The orig­in of IBD seg­ments is depict­ed via a pedi­gree of 12 indi­vid­u­als. Each box (male) and cir­cle (female) rep­re­sents an indi­vid­u­al with two homol­o­gous chro­mo­somes as bars. The top row shows three cou­ples with their chro­mo­somes col­ored dif­fer­ent­ly. Due to cross­ing over, off­spring inher­it recom­bi­nant chro­mo­somes of their par­ents. The first order cousins in the bot­tom row share one IBD seg­ment (bor­ders marked by grey lines). Both have inherit­ed this IBD seg­ment from the same indi­vid­u­al, name­ly their grand­fa­ther (orange col­ored chro­mo­some in the top row). Dia­gram cour­tesy of Gklam­bauer, Wiki­me­dia Com­mons.

The IBS (iden­ti­ty by state) def­i­n­i­tion refers to the fact that at some point two indi­vid­u­als, even if they are not relat­ed to each oth­er, present the same allele at a speci­fic locus. Because of their un-relat­ed­ness this sim­i­lar­i­ty prob­a­bly arose from a sim­i­lar muta­tion­al event. On the oth­er side, with IBD two indi­vid­u­als hap­pen to share the same allele because of their coances­try.

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