Population Genetics and COVID-19

Introduction to Population Genetics

  • Population genetics examines the genetic composition of populations and how it changes over time.
  • Focus on the variability of genetic traits across populations with respect to factors like disease resistance (e.g. COVID-19).

Basis of COVID-19 Genetic Variability

  • COVID-19 infection severity varies from asymptomatic to severe outcomes.
  • Key Concept: Genetic factors (alleles/genotypes) contribute to differential susceptibility to COVID-19.

Understanding Tolerance/Resistance Factors

  • Factors influencing tolerance/resistance involve genetic variation among populations.
  • Address the following questions:
    • What are the tolerance/resistance factors to COVID-19?
    • How are these factors distributed globally?
    • How do these factors propagate in populations?
  • A combined odds ratio (OR) for COVID-19 associated SNPs calculated using elements indexed by their respective sets.

Core Concepts in Population Genetics

  • Key topics include:
    • Hardy-Weinberg (H-W) equilibrium: A foundational principle in population genetics.
    • Chi-square test: Tool for testing genetic equilibrium.
    • 1000 Genomes Project: A large project for understanding human genetic variation.

Learning Outcomes

  • Understand terms like allele, genotype, mutation, selection, and fixation index.
  • Explore processes affecting allele frequencies:
    1. Mutation
    2. Migration
    3. Natural selection
    4. Genetic drift
    5. Gene flow

Hardy-Weinberg Equilibrium (HWE) Principles

  • Assumes:
    • Infinite population size
    • Random mating
    • No selection, migration, or mutation.
  • HWE formula for a two-allele locus:
    (p+q)2=p2+2pq+q2(p+q)^2 = p^2 + 2pq + q^2 where $p$ and $q$ are allele frequencies
  • Use as a null model for goodness of fit analysis.

Example Calculation for Allele Frequencies

  • Using beak phenotype in birds:
    • Allele frequencies defined as:
    • $p = rac{#A ext{ alleles}}{# ext{ total alleles}}$
    • $q = rac{#a ext{ alleles}}{# ext{ total alleles}}$
  • Confirm $p+q=1$ for verification.

Chi-square Test in HWE

  • Application in a newt population study:
    • Expected vs Observed counts used to validate population structure and prediction of allele distribution.

Evolutionary Forces Impacting Populations

  1. Genetic Drift: Random changes in allele frequencies, significant in small populations (e.g. bottleneck and founder effects).
  2. Natural Selection: Differential survival and reproduction influenced by inherited traits.
  3. Gene Flow/Migration: Introduction of new alleles and changes due to inter-population breeding.
  4. Sexual Selection: Affects mate choice within populations.
  5. Mutation: Birth of new genetic variations, sourcing evolution.

1000 Genomes Project Overview

  • Launched to catalog human genetic variation (2008-2013).
  • Sampled over 2,500 individuals from diverse ethnic groups.
  • Data aids research in genetics and evolution.

Insights from Genetic Variations

  • Shared variants among populations assist in understanding historical migrations and genetic relationships.
  • Variants may indicate population structure and historical interactions through patterns of allele distribution.

Fixation Index (Fst)

  • Measures genetic differentiation between populations:
    • Formula: F<em>ST=H</em>T−H<em>SH</em>TF<em>{ST} = \frac{H</em>T - H<em>S}{H</em>T}
    • Indicates how much genetic variance is shared.
  • Fst values range from 0 (no differentiation) to 1 (complete differentiation).

Fst Across Chromosomes

  • Differences observed in Fst values between autosomal vs sex chromosomes, with sexual chromosomes showing higher differentiation exemplifying lower gene flow.

Application of Fixation Index in Blue Eye Gene Variation

  • Analysis indicates higher Fst values for the allele associated with blue eyes, suggesting significant population subdivision and historical context influencing trait prevalence.