Hamer and colleagues (Science 261: 321-327 [1993 )identified 40 nuclear families in which there were two homosexual brothers In each of the 40 families, they studied the transmission of X-linked SSRs from the mother to the homosexual sons. For an X-linked SSR there are two possible genotypes in each son, and thus there are four possible combinations of genotypes in the two sons C/Y A/B AY AY Identical by descent(IBD) B B AY B Nonidentical B AYHamer and colleagues (Science 261:321-327 [1993]) identified 40 nuclear families in which there were two homosexual brothers. C/Y A/B Identical by descent (IBD) A/Y A/Y B/Y B/Y Nonidentical A/Y B/Y B/Y A/Y In each of the 40 families, they studied the transmission of X-linked SSRs from the mother to the homosexual sons. For an X-linked SSR, there are two possible genotypes in each son, and thus there are four possible combinations of genotypes in the two sons: