Consanguinity

Consanguinity refers to the genetic relationship among two or more people who are blood relatives. In other words, consanguinity describes the relationship among people who descend from a common ancestor. Parents, children, brothers, sisters, aunts, uncles, cousins, grandparents, and others all share consanguineous familial relationships. Even though the amount of shared genetic material among family members decreases as the familial line grows more distant, everyone in a given family tree is considered to share a consanguineous relationship to some extent. Marriage between people who share a consanguineous relationship, while accepted and even encouraged in some cultures, has long been a subject of controversy in the Western world. In part, this is because the genetic closeness that consanguineous couples share can lead to genetic disorders in their offspring. Because of this risk and other potential social and legal consequences, physical relationships between blood relatives, or incest, is widely considered a cultural taboo.

Background

The term consanguinity is derived from the Latin words con, which means "shared," and sanguis, which means "blood." Historically, experts can trace the issue of consanguinity and marriage all the way back to ancient Egypt. The ancient Egyptians permitted incestuous marriages between brothers and sisters. Such marriages took place primarily within royal families to protect their claim to power. Outside of royal bloodlines, however, marriage between closely related people was typically prohibited.

The Catholic Church started taking an active role in policing incestuous relationships during the Middle Ages. The extent to which the church restricted consanguineous marriage varied over time. Up until the thirteenth century, the church prohibited marriages with consanguinity to the seventh degree. This restriction was later lowered to the fourth degree to permit the marriage of more distantly related individuals. The pope also had the authority to waive the church's restrictions on consanguineous marriages in specific cases. Often, this sort of special dispensation was reserved only for royalty. In some instances, authorities granted dispensation on a cultural scale. In 1537, for example, Pope Paul III lowered the restriction on consanguineous marriage to relatives of the second degree for Native Americans and the indigenous people of the Philippines.

Since the Middle Ages, countries and cultures around the world have developed their own customs and laws relating to consanguineous marriage. Most are based on the church's guidelines or those of other religious authorities. As a result, consanguineous marriage remains restricted in most places even as the exact laws concerning such relationships have evolved over time. In some cultures, consanguineous marriage is still permitted. This practice is most common in North Africa, West Asia, and the Middle East, and it often occurs through arranged marriages between members of the same clan group. In most cases, this type of consanguineous marriage is preferred because the parents are familiar with both spouses' full family history and lineage. This practice demonstrates the varying views and perspectives on consanguinity and marriage that occur among different cultures around the world.

Overview

There are two main types of consanguinity: lineal consanguinity and collateral consanguinity. Lineal consanguinity refers to any familial relationships that occur in a direct line, such as parent to child and grandparent to grandchild. Collateral consanguinity refers to any indirect family relationship in which two or more people are related through a common ancestor but do not descend from each other. It is also important to note that consanguinity is different from affinity, which refers to any familial relationship based on marriage rather than blood. Specifically, one is related to their spouse, in-laws, step-relatives, and others through affinity.

Consanguinity is typically measured in degrees. First-degree relatives include one's parents, siblings, and children. Second-degree relatives includes one's grandparents, aunts, uncles, nephews, nieces, and half-siblings. Third-degree relatives include one's first cousins, half-uncles and aunts, and half-nieces and nephews. Beyond this point, consanguineous relationships become much more complicated. The most common of these relationships are first, second, and third cousins. First cousins are relatives with whom one shares two of the same grandparents. Second cousins are relatives with whom one shares only a set of great-grandparents. Third cousins are relatives with whom one shares a set of great-great-grandparents. Cousins are also sometimes referred to as being once removed or twice removed. These terms refer to generational separations between relatives. If a woman's father has a first cousin, that relative would be the woman's first cousin, once removed. Twice removed is used when there is a two-generation separation between relatives. Family relationships can get even more complicated when siblings from one family marry siblings from another family. Because the resulting children share all four grandparents, they are referred to as double cousins.

The degree of consanguinity also determines how much genetic material different relatives share. Identical twins, who share all of the same genes, have the greatest possible consanguineous relationship. First-degree relatives share about half of their genes, second-degree relatives share about a quarter of their genes, and third-degree relatives share about an eighth of their genes. This sharing of genes is a key part of why consanguineous marriage is problematic from a biological perspective. Many people carry recessive genes for serious genetic disorders without ever actually having these disorders or exhibiting any symptoms. The more genetic material a couple shares, the greater the chance that their children will develop such genetic disorders or suffer other birth defects. Children of closely related parents are at an especially increased risk of inheriting or developing such serious conditions as retinal and corneal degenerative diseases, Down syndrome, congenital heart defects, intellectual disabilities, and hemophilia. Because of these and other problems, such children also face an increased likelihood of neonatal or childhood death. As a result, consanguineous marriages can have serious consequences for any offspring of couples who share a significant amount of genetic material. These consequences are the main reason why consanguineous marriages are generally frowned upon and even legally outlawed in many modern societies.

Bibliography

Bittles, Alan H. Consanguinity in Context. Cambridge UP, 2012.

"Consanguinity Fact Sheet—Debunking Common Myths." LaraSig, www.larasig.com/node/2020. Accessed 10 Oct. 2017.

Eckerle, Judith K. "Consanguinity." University of Minnesota Adoption Medicine Clinic, adoption.umn.edu/consanguinity. Accessed 10 Oct. 2017.

Hamamy, Hanan. "Consanguineous Marriages: Preconception Consultation in Primary Health Care Settings." Journal of Community Genetics, vol. 3, no. 3, 2012, pp. 185–92.

Johnson Lewis, Jone. "Consanguinity and Medieval Marriages." ThoughtCo., 1 June 2017, www.thoughtco.com/consanguinity-and-medieval-marriages-3529573. Accessed 10 Oct. 2017.

"Positive Consanguinity." Florida Hospital, www.floridahospital.com/positive-consanguinity. Accessed 10 Oct. 2017.

Powell, Kimberly. "Cousin Relationships Explained." ThoughtCo., 3 Mar. 2017, www.thoughtco.com/cousin-relationships-explained-3960560. Accessed 10 Oct. 2017.

"When Parents Are Related—Consanguinity." Center for Genetics Education, www.genetics.edu.au/publications-and-resources/facts-sheets/fact-sheet-18-when-parents-are-relatives-consanguinity. Accessed 10 Oct. 2017.