INTRODUCTION

While it is generally accepted that inbreeding in humans can lead to increased incidences of genetically-determined abnormalities, due to the expression of rare, deleterious, recessive genes in the homozygous state, consanguineous marriages are common in many communities, for example, the four southern states of India, Andhra Pradesh, Karnataka, Kerala and Tamil Nadu (Kumar et al, 1967; Rao & Inbaraj, 1977a; Rami Reddy & Chandrasekhar Reddy, 1979; Radha Rama Devi et al, 1981). Although infectious diseases and nutritional disorders are still common in India, their incidences have declined markedly during the last twenty years. Therefore it seems probable that among the populations of South India (Fig. 1), which in the 1981 Census totalled over 164 millions, a transition from an almost exclusively environmental to an increasingly genetic pattern of disease is currently under way, similar to that earlier observed in countries such as Great Britain (Roberts et al, 1970). However the emerging situation in the South Indian states almost certainly will be of greater complexity than in Western countries because of the long inbreeding tradition. Indeed, it has been suggested that the high levels of inbreeding practised by the Dravidian peoples for at least 2,000 years (Centerwall et al, 1969) would have led to the gradual elimination of deleterious lethals and sublethals from the gene pool by segregation (Sanghvi, 1966). This theory has been questioned (Chakraborty & Chakravarti, 1977; Bittles, 1980) but supporting evidence was claimed in large-scale prospective and retrospective studies conducted in Tamil Nadu (Rao & Inbaraj, 1977b, 1979a,b, 1980).