Key population genetics metrics
A population cannot be described by studying a single individual. To assess the genetic status of an entire group, specific population characteristics are used:
- Gene pool and allele pool.
- Phenotype frequency, which directly reflects the frequency of the corresponding allele.
- Genetic polymorphism, i.e., the natural genetic heterogeneity of the group.
- Genetic unity against the background of the dynamic equilibrium of individual alleles and genotypes.
Gene pool and polymorphism
Gene pool is the set of absolutely all genotypes present in a population. The structure of the gene pool depends on the number of alleles of a specific gene. If only two alleles circulate in the population, the gene pool will be represented by three genotype variants: homozygotes $AA$ and $aa$, as well as heterozygotes $Aa$. If there are three alleles, the diversity of diploid genotypes increases (e.g., $A_1A_1$, $A_2A_2$, $A_1A_2$, and so on).
To quantitatively describe the gene pool, the ratio of alleles and the frequency of occurrence of specific phenotypes are used, which are recognized by specific distinguishing features — markers (phens).
The observed diversity of individuals is based on the phenomenon of genetic polymorphism. However, the concept of the gene pool is significantly broader than the concept of polymorphism. Polymorphism describes variability (e.g., the presence in a population of alleles $A_1$, $A_2$, and $A_3$ with frequencies of 60%, 30%, and 10%), whereas the gene pool includes absolutely all genes of a given population.
Allele pool and mathematical calculation of frequencies
While the gene pool evaluates genotypes (complete combinations of an individual's genes), the allele pool is the set of all population alleles individually.
The structure of the allele pool is calculated based on the ratio of various alleles. The basic rule states that the sum of the relative frequencies of all variants of a single gene equals one.
- In the presence of two alleles ($A$ and $a$), the formula looks as follows: $p(A) + q(a) = 1$, where $p$ and $q$ are the frequencies of the respective alleles.
- In the presence of three alleles (e.g., $A_1$, $A_2$, $A_3$), the formula is used: $p(A_1) + q(A_2) + r(A_3) = 1$.
Clinical significance of population characteristics
Differences in the structure of the allele pool are of direct medical importance. The frequency of hereditary pathologies strictly depends on the genetic characteristics of a specific population.
A striking example is phenylketonuria. In mass screenings, the average frequency of this pathology worldwide is 1:8,000. However, due to differences in population gene pools, the figures vary widely across countries:
- In Ireland and Turkey, the risk is highest at 1:4,500.
- In Russia, the figure is close to the average at 1:7,200.
- In Japan, the disease is extremely rare at 1:80,000.
Briefly: answers to topical questions
What equation describes the distribution of genotype frequencies in an ideal equilibrium population?
The distribution of genotype frequencies in an ideal equilibrium population is described by the Hardy-Weinberg equation. The mathematical expression of this distribution is as follows: $(p + q)^2 = p^2 + 2pq + q^2 = 1$.
In this equation:
- $p^2$ is the frequency of the homozygous dominant genotype ($AA$);
- $q^2$ is the frequency of the homozygous recessive genotype ($aa$);
- $2pq$ — frequency of the heterozygous genotype ($Aa$).
The sum of the frequencies of the alleles themselves is equal to one: $p + q = 1$.
What elementary evolutionary factors alter allele frequencies in natural populations?
In the sources, elementary evolutionary factors are classified into primary and secondary (main and additional).
The main elementary evolutionary factors include:
- mutation process;
- population waves;
- isolation;
- natural selection.
Additional elementary evolutionary factors include:
- migration (gene flow);
- meiotic drive, etc.
For additional elementary evolutionary factors, it is explicitly indicated that they alter allele frequencies and the genetic structure of the population. Natural selection also leads to a steady change in allele frequencies in a population and a change in the gene pool. The sources also mention genetic drift; the founder effect is described as a special case of genetic drift, in which the gene pool of a new population differs significantly from the original one.
To avoid confusing terms: Gene pool is the set of genotypes (paired combinations in individuals). Allele pool is the set of alleles (single gene variants).
Class and exam questions
What is a phen (marker) in genetics?
This is a distinctive feature by which a particular phenotype can be visually or laboratorily recognized in a population.
Why is the concept of the gene pool broader than genetic polymorphism?
Polymorphism describes only the presence of diversity (different alleles of the same gene and their frequencies), while the gene pool is composed of absolutely all genes present in the population.
How to calculate frequencies if there are three alleles of the same gene in a population?
The sum of the relative frequencies of all three alleles must equal one: $p(A_1) + q(A_2) + r(A_3) = 1$.
What else to study on this topic
- Mechanisms of formation of population genetic unity
- Dynamic equilibrium of individual genotypes and alleles
- Relationship between phenotype frequency and allele frequency
- Calculation of allele pool structure under multiple allelism
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