Alleles have identity by type (IBT) when they have the same phenotypic effect or, if applied to a variation in the composition of DNA such as a single nucleotide polymorphism, when they have the same DNA sequence.
Alleles that are identical by type fall into two groups; those that are identical by descent (IBD) because they arose from the same allele in an earlier generation; and those that are non-identical by descent (NIBD) because they arose from separate mutations.
NIBD can also be identical by state (IBS) though, if they share the same mutational expression but not through a recent common ancestor.
Parent-offspring pairs share 50% of their genes IBD, and monozygotic twins share 100% IBD.
This genetics article is a stub.