Allele Frequencies of Geographically Isolated Populations
Definition
Allele frequency
Allele frequency refers to how common a particular allele is within a population. It is expressed as a proportion or percentage of all alleles for a specific gene.
- When populations are geographically isolated, allele frequencies, this shows how common specific genetic variants are and how they change over time.
- This process can lead to unique traits or even new species.
In a population of 100 individuals (200 alleles total), if 60 alleles are a specific variant, the allele frequency is 60/200 = 0.30 or 30%.
Isolation Drives Changes in Allele Frequencies
Geographic isolation prevents populations from interbreeding, leading to changes in allele frequencies through the following mechanisms:
- Genetic Drift Creates Random Changes: Random events in small populations can dramatically shift allele frequencies.
- Natural Selection Shapes Traits: Different environmental pressures favor specific alleles, leading to adaptation.
- Mutations Introduce New Alleles: Unique mutations arise in isolated populations, contributing to genetic diversity.
- Founder Effect Alters Starting Frequencies: When a small group establishes a new population, its allele frequencies often differ from the original population.
Human Example: Alcohol Metabolism Genes Vary by Region
The ADH1B gene, which encodes an enzyme that breaks down alcohol, shows striking differences across populations.
Key Alleles of ADH1B
- ADH1B*1: Associated with slower alcohol metabolism.
- ADH1B*2: Associated with faster alcohol metabolism and reduced risk of alcoholism.
Geographic Distribution
- ADH1B*2 is common in East Asia but rare in Europe.
- This may have been favored in regions where rice cultivation led to increased alcohol consumption. Faster metabolism reduced the negative effects of alcohol.
- The phenomenon known as Asian flush (facial flushing after alcohol consumption) is linked to another genetic variant, ALDH2, which reduces the ability to metabolize acetaldehyde (a toxic byproduct of alcohol).
- Individuals with this variant often experience discomfort when drinking, which may reduce alcohol intake and contribute to lower rates of alcoholism in some populations.
Databases Reveal Patterns in Allele Frequencies
Modern genetic tools allow scientists to study allele frequencies across populations.
How to Use Genetic Databases
- Choose a Gene or SNP: For example, the ADH1B gene or its SNP rs2066701.
- Search the Database: Tools like AlFreD or 1000 Genomes Project provide access to genetic data.
- Analyze Patterns: Compare frequencies across populations to identify trends.
Searching for rs2066701:
- ADH1B*2 is common in Han Chinese and Japanese populations.
- ADH1B*1 is common in European populations.
Founder Effect
- Understanding allele frequencies provides insights into several critical areas:
- Tracing Human Migration: Genetic markers reveal patterns of ancient human movement and population histories.
- Understanding Evolution: Isolated populations help explain how natural selection and genetic drift shape genetic diversity.
- Improving Health Research: Knowledge of allele frequencies identifies populations at risk for specific diseases, guiding medical interventions.
- Can you think of another trait that might show different allele frequencies in isolated populations?
- How would you investigate this using a database?
- How does the study of allele frequencies challenge the idea of fixed racial or ethnic categories?
- What does this reveal about the interconnectedness of human populations?
