Inbreeding Depression Calculator

Calculate inbreeding coefficient F and expected fitness reduction from inbreeding.

Mating type

Generation of inbreeding n

Repeated inbreeding generations

Fitness of outbred population W0

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What the Inbreeding Coefficient Measures

F is the probability that both alleles at a locus are identical by descent — inherited from the same ancestral copy. It is not a measure of general similarity but of shared ancestry at that specific locus.

Relationship	F of offspring
Parent–offspring or full sibs	0.25
Half sibs, uncle–niece	0.125
First cousins	0.0625
Second cousins	0.0156

F_n = 1 − (1 − ΔF)ⁿ

Repeated inbreeding compounds. Three generations of full-sib mating gives F = 1 − 0.75³ = 0.578, not 0.75 — each generation acts on the remaining heterozygosity rather than the original.

Why Inbreeding Reduces Fitness

Mechanism	Explanation	Purgeable?
Partial dominance	Recessive deleterious alleles become homozygous	Yes, partly
Overdominance	Heterozygote advantage is lost	No

The distinction matters for conservation. If depression comes mainly from recessive deleterious alleles, slow inbreeding can purge them — selection removes homozygotes and the load declines. If it comes from heterozygote advantage, purging is impossible because the heterozygous state itself is what confers fitness.

Evidence supports partial dominance as the larger contributor in most species, which is why some captive breeding programmes tolerate slow inbreeding while avoiding rapid crashes.

Practical Thresholds

Conservation genetics commonly cites a ceiling of 1% increase in F per generation, corresponding to an effective population size of 50 — enough to limit short-term depression. Long-term retention of adaptive potential is usually put at N_e of 500, the basis of the 50/500 rule.

Note that outbreeding depression is the opposite risk: crossing populations adapted to different environments, or with chromosomal differences, can also reduce fitness. Genetic rescue programmes weigh both.

Worked Examples

Example 1: Full sibling mating, 3 generations

F=1-(1-0.25)^3=0.578

Result: Very high inbreeding — F=0.578

Comparable to parent-offspring after multiple generations

Example 2: Conservation: first cousin mating once

F=0.0625 per generation

Result: Small fitness reduction

Minimum inbreeding in managed breeding programs

Example 3: Why F compounds rather than adds

Three generations of full-sib mating at ΔF = 0.25

Result: F = 1 − 0.75³ = 0.578

Each generation acts on the heterozygosity that remains, not on the original amount. Adding would wrongly give 0.75.

Example 4: The 50/500 rule

N_e = 50 versus N_e = 500

Result: Short-term survival versus long-term potential

Fifty limits inbreeding to about 1% per generation. Five hundred is the threshold for retaining adaptive variation over evolutionary timescales.

Common Mistakes

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Adding F across generations

Inbreeding compounds multiplicatively on remaining heterozygosity. Three generations of full-sib mating gives 0.578, not 0.75.

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Assuming inbreeding depression can always be purged

Only the partial dominance component is purgeable. Depression from heterozygote advantage cannot be removed by selection, since the heterozygous state itself confers the advantage.

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Treating F as general genetic similarity

It measures probability of identity by descent at a locus, not overall relatedness or genome-wide similarity.

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Ignoring outbreeding depression

Crossing populations adapted to different environments can also reduce fitness. Genetic rescue requires weighing both risks.

Frequently Asked Questions

Inbreeding depression mechanisms?

1. Homozygosity reveals recessive deleterious alleles. 2. Heterozygote advantage lost (overdominance). Magnitude varies: outcrossing plants >50% fitness loss at F=0.5. Animals: 10-30% per 10% increase in F (typical estimate).

Conservation breeding?

Maximize mean kinship among individuals in managed populations. Pedigree-based or genomic IBD (identity by descent). SSP (Species Survival Plan) for zoos. Reintroduce individuals to avoid inbreeding in small wild populations.

What does the inbreeding coefficient measure?

The probability that both alleles at a locus are identical by descent, inherited from the same ancestral copy.

Why is F for first cousins 0.0625?

They share a set of grandparents, and the probability of both alleles descending from the same grandparental copy works out to one sixteenth.

Can inbreeding depression be purged?

Partly. Depression from recessive deleterious alleles can be reduced by selection during slow inbreeding, but depression from heterozygote advantage cannot.

What is the 50/500 rule?

An effective population size of 50 limits short-term inbreeding to about 1% per generation; 500 is the threshold for retaining long-term adaptive potential.