A Sickle-Cell Allele at 22% in a Malaria-Endemic Village
Health workers screen 800 people in a village in a malaria-endemic district of Tamil Nadu by haemoglobin electrophoresis. Results: 480 (60.0%) HbAA, 288 (36.0%) HbAS and 32 (4.0%) HbSS. The district malaria officer reports intense Plasmodium falciparum transmission and notes that HbAS children have fewer severe malaria episodes than HbAA children. In one household both parents are HbAS; their 6-year-old daughter has HbSS and has had three painful crises and one episode of acute chest syndrome. The father's brother is HbAA and asks about the risk to his own children. The couple wants to know the risk for their next child and whether it can be tested before birth. A local practitioner argues that the 22% HbS frequency simply reflects the mutation rate and will fall once the disease is treated with hydroxyurea.
1. Calculate the HbS allele frequency and the Hardy–Weinberg expectations, then say whether the population is in equilibrium and what the deviation suggests.
Read the worked answer
Allele frequencies. Count alleles, not people:
- q (HbS) = frequency of SS + half the frequency of AS = 0.04 + (0.36/2) = 0.22
- p (HbA) = 0.60 + 0.18 = 0.78 (check: 0.78 + 0.22 = 1)
Hardy–Weinberg expectations (N = 800):
| Genotype | Expected frequency | Expected number | Observed |
|---|---|---|---|
| HbAA | p² = 0.6084 | 486.7 | 480 |
| HbAS | 2pq = 0.3432 | 274.6 | 288 |
| HbSS | q² = 0.0484 | 38.7 | 32 |
Interpretation. The deviation is small in absolute terms, but its direction is consistent and biologically meaningful: heterozygotes are in slight excess (288 vs 275) and HbSS homozygotes are in deficit (32 vs 39). A deficit of the lethal homozygote with a surplus of carriers is the signature of selection acting against HbSS, partly offset by heterozygote advantage - not of random mating alone. Strictly, formal testing would need a chi-square with the caveat that the allele frequency was estimated from the same sample; the safe exam answer reports the direction of deviation and explains it rather than over-claiming statistical significance.
2. Explain why the HbS allele remains at 0.22 despite the lethality of HbSS, and predict what would happen to the frequency if malaria were eliminated from the district.
Read the worked answer
Why mutation alone cannot explain it. The β-globin mutation rate is of the order of 10⁻⁸ per gene per generation. A lethal recessive allele removed by selection each generation would reach a low equilibrium frequency determined by mutation–selection balance, far below 0.22. Something must be maintaining the allele.
Balancing selection. In this village:
- HbSS - severe anaemia, vaso-occlusion, early death, low fitness.
- HbAA - normal haemoglobin but susceptible to severe falciparum malaria.
- HbAS - mild sickling trait; the parasite's growth in these cells is impaired and infected cells are cleared efficiently, so carriers survive malaria better than HbAA.
Because the heterozygote has the highest fitness, both alleles are retained; this is heterozygote advantage (balanced polymorphism), the classic example of stabilising selection in human populations. The high frequency in a malaria-endemic region is historical evidence of natural selection, not a high mutation rate, and it is not altered by treating patients with hydroxyurea - treating illness does not change allele frequencies in the gene pool.
Prediction if malaria is eliminated. The advantage of HbAS disappears, while the disadvantage of HbSS remains. Selection will then act against HbS on both counts, and q will decline gradually over many generations - slowly, because the allele persists hidden in carriers. It will not fall in a single generation, and it will not fall to zero while mutation continues.
3. Give the recurrence risk for the couple's next child and outline genetic counselling, including the prenatal test you would offer and the molecular basis of the test.
Read the worked answer
Risk. Both parents are HbAS (carriers). Each pregnancy: 25% HbSS (affected), 50% HbAS (carrier, healthy), 25% HbAA (unaffected non-carrier). The risk is independent for each pregnancy - the earlier affected child does not change the 1 in 4 figure. The paternal uncle is HbAA, so his children cannot be affected; they can only be carriers if his wife carries HbS, so his wife should be screened before he is reassured about risks to his own children.
Counselling content.
- Explain the disease, the crises, the need for hydration, folic acid, penicillin prophylaxis and vaccination, and that hydroxyurea reduces crises.
- Emphasise that carriers are healthy and that AS status confers malaria protection - this reduces stigma and the risk of blaming the carrier parent.
- Screen siblings and offer testing to relatives; offer carrier testing to the uncle's wife.
Prenatal diagnosis. Chorionic villus sampling at about 11–13 weeks or amniocentesis at 15–18 weeks, with a decision made before the procedure about what the couple would do with the result.
Molecular basis of the test. The mutation is a single base substitution in codon 6 of the β-globin gene: GAG → GTG, glutamic acid → valine. This abolishes the MstII/DdeI recognition site, so PCR of the β-globin region followed by restriction digestion gives a larger fragment for HbS than for HbA, allowing the three genotypes to be distinguished by gel electrophoresis; ARMS-PCR with allele-specific primers or direct Sanger sequencing are alternatives.