Table 2. Expected extent of IBD and number of cousins for 1st–10th degrees of cousinship.

Degree of cousinship	Expected amount of IBD (cM)a	Chance of detecting nth cousin (%) with IBDhalf b	Expected number of cousinsc	Expected number of detectable cousins (Ndc)d
1	900	100	7.5	7.5
2	225	100	38	38
3	56	89.7	190	170.4
4	14	45.9	940	431.5
5	3.5	14.9	4,700	700.3
6	0.88	4.1	23,000	943
7	0.22	1.1	120,000	1,320
8	0.055	0.24	590,000	1,416
9	0.014	0.06	>106	NAe
10	0.0034	0.002	>106	NAe

^aTheoretical expectation of the amount of IBD across the genome shared between nth cousins, assuming 3600 cM across the entire genome. It should be emphasized this description assumes a single common ancestor for a pair of cousins; multiple shared common ancestors will increase the predicted IBD sharing.

^bThe fraction of nth degree cousins detected using our IBD algorithm and based on simulated pedigrees of up to 10th degree cousins (see Methods ).

^cAssuming a specific model of pedigree and population growth over the past 11 generations (see Methods ).

^dThe expected number of cousins detectable with our IBD algorithm (N^dc) was calculated by multiplying the probability of detecting an nth cousin by the number of nth cousins obtained from our pedigree model of population growth (see Methods ).

^eGiven the variation in population growth at >9 generations ago, combined with a low power of detection for 9th or 10th cousins, we have indicated the number of detectable cousins for those categories as not applicable, “NA”.