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Fixation Index Calculator

Last updated: December 23, 2025
Reviewed by: LumoCalculator Team

Calculate Wright's F-statistics (FST, FIS, FIT) for population genetics analysis. Measure genetic differentiation between populations, inbreeding within subpopulations, and estimate gene flow.

F-Statistics Calculator

Population genetics analysis

Expected heterozygosity of total population (0-1)

Mean expected heterozygosity within subpopulations

Observed heterozygosity in individuals (optional)

F-Statistics Results

FST (Population Differentiation)
0.1000
Moderate differentiation
Moderate differentiationGene flow Nm ≈ 2.25
FST
0.1000
Between pops
FIS
0.1111
Within subpop
FIT
0.2000
Total
HT
50.00%
HS
45.00%
HI
40.00%
💡 Interpretation

F-statistics are within normal ranges for connected populations with random mating.

Detailed Interpretation

FST: 0.1000

Moderate genetic differentiation between populations.

FIS: 0.1111

Moderate inbreeding within subpopulations.

FIT: 0.2000

Moderate overall deviation from Hardy-Weinberg equilibrium.

Wright's FST Guidelines

0 - 0.05(Little)
0.05 - 0.15(Moderate)
0.15 - 0.25(Great)
> 0.25(Very Great)

Related Calculators

Understanding F-Statistics

📊 FST - Between Populations

FST = (HT - HS) / HT

  • • Measures genetic differentiation
  • • Range: 0 (identical) to 1 (fixed)
  • • Also called GST, θ (theta)
  • • Used for population structure
🔄 FIS - Within Subpopulations

FIS = (HS - HI) / HS

  • • Measures local inbreeding
  • • Negative = excess heterozygotes
  • • Positive = inbreeding
  • • Tests Hardy-Weinberg within groups
🌍 FIT - Total Inbreeding

FIT = (HT - HI) / HT

  • • Total deviation from HW equilibrium
  • • Combines FST and FIS effects
  • • Reflects overall population structure
  • • Used in forensic genetics
🔗 Wright's Relationship

(1 - FIT) = (1 - FIS)(1 - FST)

  • • F-statistics are hierarchical
  • • Total = local × between
  • • Allows decomposition of effects
  • • Validation check for calculations

Wright's FST Interpretation Guidelines

FST RangeLevelDescriptionImplication
0 - 0.05LittleLittle genetic differentiationHigh gene flow, panmixia
0.05 - 0.15ModerateModerate differentiationSome population structure
0.15 - 0.25GreatGreat differentiationLimited gene flow
> 0.25Very GreatVery great differentiationIsolated populations

* These are general guidelines. Interpretation should consider biological context, marker type, and sample size.

Gene Flow Estimation

Wright's Island Model

Nm ≈ (1 - FST) / (4 × FST)
  • • Nm = effective number of migrants per generation
  • • Nm > 1 prevents substantial drift
  • • Nm > 4 suggests panmixia
  • • Assumes equilibrium and island model

Interpretation Guide

Nm < 1Genetic drift dominates
Nm = 1-4Some differentiation
Nm > 4Effectively panmictic

Applications of F-Statistics

Conservation Biology

Assess genetic diversity and population connectivity for endangered species management

Forensic Genetics

Account for population structure in DNA profile match probability calculations

Evolutionary Biology

Study speciation, genetic drift, and natural selection patterns

Agriculture

Evaluate breeding programs and maintain genetic diversity in livestock

Human Genetics

Correct for population stratification in genome-wide association studies (GWAS)

Heterozygosity Measures

HT - Total Heterozygosity

Expected heterozygosity of the total population

HT = 1 - Σp̄ᵢ²

Where p̄ᵢ is the mean frequency of allele i across all subpopulations

HS - Subpopulation Heterozygosity

Mean expected heterozygosity within subpopulations

HS = (1/n) × Σ(1 - Σpᵢⱼ²)

Average of Hardy-Weinberg expectations per subpopulation

HI - Observed (Individual) Heterozygosity

Actual proportion of heterozygous individuals observed

HI = (# heterozygotes) / (total individuals)

Direct count from genotype data; compared to HS to test Hardy-Weinberg equilibrium

Frequently Asked Questions

What is FST (Fixation Index)?
FST measures genetic differentiation between populations. It ranges from 0 to 1, where 0 indicates no differentiation (identical allele frequencies) and 1 indicates complete differentiation (fixed for different alleles). FST is calculated as (HT - HS) / HT, where HT is total population heterozygosity and HS is mean subpopulation heterozygosity.
How do I interpret FST values?
According to Wright's guidelines: FST < 0.05 indicates little differentiation (high gene flow), 0.05-0.15 is moderate differentiation, 0.15-0.25 is great differentiation, and > 0.25 indicates very great differentiation (isolated populations). However, interpretation should consider species biology and marker type.
What is the difference between FST, FIS, and FIT?
FST measures differentiation BETWEEN subpopulations relative to the total population. FIS measures inbreeding WITHIN subpopulations (deviation from Hardy-Weinberg within groups). FIT measures total inbreeding of individuals relative to the total population. They are related by: (1-FIT) = (1-FIS)(1-FST).
What does a negative FIS mean?
A negative FIS indicates an excess of heterozygotes within subpopulations compared to Hardy-Weinberg expectations. This can result from: negative assortative mating (preference for dissimilar mates), recent population admixture, overdominant selection (heterozygote advantage), or sampling from multiple demes.
How do I calculate gene flow from FST?
Under Wright's island model, gene flow (Nm, number of migrants per generation) can be estimated as Nm ≈ (1 - FST) / (4 × FST). An Nm > 1 generally prevents substantial differentiation. However, this is a rough estimate with many assumptions and should be interpreted cautiously.
What heterozygosity values do I need?
You need: HT (total heterozygosity) - expected heterozygosity calculated from pooled allele frequencies across all populations; HS (subpopulation heterozygosity) - mean of expected heterozygosities calculated separately for each subpopulation; HI (observed heterozygosity) - actual proportion of heterozygotes observed in individuals.