All other traits, such as hair colour, height and intelligence, varied by different degrees between each brothers in a pair and between all sets of brothers. The only trait unequivocally shared by all 818 men was being gay. The team combed through the samples, looking at the locations of genetic markers called single nucleotide polymorphisms (SNPs) – differences of a single letter in the genetic code – and measuring the extent to which each of the SNPs were shared by the men in the study.
