Table 1.

Description, number, and type of affected sibling pairs in the 242 families studied

		Type of affected sib pair
No of affected sib pairs in each family	No of families	CD	UC	Mixed	Total affected siblings	Other affected relatives
1	199	96	59	44	398	112
2	4	3	4	1	16	—
3	32	26	46	24	96	4
4	3	8	3	1	15	—
6	4	15	3	6	16	1
Total	242	148	115	76	541	117

To arrive at the total number of sib pairs in each affected family, imagine sibs A, B, C, D and E; two affected sibs are required for one sib pair, for example, AB; three affected siblings make three sib pairs, for example, AB, BC, AC; four affected siblings make six sib pairs AB, AC, AD, BC, BD, CD; if D and E are the step brother/sister of A, B and C and all are affected, there are four sib pairs AB, AC, BC, DE. See also Hauser and colleagues.⁴³

CD, Crohn's disease; UC, ulcerative colitis.