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 (just not origin). Parent-offspring pairs share 50% of their genes IBD, and monozygotic twins share 100% IBD.
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