The Effects of Inbreeding

Inbreeding is a complex issue with complex genetic consequences. Inbreeding does increase prevalence of autosomal recessive disorders. But in the most common interbred marriages in the world, cousin marriages, the percentage increase in risk is not by ver

In the United States, there is a large negative social stigma associated with consanguious marriages, known as inbreeding, but derogatorily and commonly referred to as ‘incest’. A consanguious relationship is a relationship between two people who are related through a common ancestor (at least one). Incest is normally thought to have been between siblings or parent and child (1st degree related, share ½ of their genes), but can also include the uncle-niece, aunt-nephew, and grandparent-grandchild relationships (all 2nd degree related, share ¼ of their genes) in the biological sense. Consanguious marriages can also include first cousins (share 1/8 of their genes) marriages.

History has several examples of genetics disorders and inbreeding. A famous one is the intense royal inbreeding that led to the disorder of the Habsburg Lip (jaw/chin). The royal House of Habsburg family practiced intermarriage over a time period of 200 years. It was so drastic that by the end of the regime that Charles II, the last Spanish Habsburg, was unable to properly use his jaw, limiting his speech and making it difficult for him to chew food. Charles II also had many other genetic disfunctionalities, such as distal renal tubular acidosis and pituitary hormone deficiency, which could have left him unfertile and unable to produce a child to continue the reign of his royal dynasty. Gonzalo Alvarez, a researcher at the University of Santiago de Compostela, led a study that evaluated the family. He deemed that the family inbred so much that the marriages between the close family members could have been equated to the same diseases prevalent in an incestuous relationship between siblings. The repeated and constant inbreeding within a very small gene pool resulted in the prominence of rare recessive genetic disorders, such as the jaw disorder mentioned above.

It is commonly believed today that any and all inbreeding can result in genetic disorders. It is true that inbreeding between cousins (which is more common in Eastern countries and Europe) can result in an increased physical prevalence of more recessive disorders, but not by a significant amount for most diseases. 

Currently, about 24 states in the United States do not allow consanguious marriage between first cousins. Although associated with a strong negative stigma and deemed by society to be ‘wrong’ in most North American countries, the practice of inbreeding is common in some eastern cultures, such as in the Middle East, Asia, and Africa, and is allowed in Europe. In a report conducted by the National Society of Genetic Counselors, it was determined that ‘‘in some parts of the world, 20 to 60 percent of all marriages are between close biological relatives.''

The data from the report, published in The Journal of Genetic Counseling, showed that cousin marriages are actually not as bad as they are commonly thought to be and for most diseases, only add a small percentage increase of risk. There is a 1.7-2.8% increase in risk for major congenital defects and a 4.4% increase for mortality (before the child is born). Due to the assumed significant increase in risk and misinformation about cousin marriages, the researchers recommend the same genetic counseling, advising, and tests that other couples go through and suggest that laws preventing marriage between cousins should be eliminated. Risk of deafness and uncommon metabolic disorders may be more likely in cousin marriages. These problems, although not very prevalent, may arise due to similar genes and both cousins can be carrying the autosomal recessive gene, leading to a 1 in 4 chance that the child inherits the disease, such as cystic fibrosis. The risk is only slightly lower in people who are unrelated (Bennet et al, 2002). The report did not discuss relationships of repeated familial inbreeding relationships with a limited gene pool.

Although there has been a negative perception, with historic examples, of inbreeding, it is and has been common for centuries. Cousin inbreeding is the most common noted form of inbreeding in the world today, popular in the UK, Middle East, Africa, and Asia. Inbreeding does increase the risk of autosomal recessive disorders, but only slightly in cousin marriages. The more intense inbreeding of the House of Habsburg, with its very limited gene pool, resulted in severe jaw impairment, known as the Habsburg lip. More research needs to be conducted in regards to other consanguious relationships for information to be learned about the genetic implications that may result, for example the genetic consequences of incestual relationships.  


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