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The Dangers of Human Inbreeding
Human inbreeding can be a huge health hazard because it makes people more susceptible to genetic diseases, and in nature populations with fewer members are usually less fit.
For example, our genomes are not perfect, they are dotted with mutations. Most of these are harmless and recessive, which means that two copies of the mutated gene are required for the trait to be expressed. If two people with the recessive mutation mate and have children, then there is a 25% chance of their offspring contracting the condition. In smaller populations the likelihood of two carriers for a condition - that is unaffected people with the recessive gene - mating increases. The consequences of human inbreeding can include miscarriages, and post neonatal death.
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India's Caste System
Genetic studies of India's caste system have revealed that a small group of ancestors found most Indian groups. That was the conclusion of a genetic study conducted on the DNA from 25 groups across India by the Broad Institute in Cambridge, Massachusetts. The researchers found that inbreeding created many distinct genetic differences between the castes, and that genetic groups had been isolated from each other for thousands of years.
These groups are a mixure of genes from just two ancestral populations. The first group is related to people from Europe, the Middle East, and Central Asia, and the second group to people who hail from the subcontinent.
Whilst the research revealed that the caste system was not an invention of colonialism, as the current distinctions are much older it also has health implications because of the potential hazards of being a member of an isolated group. For example Ashkenazi Jews are a genetically isolated group that suffer from a higher incidence and increased risk of genetic diseases such as Tay-Sachs.
This research may help to explain why some genetic conditions are more common in India than elsewhere. For example a 25 base pair mutation in the MYBPC3 (cardiac myosin binding protein C) gene raises the risk of heart failure by seven times, and is found in about 4% of the Indian population. It is rare elsewhere in the world.
There is plenty of anecdotal evidence that recessive diseases are quite common in India, but they haven't been studied in any systematic way. The next steps are to look for the causal genes amongst these groups, and to study and assess more Indian populations.