Consanguinity

Consanguinity ("blood relation", from the Latin "") means being of the same kinship as another person. Consanguinity also means being descended from the same ancestor as another person. The laws of many jurisdictions set up degrees of consanguinity when prohibiting sexual relations and marriage. Rules of Consanguinity are also used to determine heirs of an estate. These are according to laws that govern intestate succession. These vary from jurisdiction to jurisdiction.
History.
Most cultures have prohibited marriages between individuals who are closely related by blood (consanguinity). There were some exceptions. Brothers and sisters, as well as fathers and daughters, often married in ancient Egypt. These were primarily limited to the royal family from the earliest times. Some theories hold that it strengthened a King's claim to the throne. In biblical history, Abraham married his (half-)sister Sarah.
Most early societies had rules or laws that limited consanguineous marriages. So the dangers of inbreeding must have been known to them. 
Roman civil law.
In ancient Rome, marriage was governed by civil laws. Under Roman civil law couples were forbidden to marry if they were within four degrees of consanguinity. The Germanic peoples also had rules against such marriages but were not as strict. 
Medieval Europe.
In the 5th century, after the Roman empire collapsed, the church took over regulating marriage. At first they did not have a great deal of influence. The Church's power over marriage grew gradually. At first the Church followed Roman civil law. In the ninth century the church raised the number of prohibited degrees to seven. They also changed the method by which they were calculated. The nobility of Europe usually married from within their own social class. Eventually they became too interrelated to marry by the Church's rules. Many found it necessary to defy the Church. In 1215 the Fourth Lateran Council reduced the number of prohibited degrees of consanguinity from seven back to four. The pope had the power to waive the rules for particular couples. Frequently, papal dispensations were given allowing closely related couples to marry. These were almost always reserved for the royalty and nobility. Sometimes the Church used consanguinity to block marriage as in the case of William the Conqueror and Matilda of Flanders. In 1049 Pope Leo IX refused to allow the marriage. He did not give a reason but the two were cousins. They married anyway. Ten years later, in 1059, Pope, Nicholas II finally granted a dispensation for their marriage. Despite restrictions and laws, up to the 20th century consanguineous marriages were not uncommon in Europe.
Common ancestor.
The degree of consanguinity can be illustrated with a consanguinity table. Each level of lineal consanguinity ("i.e.", generation or meiosis) appears as a row. Individuals with a collaterally consanguineous relationship share the same row. Consanguinity (blood relationship) is different from Affinity (relationship through marriage) but have been prohibited in the same degrees in the past.
References.
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