks)
- A gene mutation is a change in the DNA / base sequence of a gene
- The changed gene codes for a different / abnormal protein (haemoglobin), so the red blood cells have an abnormal shape
- Carriers have one recessive allele and one dominant (normal) allele, so they do not have the disease
- Each parent can pass on either the normal or the mutant allele to the child
- There is a 1 in 4 / 25% chance the child inherits the recessive allele from both parents, so the child has two copies of the mutant allele and has sickle cell disease
Key terms in this question
gene mutation · recessive · carrier
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →