Mendelian Genetics in Populations

Mendelian Genetics in Populations

Population genetics combines Darwin's theory of evolution via natural selection with Mendelian genetics, focusing on changes in allele and genotype frequencies in a population over generations. Evolution is defined as an alteration in allele frequency.

Alleles and Genotypes

In a locus of interest, there are typically two alleles (A and a) with three potential genotypes: homozygous dominant (AA), heterozygous (Aa), and homozygous recessive (aa). Assuming random mating, gametes unite, yielding these genotypes at predictable ratios based on allele frequencies.

Hardy-Weinberg Equilibrium (HWE)

HWE states that in an ideal population with no evolution (no selection, mutation, migration, or non-random mating), allele frequencies remain constant across generations. Key equations:

1. $p + q = 1$ (with $p$ = frequency of A, $q$ = frequency of a)

2. Genotype frequencies can be predicted via:

   • $p^2$ for AA

   • $2pq$ for Aa

   • $q^2$ for aa

3. $p^2 + 2pq + q^2 = 1$
   Genotype frequency examples illustrate the validity of these predictions against observed frequencies and help assess if a population is in Hardy-Weinberg equilibrium.

Evolution and Genetic Drift

Genetic drift, distinct from selection, leads to random changes in allele frequencies, especially in small populations. Notable concepts include population bottlenecks and the founder effect, which increase genetic drift. Additionally, persistent selection can induce significant long-term changes in allele frequencies.

Inbreeding Depression

Inbreeding can result in increased homozygosity for deleterious mutations, lowering fitness. Heterozygosity typically confers an advantage, preserving genetic diversity and enhancing vitality.

Assumptions of Hardy-Weinberg Principle

1. No selection

2. No mutation

3. No migration

4. No chance events

5. Random mating