We analysed microsatellite genotypes sampled from the Pakistani and Indian communities in Nottingham, UK, to investigate the genetic consequences of substructuring mediated by traditional marriage customs. The application of a recently developed likelihood approach identified significant levels of population substructure within the Pakistani community as a whole, as well as within the finer divisions of castes and biradheri. In addition, high levels of cryptic or unacknowledged consanguinity were detected within subgroups of this community, including biradheri. The Indian sample showed no significant evidence of either substructure or consanguinity. We demonstrate that estimates of disease gene frequencies can be inaccurate unless they are made jointly with estimates of population substructure and consanguinity ((θ≡FST) and C). The magnitude of these estimates also highlights the importance of accounting for the finer scale of social structuring when making decisions regarding the risk of recessive disorders in offspring.