Population Genetics

Department of Biological and Environmental Sciences: Samford University

Basic Population Genetics

Population genetics deals with terms like gene pools, allelic frequencies, genotypic frequencies, phenotypic frequencies. Instead of studying the gene from the level of the molecule, cell, or organism, it looks a gene from the population point of view. A population is a local group of organisms of the same species (interbreeding or potentially so).

Hardy-Weinberg Equilibrium: This mathematical expression predicts what proportions to expect in each of the three genotypic classes (looking at only one gene and assuming there are only two alleles).

Genotypic Frequencies Are Determined by Allelic Frequencies (Gene Frequencies): The frequency of AA individuals should be p², Aa 2pq, and aa q². (p is the allelic frequency of A in the gene pool and q is the allelic frequency of a in the gene pool). If we assume HW equilibrium, we can estimate the gene and genotypic frequencies from the homozygous recessive frequency (q²=[aa]). HW equilibrium applies to autosomal genes and to X linked genes in females.
Assumptions of Hardy-Weinberg Equilibrium: These frequencies will be observed if we assume the following.

Large, Randomly Mating Population
No Selection
No Mutation
No Migration (no immigration or emigration)

Changes in Gene Frequency: The gene and genotypic frequencies will not remain at HW equilibrium if the above condition are not met.

Selection: With selection, all genotypes are not equally viable. Fitness values (W) and selection coefficients (s) are assigned to different genotypes

Complete Selection: With complete selection against aa, it is assigned an s value of 1.

q_new=q_old/(1+q_old)
q_n=q₀/(1+nq₀)

Partial Selection: With partial selection against aa, it is assigned an s value less than 1 but greater than 0.

q_new=(q_old(1-sq_old))/(1+sq_old)

Heterozygote Advantage: With this type of selection, both homozygotes are selected against (have an s value greater than 0). Sickle-cell anemia in a malaria environment is an example of this kind of selection. The value of q will reach an equilibrium independent of the initial genotypic frequencies.

q at equilibrium = s₁/(s₁+s₃)

Drift: If the mating population is small, dramatic fluctuations in gene and genotypic frequencies may be seen due to sampling error. Founder's Principle is a "one-time" drift event due to a new population being founded from a small sample of the original population.
Inbreeding: Inbreeding does not change the gene frequency but changes the genotypic frequencies.
Migration

Balanced Polymorphisms: In nature, population often exist with genotypic frequencies balanced at some level. This may be due to neutral mutations, heterozygous advantage, equilibrium between mutation and selection, equilibrium between forward and backward mutation, or density-dependent selection.