Genetic Drift Practice Problems- Explained
What Genetic Drift Actually Is
Genetic drift is random change in allele frequencies across generations. Unlike natural selection, it doesn't care about fitness. Traits that are harmful can spread. Beneficial alleles can vanish. It's pure chance, and that's the whole point.
This happens fastest in small populations. The smaller the gene pool, the more volatile allele frequencies become. One individual with a rare allele dies, and that allele drops from 10% to 0% in a single generation.
You need to understand this for exams. Here's how to actually solve these problems.
The Two Types You Must Know
Founder Effect
A small group breaks off from a larger population and establishes a new colony. That new group only carries part of the original genetic diversity.
Example: 100 people start a colony on an island. The mainland had 50% allele A and 50% allele a. The colonists happen to have 80% allele A and 20% allele a. The new population is instantly different from the source.
Bottleneck Effect
A disaster kills most of the population. Survivors rebuild, but they represent only a subset of the original genes.
Example: A population of 10,000 birds gets reduced to 50 by a hurricane. Those 50 birds determine all future allele frequencies, regardless of what the original 10,000 looked like.
Core Formulas You Need
Most problems ask you to calculate allele frequencies or genotype frequencies after drift occurs.
Allele frequency:
p = (2 × AA individuals + Aa individuals) ÷ (2 × total individuals)
Expected genotype frequencies under Hardy-Weinberg:
p² + 2pq + q² = 1
But here's the catch: genetic drift breaks Hardy-Weinberg assumptions. In small populations, observed frequencies won't match expected values. That's the point of these problems.
Practice Problem 1: Founder Effect Calculation
A mainland population has 500 individuals. 200 are homozygous dominant (AA), 200 are heterozygous (Aa), and 100 are homozygous recessive (aa). A group of 50 individuals colonizes a new island. If the colonists are randomly selected, what are the expected allele frequencies on the island?
Step 1: Calculate mainland allele frequencies
Total A alleles = (200 × 2) + 200 = 600
Total a alleles = (100 × 2) + 200 = 400
Total alleles = 1000
p = 600/1000 = 0.6
q = 400/1000 = 0.4
Step 2: Apply to founder population
If 50 individuals are randomly selected, expect roughly:
Allele A: 50 × 2 × 0.6 = 60 copies
Allele a: 50 × 2 × 0.4 = 40 copies
Island allele frequencies: p = 0.6, q = 0.4
But reality check: Random sampling means the actual frequencies might be different. If 25 of the 50 colonists happened to carry the recessive allele more often, q could easily be 0.5 or 0.3. That's genetic drift in action.
Practice Problem 2: Bottleneck and Allele Loss
A population of 1000 mice has 1600 B alleles and 400 b alleles. A predator outbreak kills 90% of the population, leaving 100 survivors. What happens to the b allele?
Step 1: Find original frequencies
Total alleles = 1000 × 2 = 2000
B = 1600 → p = 0.8
b = 400 → q = 0.2
Step 2: Assume survivors reflect original ratios (simplified model)
If 100 survivors are a random sample:
Expected B carriers: 100 × 2 × 0.8 = 160 B alleles
Expected b carriers: 100 × 2 × 0.2 = 40 b alleles
Step 3: The brutal reality
With only 100 survivors, there's a real chance the b allele disappears entirely. If none of the 100 survivors happen to carry b, q drops from 0.2 to 0.0. That's extinction of that allele through drift.
Calculate the probability of losing b: (0.8)^200 ≈ astronomically small but not zero. In real bottlenecks, rare alleles die first.
Practice Problem 3: Probability of Allele Fixation
In a population of 50 individuals, one individual is heterozygous (Aa). What's the probability that the 'a' allele eventually fixes in the population?
The fixation probability formula:
For a neutral allele (no selection pressure):
Probability of fixation = 1/(2N) for a single copy
N = 50, so 2N = 100
Probability = 1/100 = 0.01 or 1%
That means the rare allele has a 99% chance of being lost. This is why rare alleles drift out of small populations fast. The math is merciless.
Comparison: Genetic Drift vs Natural Selection
| Factor | Genetic Drift | Natural Selection |
|---|---|---|
| Driver | Random chance | Environmental pressure |
| Population size | Critical - more effect in small groups | Doesn't depend on size |
| Direction | Unpredictable | Toward beneficial traits |
| Allele fitness | Doesn't matter | Determines outcome |
| Speed | Faster in small populations | Depends on selection strength |
| Hardy-Weinberg | Assumption violated | Assumption violated (selection) |
How to Solve Any Genetic Drift Problem
Follow this sequence every time:
- Identify the population size (N) - This determines everything else. Small N = strong drift.
- Count the alleles - How many copies of each allele exist? Get your p and q.
- Determine the event type - Founder effect (splits off) or bottleneck (population crashes)?
- Apply sampling - The survivors or founders represent a subset. Calculate expected vs actual possible outcomes.
- Check for allele loss - Rare alleles in small populations often go extinct. Calculate the probability if asked.
- Consider fixation - If an allele survives, what's its chance of taking over? Use 1/(2N) for neutral alleles.
Common Mistakes Students Make
Assuming Hardy-Weinberg equilibrium applies. It doesn't when drift is the mechanism. Hardy-Weinberg requires infinite population size. Small populations violate this.
Forgetting that drift is random. Many students assume drift always reduces genetic diversity. It can increase it temporarily, but over time, diversity drops.
Confusing drift with selection. If a problem mentions fitness, survival rates, or adaptation, it's selection. Drift problems focus on random sampling and population size effects.
Miscalculating allele copies. Each homozygous individual contributes 2 copies. Each heterozygous contributes 1. Don't forget to multiply.
Quick Reference: Key Numbers to Memorize
- Fixation probability for single allele copy: 1/(2N)
- Time to fixation: approximately 4N generations for neutral alleles
- Drift is significant when N < 100
- Heterozygosity decreases by 1/(2N) per generation
Final Warning
These problems show up on AP Biology, college genetics exams, and GRE Biology. The calculations are straightforward. The hard part is recognizing when drift is the mechanism versus selection.
Read the problem. If it mentions small populations, founder groups, bottlenecks, or random sampling—it's drift. If it mentions fitness, adaptation, or environmental pressure—it's selection. Don't mix them up.