Selective pressure and infections
Long-term studies have revealed a strict correlation between the distribution of blood groups and human resistance to various infectious agents. Throughout evolution, populations were exposed to selective pressure from pathogenic microorganisms. Infections acted as a rigorous selection factor: individuals with less advantageous genetic combinations perished more frequently, which directly affected the frequency of specific alleles in subsequent generations.
Impact of smallpox pandemics
Smallpox played a colossal role in shaping the genetic landscape of humanity. It has been proven that individuals with blood group A (II) are significantly more susceptible to this disease.
- During major pandemics, carriers of the $I^A$ allele exhibited extremely high mortality rates.
- A natural consequence of this selective vulnerability was a noticeable decrease in the frequency of the $I^A$ allele in historical regions that served as major foci of smallpox spread.
Plague in the Asian region
Asian plague epidemics demonstrate a different pattern of genetic selection. An analysis of territories that historically served as natural and epidemic plague foci in Asia revealed a specific genetic anomaly.
In these regions, the geographic distribution of alleles differs radically from the global average: there is a marked overlap between infection foci and areas of abnormally low frequency of the $I^0$ allele. Concurrently, a stably high frequency of the $I^B$ allele is recorded in these locations.
Geographic distribution of alleles
Due to the uneven impact of infectious diseases, genetic equilibrium in the AB0 system always shifts when changing geographic regions. Today, the following distribution pattern has emerged:
- Western Europe: Characterized by a high frequency of the $I^A$ allele. Meanwhile, the $I^B$ allele is less prevalent here, with a frequency of less than 10%.
- Central Asia: Demonstrates a steadily high frequency of the $I^B$ allele, reaching 20–30%.
- American Indians: This population exhibits an extremely high frequency of the $I^0$ allele. At the same time, the $I^B$ allele is extremely rare or entirely absent.
Thus, local conditions and historical contacts with pathogens strictly dictate a specific allele balance in particular populations.
Briefly: answers to topical questions
What infectious diseases are considered the main factors of historical selective pressure on the AB0 system?
The plague and smallpox are considered the main factors of historical selective pressure on the AB0 system.
- Plague: plague foci in Asia coincide with areas of low $I^0$ allele frequency and high $I^B$ allele frequency.
- Smallpox: individuals with blood group A (II) are more susceptible to smallpox; this resulted in a decreased frequency of the $I^A$ allele in historical smallpox spread foci.
Why did an abnormally low frequency of the $I^0$ allele form in historical plague epidemic foci?
An abnormally low frequency of the $I^0$ allele in plague foci developed due to antigenic similarity between the pathogen and human antigens.
The plague pathogen (Yersinia pestis) possesses an H-like antigen that is structurally similar to the $I^0$ (0) blood group antigen. As a result, the immune system of individuals with type I blood poorly recognizes the bacterium as a foreign agent. This makes individuals with blood group $I^0$ (I) less resistant to the infection, which historically led to their elimination in epidemic foci.
For quick memorization of disease-allele links: "Smallpox" hits group A (decreases $I^A$), while plague foci are dominated by the letter "B" ($I^B$ increases).
Class and exam questions
What is selective pressure in the context of the AB0 system?
This is an evolutionary process in which infectious diseases promote the survival of carriers of certain blood groups and the death of others, thereby altering allele frequencies in the population.
How did smallpox affect the spread of the $I^A$ allele?
Due to high susceptibility and mortality among people with blood group A (II) during smallpox pandemics, the frequency of the $I^A$ allele in historical foci of this infection decreased significantly.
What genetic feature is characteristic of American Indians?
American Indians exhibit a very high frequency of the $I^0$ allele, whereas the $I^B$ allele is extremely rare or completely absent.
Why do blood group frequencies differ across various regions of the Earth?
Equilibrium between blood types shifts depending on the geographic region due to varying historical pathogen exposure and the formation of unique selective pressure in each region.
What else to study on this topic
- Genetic drift and its impact on small populations
- The founder effect in isolated groups
- Isolation as a factor in allele fixation
- Genetics of the AB0 system: interaction of $I^A$, $I^B$, and $I^0$ alleles
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