FST Calculator
Calculate Wright's FST (fixation index) to measure genetic differentiation between populations. FST ranges from 0 (identical) to 1 (completely different). Uses Nei's formula.
🧬 Genetics📐 FST = (HT - HS) / HT🧬 Biology
Expected heterozygosity population 1 (H1)
Expected heterozygosity population 2 (H2)
Allele frequency p1 in pop 1
Allele frequency p1 in pop 2
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| FST Calculator | — | FST = (HT - HS) / HT | 0 to 1 |
Step-by-Step Examples
Example 1
Two populations
Pop1: p=0.60, Pop2: p=0.40. Calculate FST.
- p̄ = (0.60+0.40)/2 = 0.50
- HT = 2×0.50×0.50 = 0.50
- HS = (2×0.60×0.40 + 2×0.40×0.60)/2 = 0.48
- FST = (0.50-0.48)/0.50 = 0.04
✓ FST = 0.04 (little differentiation)
Example 2
High differentiation
Pop1: p=0.90, Pop2: p=0.10.
- p̄ = 0.50, HT = 0.50
- HS = (2×0.9×0.1 + 2×0.1×0.9)/2 = 0.18
- FST = (0.50-0.18)/0.50 = 0.64
✓ FST = 0.64 (very high differentiation)
Example 3
Human populations
Typical human FST across continental groups ≈ 0.12-0.15.
- Most genetic variation is within populations (85-90%)
- Only 10-15% is between continental groups
- Low FST = humans are genetically similar worldwide
✓ Human continental FST ≈ 0.12 (Lewontin 1972)
Real-World Applications
Evolutionary Biology
FST quantifies population structure and genetic differentiation driven by drift, selection, or reduced gene flow.
Conservation Genetics
High FST between subpopulations may indicate conservation units. Low FST: gene flow is high, manage as one unit.
Molecular Epidemiology
FST of pathogen populations tracks disease spread: high FST = local epidemics; low FST = global spread.
Forensic Genetics
FST used in Bayesian kinship and match probability calculations in DNA databases.
Common Mistakes to Avoid
⚠️
FST = 0: no differentiation
Populations have identical allele frequencies. FST = 1: one allele fixed in each population, completely different.
⚠️
Rule of thumb: FST < 0.05 little, 0.05-0.25 moderate, > 0.25 great
Wright's guidelines widely used but not universal. Some organisms show high FST even with moderate gene flow.
⚠️
Use multiple loci for reliable FST
Single-locus FST is noisy. Average over 10+ loci (or SNPs) for a robust estimate of overall population differentiation.
Frequently Asked Questions
What is FST? ▾
Fixation index measuring proportion of total genetic variation that is due to differences between populations. FST = (HT-HS)/HT using Nei's heterozygosity formula.
What causes high FST? ▾
Low gene flow (geographic barriers), genetic drift in small isolated populations, or strong local adaptation (divergent natural selection).
What is the difference between FST, GST, and QST? ▾
FST (Wright): biallelic loci. GST (Nei): generalization to multiple alleles and populations. QST: quantitative trait analog of FST. QST > FST suggests divergent selection.
What FST do human populations show? ▾
Continental groups: FST ≈ 0.10-0.15. Neighboring populations: FST ≈ 0.01-0.05. Isolated groups (island, linguistic): up to 0.30+. Overall, humans are genetically very homogeneous.
What is an FST outlier test? ▾
Compare FST at each locus to genome-wide average. Loci with unusually high FST are candidates for local adaptation (selection). Used in population genomics scans.