Table 1.

Expected Mean IBD Proportions for the Outbred Familial Relationship Categories

Familial Relationship	IBD0	IBD1	IBD2
Parental	0	1	0
Full-sibling	0.25	0.5	0.25
Half-sibling, avuncular, and grandparental	0.5	0.5	0
First-cousin, great-grandparental, great-avuncular, and half-avuncular	0.75	0.25	0
Distantly related	varies	varies	0
Unrelated (includes relationships beyond the third degree)	1	0	0

IBD0, IBD1, and IBD2 are the genome proportions shared on 0, 1, and 2 chromosomes, respectively, between two individuals. Many relationships share the same expected mean IBD proportions; however, for full-sibling, second-degree, and third-degree relationships, a variance around the expected mean is due to the random nature of recombination events. Genotyping and other technical errors can contribute to this variance.