Inbreeding, the mating of biologically related people, can lead to genetic defects and vulnerability to diseases. The degree of relatedness varies, but marriages between cousins or second cousins were common in the past. Inbreeding can cause a shrinking gene pool and increase the likelihood of genetic defects. Culling can reduce negative traits, but genetic testing can eliminate concerns about passing on certain diseases. Incest taboos exist in many cultures, but some allow marriage between close relatives. The European monarchy and ancient Egyptians practiced inbreeding, leading to serious diseases and infant mortality.
In human populations, inbreeding refers to two biologically related people mating and having children. The degree of relatedness isn’t always fully defined, but a marriage between two cousins or second cousins is one example, and these marriages are still legal in many places today and were considerably common in most parts of the world several hundred years ago. From a health perspective, inbreeding poses problems because it can cause a higher likelihood of genetic defects or vulnerability to certain diseases. This is why many countries and cultural groups discourage its practice.
The actual health risks to consanguineous children depend on the degree of relatedness of the parents and past history of family consanguinity. Where it is common for people with many connections to each other to have children, the available gene pool is also more likely to shrink with each generation. In areas geographically separated from other localities or where, for other reasons, inbreeding is most likely to occur, certain genetic defects begin to show up more frequently, and the overall blood relationship between mates often leads to offspring with similar characteristics such as being more small in size and with lower fertility levels.
If an entire population area or other group practices inbreeding on a regular basis, one thing that occurs in trying to correct this is a process called culling. A high infant mortality rate, infant mortality rate, or inability to reproduce can reduce the amount of negative traits that are passed on, leaving only those people who have the strongest genetic makeup. This could increase the survival rates of future generations and select for more desirable genes.
There are well-known examples of systemic inbreeding. For many centuries, the European monarchy typically only married into the nobility, and many marriages occurred between people with one or more relational ties to each other. This practice, repeated over time, led to the expression of serious diseases that often affected infant mortality. Long before the existence of a European monarchy, other cultural groups such as the ancient Egyptians practiced marriage between much closer relatives. Egyptian kings often married their sisters.
Culturally, taboos have arisen regarding inbreeding, and many of these have existed long before the genetics of matter were fully understood. Strong incest taboos exist in many cultures, prohibiting the routine practice of mating between siblings or parents. Some cultures extend this and consider cousin marriage to be unacceptable as well. Conversely, in other cultures past and present, marriage is acceptable only if the two people have a defined level of relationship; for example, nieces must marry uncles. Alternatively, some groups appear to be so aware of the potential genetic risks of inbreeding that people cannot marry anyone from their geographic area and have to find mates in other cities or tribes.
Related people who want marriage could use genetic testing to determine some risks to the offspring. The tests don’t check everything, but they could eliminate concerns about passing on some diseases, such as autosomal recessive inherited diseases, which are passed on to children as high as 25% when both parents have a gene for the condition. With related mates, the likelihood that both carry a gene for these types of disorders is high.
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