A DNA segment is Identical by state (IBS) in two or more individuals if they have identical nucleotide sequences in this segment. An IBS segment is identical by descent (IBD) in two or more individuals if they have inherited it from a common ancestor without recombination, that is, the segment has the same ancestral origin in these individuals. (https://en.wikipedia.org/wiki/Identity_by_descent)
공통조상으로부터 유래했기 때문에 같은 DNA 조각을 IBD라고 한다. IBS는 우연히 같은 것으로, IBD 때문일 수도 있고 아닐 수도 있다.