Population Genetics

Population genetics is a field in genetics that is concerned with the differences in the gene pool between different populations and how this underlies phenotypic differences between populations. The Hardy-Weinberg equilibrium serves as a basis for studying genetic variation within a population and allows for the calculation of allelic frequency. The process of natural selection is what determines the allelic frequencies in the population and the variability in the genotype-phenotype relationships between species.

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Hardy-Weinberg Equilibrium

Introduction

Population genetics studies genetic variation within a group.

  • Depends on genetic, environmental, and societal factors
  • These factors determine the frequency and distribution of alleles and genotypes.

A given population possesses a common gene pool.

  • May contain several alleles of 1 gene
  • Relative proportions of these alleles are referred to as gene frequency.

Gene flow is created when individuals migrate into or away from the population.

Definition

The Hardy-Weinberg equilibrium says that within a given population, both allele and genotype frequencies remain constant, without evolutionary influences.

Assumptions of the Hardy-Weinberg equilibrium

The Hardy-Weinberg equilibrium relies on 7 assumptions:

  • Organisms must be diploid.
  • Sexual reproduction produces new members of the population (no migration).
  • Generations do not overlap.
  • Random mating (without selection)
  • Infinitely large population size
  • Allele frequencies are equal between the sexes.
  • Within a population, there is no gene flow or mutation.

Evolutionary influences

  • Genetic drift: Random sampling of organisms leads to a change in genetic frequency.
  • Assortative mating: Individuals with similar phenotypes mate more commonly.
  • Natural selection: Phenotypes that provide an advantage perpetuate an increased frequency of the corresponding genotype.
  • Sexual selection: a drive to mate with the opposite sex for reproductive purposes
  • Mutation: change in nucleotide sequence
  • Gene flow: transfer of genetic material between populations
  • Meiotic drive: 1 allele may be favored to be passed.
  • Population bottleneck: event that reduces population (e.g., natural disaster)
  • Inbreeding: mating between individuals or organisms that are closely related
  • Founder effect: a new population leading to a decrease in genetic variation
Concept of founder effect

The concept of founder effect is shown in the diagram. There is a reduction in genetic variation when a small group of individuals forms a new population.

Image: “Founder effect” by Zerodamage. License: Public Domain

Hardy-Weinberg Equation

Introduction

The Hardy-Weinberg equation allows for the calculation of genetic variation of a population.

  • This equation relies on the assumption that genetic variation in a population will remain constant between generations.
  • Permits a genetic locus to have 2 alleles
  • The Hardy-Weinberg equation relies on the absence of sexual selection, the absence of genetic flow or mutation, and a large population such that the probabilities are equal to the frequencies.

Equation

The Hardy-Weinberg equation:

$$p^{2}+2pq+q^{2}=1$$
  • Components:
    • The “p” stands for the frequency of 1 allele and “q” stands for the frequency of the other allele.
    • Genotype AA: frequency p2
    • Genotypes Aa, aA: frequency 2pq
    • Genotype aa: frequency q2
  • With the aid of the Hardy-Weinberg equation, population geneticists are able to calculate what percentage of a certain disease is contained in a gene. If gene A and gene a are distributed in a population that is constant, the following applies: 
$$p+q=1 (=100\%)$$

Sample calculation

In a population, the dominant allele is present with a frequency of 60% in the gene pool. What is the distribution of the possible genotypes within the population?

  • p = 60%, q = 40%, because q + p = 100%
  • p2 + 2pq + q2 = 1
  • (0.6)2 + 2 × (0.6 × 0.4) + (0.4)2 = 1
  • 0.36 + 0.48 + 0.16 = 1
  • p2 = 36%, pq = 48%, q2 = 16
  • Answer: AA is 36%; Aa is 48%, and aa is 16%.
Hardy-weinberg proportions

The calculated Hardy-Weinberg proportions given 2 alleles in a genetic pool

Image: “Hardy-Weinberg” by Johnuniq. License: CC BY-SA 3.0

Natural Selection

  • Over longer periods of time, the gene pool of a population changes via several mechanisms, most commonly natural selection.
  • Natural selection favors individuals with a genetic composition that improves the chances of survival and reproduction:
    • A trait that gives a reproductive advantage will be passed down at a higher rate than traits that do not give a reproductive advantage.
    • These genes will occupy a growing share of the gene pool over time.
    • If the genes that offer survival advantages are dominant, they spread rapidly.
    • Dominant genes that are disadvantageous to the individual disappear quickly.
    • Recessive genes persist longer in a population.
Natural selection

An example of natural selection occurring in bacteria to perpetuate antibiotic resistance

Image: “Antibiotic resistance” by Wykis. License: Public Domain

Clinical Relevance

  • Sickle cell anemia: an example of a regionally frequent gene defect that offers its carriers a selective advantage. Sickle cell anemia is especially prevalent in sub-Saharan Africa, where around 80% of the disease occurs. Sickle cell anemia is an autosomal recessive condition. Heterozygous carriers, who carry the HbS gene, have a higher resistance to malaria than non-carriers do. Thus, through selective advantage, the carriers in malaria-endemic regions receive a high share of the HbS gene in the gene pool of the population. These heterozygous individuals are able to produce enough hemoglobin for normal function while receiving the benefit of less-severe malaria infections. 
  • Tay-Sachs disease: founder effect is displayed through Tay-Sachs disease. The founder effect is seen when a smaller group isolates itself from a population, splits off, and reproduces, thereby decreasing genetic variation. Ashkenazi Jews have a higher-than-normal chance of Tay-Sachs disease and other lipid storage disorder, which is partially attributed to the high incidence of a certain chromosome with a high allele frequency in the early founding population.
  • Antibiotic resistance: commonly occurs through horizontal gene transfer, which refers to the exchange of genetic information from 1 organism to another. Horizontal transfer of genetic information occurs between concurrently living organisms, not through sexual reproduction.

References

  1. Griffiths, AJF, Miller, JH, Suzuki, DT, et al. (2000). Chapter 24: Population genetics. In An Introduction to Genetic Analysis. 7th edition. New York: W. H. Freeman. https://www.ncbi.nlm.nih.gov/books/NBK21961/
  2. Charlesworth, B, & Charlesworth, D. (2017). Population genetics from 1966 to 2016. Heredity. 118(1), 2–9. https://doi.org/10.1038/hdy.2016.55
  3. Casillas, S, & Barbadilla, A. (2017). Molecular population genetics. Genetics. 205(3), 1003–1035. https://doi.org/10.1534/genetics.116.196493
  4. Belsky, DW, Moffitt, TE, & Caspi, A. (2013). Genetics in population health science: Strategies and opportunities. American Journal of Public Health. 103 Suppl 1(Suppl 1), S73–S83. https://doi.org/10.2105/AJPH.2012.301139

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