Calculate Wright’s inbreeding coefficient F from common ancestor paths in a pedigree. The calculator helps students, breeders, and conservation genetics learners estimate expected identity by descent for offspring from first cousins, full siblings, half-siblings, parent–offspring matings, and custom loops.
Select a relationship preset or enter each common ancestor loop. The result updates immediately with F as a percentage, decimal value, and path-by-path contribution.
Start with a common mating pattern, then edit the loop paths when your pedigree has a different structure.
First cousins
Two shared grandparents, with two generations from each parent to each common ancestor.
Each row represents one independent path through a common ancestor shared by the two parents.
Loop path 1
Contribution: 3.1250%
Loop path 2
Contribution: 3.1250%
The diagram shows how both parents can trace back to the same ancestor, creating identity-by-descent risk.
Wright path formula
FX = Σ [(1/2)n₁+n₂+1 × (1 + FA)]
n₁ and n₂ count generations from each parent to the common ancestor. FA is that ancestor’s own inbreeding coefficient.
Live result
This value matches or approaches the expected offspring F from a first-cousin mating.
Decimal F
0.0625
Parent r estimate
12.50%
Paths
2
Larger bars show the loop paths that add most to the offspring inbreeding coefficient.
n₁ = 2, n₂ = 2, ancestor F = 0%
n₁ = 2, n₂ = 2, ancestor F = 0%
F estimates the probability that two alleles in the offspring are identical by descent from a shared ancestor.
A value of 6.25% means 0.0625 of the offspring genome has expected autozygosity under this pedigree model.
Pedigree F predicts expected inheritance. DNA-based runs of homozygosity can differ because real recombination is random.
An inbreeding coefficient, usually written as F, estimates the probability that two alleles in one individual are identical by descent. Identical by descent means both alleles trace back to the same ancestral allele copy. The value comes from pedigree structure, not from the visible trait.
Sewall Wright formalised coefficients of inbreeding and relationship in the early twentieth century. His path method counts each independent route from one parent to a common ancestor and back down to the other parent. Boucher later proved Wright’s rule for arbitrary pedigrees covering autosomal and X-linked loci. View the PubMed record.
F connects pedigree analysis with population genetics. Small effective population size raises random mating between relatives, so the effective population size calculator can help explain why inbreeding accumulates faster in small breeding populations.
Select first cousins, half-siblings, full siblings, parent–offspring, or another common pedigree pattern.
Enter each shared ancestor and count generations from parent 1 and parent 2 to that ancestor.
Type F_A as a percentage if the common ancestor is already inbred; otherwise leave it at zero.
Use the percentage, decimal F, and contribution bars to see which pedigree loop drives the result.
Count generations carefully. A parent to its own parent counts as one generation. A parent to a grandparent counts as two generations. If a common ancestor appears in more than one independent loop, enter each loop separately.
These buttons load common pedigree structures such as first cousins, half-siblings, and full siblings. They save time when you need a standard textbook value.
Each input card represents one common ancestor path. The generation counts control how strongly that ancestor contributes to F.
This field adjusts the formula when the shared ancestor already has inbreeding. Leave it at 0% when you do not know that ancestor’s own pedigree F.
The bars show which ancestor path adds the most to the final value. Short loops usually dominate the calculation.
The calculator applies Wright’s path formula to every common ancestor loop. One path contributes (1/2)n₁+n₂+1 × (1 + FA). The total offspring F equals the sum of all valid path contributions.
n₁ counts generations from parent 1 to the common ancestor. n₂ counts generations from parent 2 to the same ancestor. FA represents the common ancestor’s inbreeding coefficient, so an inbred ancestor increases the final offspring value.
First cousins share two grandparents. Each grandparent sits two generations from parent 1 and two generations from parent 2. Each path contributes (1/2)5, or 1/32.
Two grandparents produce 1/32 + 1/32 = 1/16. The offspring F equals 0.0625, or 6.25%.
Full siblings share both parents. Each shared parent sits one generation from parent 1 and one generation from parent 2. Each path contributes (1/2)3, or 1/8.
Two shared parents produce 1/8 + 1/8 = 1/4. The offspring F equals 0.25, or 25%.
Inbreeding increases homozygosity because both alleles can descend from the same ancestor. That shift can expose recessive variants, reduce fitness in small populations, and make family-based carrier risk more predictable. The effect depends on pedigree depth, founder diversity, and the distribution of harmful alleles.
Conservation geneticists track F beside drift, bottlenecks, and loss of heterozygosity. You can compare this page with the genetic drift simulator to see how small populations lose variation even without selection. For family risk involving recessive conditions, the carrier probability calculator handles a different question: whether a person likely carries a specific allele.
The calculator does not read DNA markers, detect runs of homozygosity, or identify harmful variants. It estimates expected identity by descent from pedigree paths. Real genomes can differ from the expectation because meiosis samples chromosome segments randomly.
It also does not judge breeding ethics, animal health, or human clinical risk. Those decisions need full pedigree review, trait records, DNA testing, and professional guidance. Treat this result as an educational pedigree-genetics estimate.
Use these tools to connect pedigree inbreeding with population size, genetic drift, and allele-frequency change.