Consanguineous marriages 1-2

Universal incest taboo between parents and children. Nearest consanguineous marriages are are between first cousins or uncle and niece marriages. Among khasis muslims Reddys. 

First cousins share 12.5% of genetic material uncle and niece share 25%.

Increases homogeneity of genetic traits. Likelihood of expression of deleterious genes which are filtered over generations. Population is sustained by large numbers. Traits like thalassemia, webbed feet, sickle cell anaemia, Tay Sachs disease, cystic fibrosis.

Is population is small it can create lower fertility and more genetic defects.

Afzal and Sinha 1984 study on Ansari Muslims Bhagalpur Bihar. Child mortality among inbred children was 36% while 28% in outbred population.