Marrying within the family not only ups the risk that your children will have a genetic disorder, but also makes them more susceptible to some infections. William Amos at the University of Cambridge and colleagues analysed genetic data from communities in Gambia and India, where marriage within the extended family is common. In Gambian families where children had TB or hepatitis B, parents were significantly more likely to be related than in families with healthy children (Biology Letters, DOI: 10.1098/rsbl.2009.0133). Amos says this is probably because the children have less diversity in the genes giving rise to the immune system.
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