Identity by descent
Two or more alleles are identical by descent (IBD) if they are identical copies of the same ancestral allele. This property is often used in genetic linkage to identify alleles which are potential candidates for harboring mutations causing a genetic disease.
A common way to identify alleles as identical by descent is usually carried out using single-nucleotide polymorphism (SNP) data. If enough SNPs in two alleles are observed as the same, then the two alleles are inferred as IBD, although this does not guarantee that the two alleles are identical. Even if they are indeed derived from the same ancestral allele, they could harbor different mutations which arose during the inheritance process, like single nucleotide mutations, insertion, deletions or others.
Given the new importance that IBD alleles are assuming in genetic association studies, a new definition of IBD that would better suit the underlying inheritance property would be that two or more alleles are identical by descent (IBD) if they have been inherited from the same ancestral allele without recombination events.