Human Inbreeding: What are the consequences?

“Inbreeding is breeding between close relatives, whether plant or animal” (1). Inbreeding can occur naturally in plants via self pollination. It is also carried out in animals by breeders to produce pure strains or to amplify a desirable trait. Inbreeding limits genetic diversity in addition to increasing the probability of inheriting any deleterious recessive genes from a parent or ancestral population. The level of inbreeding is determined by ‘inbreeding coefficient’ meaning that the closer the relatives are the higher the inbreeding coefficient is and thus riskier.

Inbreeding is not only limited to plants and animals. Inbreeding was practiced in western societies (before 19th and 20th centuries) and it is still common in some parts of the world. While Muslims of India, Middle-East, and Africa practice inbreeding for cultural and religious reasons, Amish, Mennomites and Hutterites may have practiced it because they were isolated. The high inbreeding coefficient in Dammam, Saudi Arabiais (0.0312) an be attributable to beliefs, traditions, and the desire to keep property within the family. The practice of endogymy in Hindus may arguably produce similar effects as inbreeding over long periods of time. Another reason responsible for persistence of inbreeding practices is socioeconomic status—the rich wanting to marry only the rich to maintain their status and thus looking in their own families for partners and poor only being able to afford a marriage among themselves. Interestingly, some royal societies have also been known to practice inbreeding to protect royal blood lines. For example, the English Royal family has had many hemophiliac members due to inbreeding. Inbreeding was common in Spanish Royal family as well. Historical events also promote inbreeding. After the United States devastated Hiroshima and Nagasaki in Japan, “there was an increase in the number of consanguineous marriages (first cousin level) in the areas surrounding Hiroshima and Nagasaki.”

While some propose that long-term practice of inbreeding can be beneficial to a population as harmful genes are elimination by selection, there have been ample studies suggesting that inbreeding poses threats to a population in a variety of ways such as reduced fertility and increased child mortality/morbidity. A study investigating the effects of inbreeding on the fertility of adult women demonstrated that deleterious recessive alleles received from inbreeding can lower the fertility rates of adult woman. Another study investigated five effects of inbreeding: fertility, mortality and morbidity of the offspring, reproductive performance of the offspring, and characteristics of the offspring. In thus study inbreeding did not have an adverse effect on the fertility of the marriages; however, results showed that inbreeding significantly increases childhood mortality in the first year of life, increases morbidity, and significantly increases numbers of disabled offspring. Moreover, development also seemed to be affected for “the children of consanguineous marriages were significantly older than the control group when they first walked and talked.” However, inbreeding seemed to be beneficial when examining some aspects of health. Offspring from the inbreeding group showed a 14.3% decrease in allergies and a 23.9% decrease in nephritis, a rare genetic disorder that causes inflammation of the kidneys. While these are a studies that investigate effects of human inbreeding, many more animal studies have demonstrated that inbreeding creates health problems. For example, all pure bred dogs have significantly higher and devastating medical conditions.

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