A gene, a parasite and a cure
365 words over 34 numbered lines, then 6 parts worth 13 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice.
A gene, a parasite and a cure
Sickle cell disease is one of the most common inherited disorders in
the world. It is caused by a mutation in the gene for haemoglobin, the
protein that carries oxygen in red blood cells. The mutation changes a
single base in the DNA, which changes one amino acid in the protein.
About 300 000 babies are born with the disease every year, most of
them in Africa.
When the faulty haemoglobin gives up its oxygen, its molecules stick
together into long fibres. These pull the red blood cells into a
curved, stiff sickle shape. Sickle cells cannot squeeze easily through
capillaries and can block them, causing severe pain. They also break
down quickly, so people with the disease often have too few red blood
cells.
A person only has the disease if they inherit two copies of the sickle
cell allele, one from each parent. People with one copy are called
carriers. They usually have no symptoms, but they can pass the allele
on to their children. When two carriers have a child, there is a
chance that the child will have the disease.
The sickle cell allele is most common in parts of Africa, the Middle
East and India where malaria is, or used to be, widespread. Carriers
are much less likely to die from malaria than people without the
allele. In an illustrative survey of one village, 180 of the 600
adults tested were carriers.
Until recently the only cure was a bone marrow transplant from a
closely matched donor, which few patients could have. In 2023, a new
treatment using gene editing was approved. Stem cells are taken from
the patient’s own bone marrow and edited in a laboratory so that they
make a form of haemoglobin that does not sickle. The edited cells are
then put back into the patient.
The treatment can stop the painful attacks, but it costs more than a
million dollars per patient. Doctors hope that cheaper methods will be
found for the countries where the disease is most common. Meanwhile,
simple measures such as screening newborn babies, giving antibiotics
to prevent infections and teaching families to recognise early warning
signs have greatly reduced deaths in childhood.
Read the article. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term mutation.
(1)Explain why people with sickle cell disease often feel tired and short of breath.
(2)Use a genetic diagram to explain the probability that a child of two carriers will have sickle cell disease. Use H for the normal allele and h for the sickle cell allele.
(3)Calculate the percentage of the adults tested in the village who were carriers.
(2)Explain why the sickle cell allele is common in areas where malaria is widespread.
(3)Suggest why the stem cells are taken from the patient’s own bone marrow rather than from a donor.
(2)Total for this question: 13 marks
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