If the region of the X chromosome being tested plays no causal role in male homosexuality, then the four possible combinations should be equally likely, and identity by descent and nonidentity should be equally likely IBD Nonidentical Expected 20 20 On the distal long arm of the x chromosome hamer and colleagues observed a dramatic departure from random expectations among the 40 families IBD Nonidentical Observed 33 O-E Chi-square (3320)2+(720 16.9 20 EIf the region of the X chromosome being tested plays no causal role in male homosexuality, then the four possible combinations should be equally likely, and identity by descent and nonidentity should be equally likely. Expected IBD Nonidentical 20 20 On the distal long arm of the X chromosome, Hamer and colleagues observed a dramatic departure from random expectations among the 40 families: Observed IBD Nonidentical 33 7 Chi-square = (33-20)2 + (7-20)2 20 20 = 16.9 (O − E) 2 E ∑ =