Reply #151: Yes - this is very likely the reason for the taboo [View All]

[LeftishBrit]

First cousin marriages do result in an increased risk of genetic disorders in the offspring, due to the greater chance of a child being born with both of the recessive genes linked to a disorder. We all have several abnormal recessive genes, but usually they are balanced by normal genes. Children of first-cousin marriages are more likely to inherit the same abnormal recessive gene from both parents, and so to exhibit the disorder.

For first cousins, the risk is increased but not enormous. For closer relatives, it is much greater. Children of brother-sister or parent-child unions may have as high as a 50% chance of perinatal death or serious disorder; and this may explain why such unions are forbidden in almost all societies.

By the time you get to second cousins, there is only a very slightly increased risk for the offspring, and most societies will accept such marriages.