On differing aspects and perception of consanguineous marriages

Consanguineous unions are important because they are very common, accounting for approximately 10% of all marriages in the world. This prevalence is significantly higher than commonly perceived. Consanguinity is often associated with higher frequencies of congenital malformations and increased mortality among the offspring of such marriages. However, the harmful consequences of inbreeding may have been overestimated because the inherent taboos against consanguinity tend to bias the assessment of its genetic dangers. Inbreeding increases the speed of selection of many recessive and codominant alleles. Notably, among such alleles are α⁺_·-thalassemia, hemoglobin C, and Duffy antigen negative red blood cells, which protect against the death from malarial parasites. α⁺_·-Thalassemia, the most common monogenic condition in humans, is conspicuously superimposed on both geographic distributions of P. falciparum and high rates of consanguinity in populations, which suggests a causal link. Computer simulation studies have shown that in regions where malaria is prevalent, inbred populations with α⁺_·-thalassemia grow faster than outbred populations. Whether this faster growth compensates the excess of deaths in consanguineous populations due to harmful recessives remains to be determined. Nonetheless, it is possible that in these regions, consanguinity was genetically beneficial and evolved as a survival strategy - to counter high population mortality from malaria and perhaps other infectious diseases. In general, multiple social benefits are thought to be the reason for the persistence of high levels of consanguinity in populations. However, the social benefit theory does not explain why non-consanguineous societies, despite the same benefits, do not practice consanguinity. Furthermore, results of computer simulation of the genetic benefits of consanguinity fits into the historical conditions which existed during the agrarian revolution, when multiple epidemic infections emerged in rapidly enlarging populations. Besides malaria, this theory of genetic benefits is also applicable to many other infectious diseases like tuberculosis and HIV, the protection being due to the codominant MCP-1 and recessive CCR5δ32 alleles, respectively. This article is an attempt to give a rational and objective view of consanguinity integrating different aspects from human genetics, history, psychology, anthropology and clinical medicine.