Research updates
7 - The sickling disease page 3
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2. What causes sickle cell anaemia? Link to the Medical Research Council web site


Figure 3. Distribution of sickle cell anaemia and malaria in Africa

The sickle cell trait combats malaria
Around 250 million people suffer from malaria world-wide and around 2.5 million die each year. A severe and often fatal form of the disease - malignant malaria - is caused by the parasite Plasmodium falciparum. The parasite has become adapted to spend part of its life cycle inside human red blood cells, where it multiplies by asexual reproduction.
Questions 3 and 4

3. Refer to Figure 3. What relationship can you detect between the geographical distribution of the sickle cell allele (S) and the geographical distribution of malaria?

4. a) In some regions of Africa, the incidence of sickle cell anaemia may be as high as 1 in 25 births. Use the Hardy-Weinbery equation (p2 + 2pq + q2 = 1) to calculate the frequency of the sickle cell allele (S).

b) In rural Africa, health care is poor and people suffering from sickle cell anaemia are unlikely to live long or produce children. In the light of this, explain the source of the high frequency you calculated for the S allele in part a).

In the early years of life, individuals with sickle cell trait, i.e. heterozygotes, are less susceptible to malignant malaria than people who lack the S allele or who have sickle cell anaemia. Their single S allele affords up to 80% protection against the malarial parasite. This is a case of heterozygote advantage.

Why does a single S allele protect against malaria? Research indicates that the presence of the parasite in red blood celle lowers the oxygen concentration sufficiently to result in sickling of the cells. Consequent damage to the cell membrane allows leakage of potassium ions and the generation of an ion balance in the red cell cytoplasm which is lethal to the parasite.

Much research on abnormal haemoglobins has been carried out by MRC scientist Professor Sir David Weatherall and his team (see Figure 1) at the Institute of Molecular Medicine, part of the John Radcliffe Hospital in Oxford.

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