Sickle Cell Anaemia and Allele Frequency in Africa

This question discusses the sickle cell anaemia allele ($\text{S}$) being maintained at a high frequency in some populations in Africa, where malaria is prevalent. Let's break down the genotypes and their implications:

• The allele for normal haemoglobin is $\text{A}$.
• The allele for sickle cell haemoglobin is $\text{S}$.
• Genotype $\text{AA}$: Individuals have normal haemoglobin. They are susceptible to malaria.
• Genotype $\text{SS}$: Individuals have sickle cell anaemia. This is a severe, often fatal condition.
• Genotype $\text{AS}$: Individuals are carriers of the sickle cell trait. They generally do not suffer from severe sickle cell anaemia, and importantly, they have increased resistance to malaria.

Understanding Heterozygote Advantage

In regions where malaria is common, the fitness (ability to survive and reproduce) of the different genotypes varies significantly:

• Individuals with genotype $\text{AA}$ (normal) are prone to contracting severe malaria, which can reduce their survival.
• Individuals with genotype $\text{SS}$ (sickle cell anaemia) suffer from a debilitating disease which significantly reduces their survival and reproductive success.
• Individuals with genotype $\text{AS}$ (carriers) have the advantage of being resistant to severe forms of malaria. They also do not suffer from the full severity of sickle cell anaemia like $\text{SS}$ individuals. Therefore, in a malaria-prone environment, $\text{AS}$ individuals tend to have higher survival rates than both $\text{AA}$ and $\text{SS}$ individuals.

This situation, where the heterozygote genotype ($\text{AS}$) has a higher fitness than either of the homozygote genotypes ($\text{AA}$ or $\text{SS}$), is known as heterozygote advantage.

Why Allele S Persists

Heterozygote advantage explains why the sickle cell allele ($\text{S}$) is maintained at a relatively high frequency in malaria-endemic regions despite causing sickle cell anaemia in the homozygous ($\text{SS}$) state. The survival benefit conferred by the $\text{AS}$ genotype in resisting malaria outweighs the negative impact of the $\text{S}$ allele when present in the homozygous state ($\text{SS}$), preventing the allele from being eliminated by natural selection.

Evaluating Other Options

• Homozgyote advantage: This would mean either $\text{AA}$ or $\text{SS}$ genotype has the highest fitness. This is not the case in malaria-prone areas, as both homozygotes face significant disadvantages (malaria susceptibility for $\text{AA}$, sickle cell anaemia for $\text{SS}$).
• Dominance: Dominance refers to how one allele masks the effect of another in a heterozygote. While there are dominance relationships at play (e.g., $\text{A}$ is dominant over $\text{S}$ for preventing severe anaemia, but $\text{AS}$ shows incomplete dominance or co-dominance regarding malaria resistance and haemoglobin type), dominance itself doesn't explain the maintenance of a detrimental allele at high frequency; fitness differences do.
• Genetic drift: Genetic drift is random fluctuation in allele frequencies, particularly strong in small populations. While drift can influence allele frequencies, the consistent high frequency of the $\text{S}$ allele across many populations in malaria-prone regions is a pattern driven by strong selection pressure (malaria), not purely random chance.

Thus, the phenomenon described, where the sickle cell allele ($\text{S}$) is maintained at a high frequency due to the carrier state providing resistance to malaria, is a classic example of heterozygote advantage.