The inbreeding coefficient (F) is the probability that both alleles at a locus in an individual are identical by descent (IBD) — i.e. both copies are inherited from the same ancestral allele through different paths in a pedigree. It ranges from 0 (no inbreeding) to 1 (complete inbreeding). High inbreeding increases homozygosity and the expression of deleterious recessive alleles, reducing fitness (inbreeding depression).