What is polymorphism?
Polymorphism describes the heterogeneity of a population in which two or more variants of a single trait persist for a long time. An important criterion is that the rarest variant must occur in more than 1% of individuals to rule out the influence of sporadic mutations.
There are two main types of polymorphism:
- Genetic (allelic) — the presence of different alleles of a single gene (ensures the genetic uniqueness of all humans except monozygotic twins).
- Chromosomal — variations in chromosome structure, including rearrangements (e.g., inversions) or differences in sex chromosomes.
Examples of genetic polymorphism in humans
Genetic diversity manifests in a wide variety of traits, from metabolism to susceptibility to infections:
- Phenylketonuria. This is an inherited metabolic disorder (inherited in an autosomal recessive manner). About 300 different mutations of the gene responsible for this pathology have been described.
- HIV resistance (CCR5 gene). Normally, the gene encodes a cellular receptor through which the virus enters the lymphocyte. With a deletion of this gene, the receptor is absent, and the person becomes resistant to HIV. This mutation is most common in Scandinavia (13.7%) and Russia (13%), while it is practically absent in African countries.
- ABO blood group system. Each population has its own unique composition of alleles of the I gene. Interestingly, polymorphism in this system arose even before the appearance of Homo sapiens (for example, chimpanzees have groups A and O, orangutans have A, B, and AB).
Evolutionary significance of polymorphism
Studying the frequencies of specific alleles allows us to track evolutionary processes and build phylogenetic trees. Recently separated populations have a similar gene frequency profile, whereas reproductively isolated groups accumulate significant differences (divergence) over time.
A striking example of the action of evolutionary factors (natural selection) is skin pigmentation polymorphism. Skin color depends on the polymeric interaction of at least three pairs of genes regulating the production of melanin, hemoglobin, and carotene.
- In hot regions, selection fixes dark skin: melanin protects against mutagenic ultraviolet radiation, preventing skin cancer.
- In northern latitudes, light-skinned individuals have an advantage: this is necessary for the synthesis of vitamin D under weak sunlight and the prevention of rickets.
Briefly: answers to topical questions
What types of structural rearrangements belong to human chromosomal polymorphism?
Human chromosomal polymorphism includes structural rearrangements such as inversions.
Chromosomal polymorphism can be:
- By chromosome types (e.g., by sex chromosomes).
- By rearrangements (e.g., inversions).
Which specific genes control the polygenic inheritance of human skin pigmentation?
The genetic determination of skin pigmentation is determined by the interaction of at least three pairs of non-allelic genes. These polymeric genes are designated as $S_1$, $S_2$, $S_3$. The degree of trait manifestation (amount of pigment) depends on the total number of dominant alleles in the genotype.
What are some examples of balanced polymorphism providing resistance to malaria?
An example from the sources is sickle cell anemia (S-hemoglobin).
In malaria-endemic areas, selection maintains the existence of a lethal allele:
- homozygotes for the recessive gene die in the early stages from severe anemia;
- heterozygotes have a selective advantage; heterozygotes for sickle cell anemia (Aa) exhibit overdominance and do not contract malaria, unlike normal homozygotes (AA).
Mechanism of resistance: the pathogen enters the heterozygote's erythrocyte, the erythrocyte changes shape, and the spleen destroys the altered infected erythrocytes along with the plasmodium.
Class and exam questions
A mutation is a single change in DNA. If a mutation becomes fixed in a population and its frequency exceeds 1%, this state is called genetic polymorphism.
A mutation is a single change in DNA. If a mutation becomes fixed in a population and its frequency exceeds 1%, this state is called genetic polymorphism.
The mutation conferring HIV resistance probably became fixed in Europeans under pressure from other historical infections, whereas in African populations this variant did not gain an evolutionary advantage.
The mutation conferring HIV resistance probably became fixed in Europeans under pressure from other historical infections, whereas in African populations this variant did not gain an evolutionary advantage.
Skin color is inherited via polymerism — the trait is determined by the cumulative effect of several (at least three) pairs of non-allelic genes.
Skin color is inherited via polymerism — the trait is determined by the cumulative effect of several (at least three) pairs of non-allelic genes.
What else to study on this topic
- Types of gene interaction (polymerism)
- Population phylogenetic trees
- Structure of the human immunodeficiency virus (HIV)
- Human blood group systems (other than ABO)
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Genetic polymorphism" and 3,793 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the section "Population — the elementary unit of evolution"
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.