Impact of selection on blood antigens
Natural selection exerts a direct influence on the frequency of alleles controlling blood antigens in the human population. This effect can be traced through two main systems:
- Rhesus factor: A statistical gradual decrease in the prevalence of individuals with negative Rhesus factor is observed.
- ABO system: A reliable decrease in the number of children is recorded in marriages characterized by immunological incompatibility between parents. The primary risk group includes families where the woman has blood type 0 (I) and the man has type A (II), B (III), or AB (IV).
The paradox of lethal allele preservation
Under certain specific conditions, natural selection is capable of maintaining the existence of lethal alleles in a population. The most famous example of this mechanism is the inheritance of abnormal hemoglobin S, which causes sickle cell anemia.
The fate of mutation carriers depends crucially on their genotype:
- Homozygotes for the recessive gene: Suffer from an extremely severe form of anemia. As a rule, they die at the earliest stages of development (before reaching 2 years of age).
- Heterozygotes: Possess a marked selective advantage in natural malaria foci. Due to this advantage, the frequency of this allele is extremely high among Black populations in Africa, reaching 20%.
Mechanism of malaria resistance
The selective advantage of heterozygotes is based on a specific cellular defense mechanism against the malaria pathogen — Plasmodium falciparum.
The defense process proceeds in several stages:
- The pathogen penetrates inside the erythrocyte of a heterozygous individual.
- In response to the parasite's invasion, the erythrocyte physically changes its normal shape.
- The blood passes through the spleen, which recognizes, filters, and completely destroys only the altered (infected) erythrocytes along with the plasmodium inside.
Conclusion: Heterozygous carriers survive in malaria-endemic areas much more often than homozygotes for the dominant allele (i.e., completely healthy individuals who, however, have no resistance to malaria infection).
Environmental changes and selection vector
When environmental conditions change, the direction of natural selection also transforms. This is clearly demonstrated by the statistics of the African-American population in the USA.
For African-Americans, malaria is no longer a selection factor, since this disease is rare in the USA. Consequently:
- Direction of selection: Since heterozygotes lose their selective advantage, selection becomes directed solely at eliminating the recessive lethal allele.
- Statistics: Today, only 1 in 375 African-Americans suffers from sickle cell anemia, which is significantly lower than the rates in Central Africa. In general, in human populations living outside malaria-endemic areas, the allele frequencies of any abnormal hemoglobins typically do not exceed 1%.
Briefly: answers to topical questions
What amino acid substitution in the hemoglobin molecule leads to the formation of hemoglobin S?
The formation of hemoglobin S (HbS) occurs as a result of the substitution of glutamic acid for valine in the polypeptide chain. The mutation is localized in the 6th triplet of the gene encoding the hemoglobin β-chain. Replacing a hydrophilic amino acid with a hydrophobic one alters the protein structure: the hydrophobic valine residues interact with valine of the neighboring chain, which disrupts charge distribution and causes molecular polymerization. As a result, erythrocytes acquire a pathological sickle shape, leading to the development of sickle cell anemia.
What other abnormal forms of hemoglobin, besides HbS, provide resistance to malaria?
Resistance to malaria, besides HbS, is provided by the following abnormal forms of hemoglobin:
- Hemoglobin C (HbC) — formed as a result of the substitution of glutamic acid for lysine; protects against malaria by 90% in the homozygous state and 30% in the heterozygous state.
- Hemoglobin E (HbE) — arises from the substitution of glutamic acid for lysine at position 26; provides resistance to malaria quartana.
Carriers of HbS, HbC, and HbE hemoglobinopathies rarely present with severe clinical manifestations of tropical malaria.
Which specific cells of the spleen are responsible for destroying deformed erythrocytes?
The destruction of deformed erythrocytes is carried out by macrophages of the splenic red pulp.
The elimination process of altered cells is implemented in several stages:
- Erythrocyte membrane defects lead to a decrease in their plasticity and deformability.
- Abnormal erythrocytes are trapped in the splenic cords (cords of Billroth) that form the red pulp.
- Splenic macrophages perform phagocytosis, capturing and destroying the trapped cells.
Splenomegaly may be observed during intensive erythrocyte destruction.
The secret of heterozygote survival: The spleen devours sickle cells.
Class and exam questions
Why does the recessive sickle-cell anemia allele not disappear in Africa?
Under conditions of constant malaria exposure risk, heterozygous carriers of this allele have a marked selective advantage. They survive more often than healthy individuals (dominant homozygotes), which is why the mutation persists in the population at a high frequency.
What happens to an infected erythrocyte in an S-hemoglobin heterozygote?
Upon penetration of the pathogen Plasmodium falciparum, the erythrocyte changes its shape. Then the spleen filters out and destroys such deformed cells along with the parasite inside.
How does the mutation frequency change when a population relocates to a region without malaria?
Malaria ceases to be a selection factor, and heterozygotes lose their advantage. Selection begins to work towards eliminating the lethal allele, sharply reducing the disease frequency in the population.
In which ABO marriages is a decrease in the number of children observed?
A decrease in the number of children due to immunological incompatibility is most characteristic of marriages where the woman has blood type 0 (I) and the man has type A (II), B (III), or AB (IV).
What else to study on this topic
- Genetics and inheritance of the ABO system
- Immunological conflict and Rhesus factor
- Life cycle of the malaria parasite (Plasmodium falciparum)
- Pathogenesis of sickle cell anemia in homozygotes
- Genetic drift and other factors of microevolution
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