Inbreeding Coefficient Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
The inbreeding coefficient (F) measures the probability that an individual inherited two identical copies of the same ancestral gene, due to the relatedness between their parents. For first-cousin parents, F = 1/16 (6.25%); for sibling parents, F = 1/4 (25%).
Explanation
The inbreeding coefficient F represents the probability, at a given location in the genome, that an individual inherited two IDENTICAL copies of the same ancestral gene — one passed down from their mother, the other from their father — because their two parents themselves share a recent common ancestor. The higher this coefficient, the greater the risk of homozygosity across the entire genome, which statistically increases the chance that harmful recessive mutations, normally masked by a single functional copy of the gene, end up present in duplicate and expressed clinically. This coefficient structurally halves at each generation of distance between the parents: sibling parents pass an F of 1/4 to their child, first cousins (who share common grandparents, a more distant degree of relatedness) pass an F of 1/16 — four times lower — and second cousins (sharing common great-grandparents) drop to 1/64, sixteen times lower than for siblings. This purely probabilistic calculation never predicts a certain health problem for a given individual: it quantifies an increased statistical risk at the population level, not an individual diagnosis. This is why professional genetic counseling, which takes into account the actual family medical history and not just this theoretical coefficient, remains the only appropriate approach for assessing a concrete risk in a personal situation. This same increased-homozygosity risk can be approached at the scale of an entire population, rather than an individual, with our Hardy-Weinberg equilibrium calculator — and the loss of genetic diversity that repeated inbreeding causes across a population is exactly what our Shannon diversity index calculator is built to quantify, applied there to species or ecosystems rather than a single gene pool.
Example: first-cousin parents
Inputs
Family relationship between the parents: first cousins.
Calculation
F = 1/16 = 0.0625, or 6.25%.
Result
A child born to first-cousin parents has an inbreeding coefficient of 6.25%, versus 0% for parents with no known family relationship.
Frequently asked questions
Does a high inbreeding coefficient guarantee a health problem?
No, absolutely not: this coefficient measures an increased statistical probability of homozygosity across the whole genome, not the certainty that a specific recessive disease will appear. Most children born to related parents, even fairly closely related ones, develop no particular genetic disease — the actual risk also depends on the specific recessive mutations actually present in the family concerned, information this theoretical coefficient alone can't provide.
Why does the coefficient halve at each generation of distance?
Because at each additional generation between two individuals and their common ancestor, the probability that each one inherited the same ancestral allele (rather than the other possible allele from the ancestor pair) is halved at each step of transmission — a direct principle of Mendelian genetics. First cousins and siblings differ by exactly one additional generation of distance from the common ancestor, hence the factor of 4 (2×2) observed between 1/4 and 1/16.
Does this coefficient apply only to humans?
No, the same principle and formulas apply to any sexually reproducing species, and are widely used in breeding (cattle, dogs, horses) to limit inbreeding in selected populations, which can reduce overall vigor and increase the frequency of recessive hereditary diseases within a line or breed — the same inheritance mechanism illustrated at the level of a single cross by Mendelian genetics.