Population Genetics

Population: A population is defined as a group of organisms belonging to the same species that inhabit the same geographic area and have the capability to interbreed with one another.

Population Genetics: The study focuses on understanding the genetic composition of populations and how it changes over time.
- The genetic variation in populations arises due to the existence of different alleles at various genetic loci.
- Example: Shell color polymorphism in a snail is an illustration of a polymorphic trait within a population, indicating variation.

A gene is considered polymorphic if there are two or more alleles present within a population.
The goal is to comprehend the prevalence of polymorphic genes within populations and their link to phenotypic functions.

Key measurements in population genetics include:
- Phenotype frequencies: The proportion of different phenotypes in a population.
- Genotype frequencies: The proportion of individuals of each genotype in a population.
- Allele frequencies: The frequency of specific alleles in a population.

Example Calculation:
- Total flowers = 1000
- White: 200/1000 = 0.2
- Pink: 500/1000 = 0.5
- Red: 300/1000 = 0.3

Example Calculation:
- For the genotypes:
- rr: 200/1000 = 0.2
- Rr: 500/1000 = 0.5
- RR: 300/1000 = 0.3

Definition: The frequency of a specific allele is calculated as the number of copies of the allele in the population divided by the total number of alleles for that gene.
Example Calculation:
- Total alleles = 2000
- Frequency of allele r: 900/2000 = 0.45
- Frequency of allele R: 1100/2000 = 0.55
- More detailed example:
- Frequency of allele r: ext{frequency of r} = rac{200 + (0.55 imes 1000)}{2000} = 0.45
- Frequency of allele R: ext{frequency of R} = rac{300 + (0.55 imes 1000)}{2000} = 0.55

For polymorphic genes, the sum of all allele frequencies must equal 1.
- Example:
- Frequency of R = 0.55
- Therefore, frequency of r = 1 - frequency of R = 1 - 0.55 = 0.45

Hardy-Weinberg Equilibrium: This principle predicts that allele and genotype frequencies will remain constant across generations in a population, given certain conditions are met:
- No new mutations.
- No genetic drift.
- No migration.
- No natural selection.
- Random mating.

The sum of the allele frequencies: R + r = 1.
If each individual has two alleles for each gene, then: (R + r) imes (R + r) = 1
This can be expressed as (p + q) imes (p + q) = 1 where p is the frequency of one allele and q is the frequency of the other allele.

The general formula is:
- R^2 + 2Rr + r^2 = 1
- Where:
  - R^2 = frequency of genotype RR
  - 2Rr = frequency of genotype Rr
  - r^2 = frequency of genotype rr
In standard terminology the equation is often expressed as:
- p^2 + 2pq + q^2 = 1
- Where p and q represent allele frequencies.

Given an allele frequency of r = 0.45 , if the population is in Hardy-Weinberg equilibrium, we can calculate:
- Frequency of genotype rr: 0.45^2 = 0.2025
Population genotype frequencies can be calculated similarly by using the allele frequencies in the H-W equation.

To determine if a population is in Hardy-Weinberg equilibrium:
- Chi-square tests can be applied to compare observed vs expected genotype frequencies.
- Degrees of freedom (df) for chi-square: ext{#genotypes} - ext{#alleles} .

Importance of genetic variation:
- Vital for adaptation to environmental changes.
- Plays a crucial role in conservation and biodiversity.

Mutation:
- Spontaneous changes in DNA leading to new alleles.
- It's the ultimate source of genetic variation.
Migration:
- Also known as gene flow, it involves the genetic exchange due to migration of fertile individuals or gametes between populations.
- Example of genetic shifts in different populations.
Natural Selection:
- Certain genotypes contribute to higher offspring production.
- Increased fitness is associated with better survival and reproductive rates, leading to changes in allele frequencies.
- Example: Resistance to antibiotics can be an outcome of natural selection over multiple generations.
Genetic Drift:
- Changes in allele frequencies due to random chance, notably in small populations.
- Can result in fixation (all individuals have an allele) or loss of an allele.
Non-random Mating:
- Influences the arrangement of alleles into genotypes.

Natural selection can lead to population divergence based on environmental pressures.
Factors contributing to genetic drift include:
- Bottleneck effect: A catastrophic reduction in population size.
- Founder effect: A new population arises from a small number of colonizing individuals, carrying limited genetic variation.
Demographic stochasticity contributes to variations in survival and reproduction rates.

Random Mating: Occurs without regard to genotype and results in expected proportions of offspring genotypes.
Positive Assortative Mating: Individuals select genetically similar mates, leading to an increased frequency of homogenous genotypes.
Negative Assortative Mating: Individuals prefer different genotypes, maintaining genetic diversity.

Changes in population allele frequencies are influenced by mutation, migration, natural selection, genetic drift, and mating patterns.
The dynamics of these processes play critical roles in shaping genetic structures over time.