First degree relative

Consanguinity ("blood relation", from the Latin ) is the property of being from the same kinship as another person. In that aspect, consanguinity is the quality of being descended from the same ancestor as another person. The laws of many jurisdictions set out degrees of consanguinity in relation to prohibited sexual relations and marriage parties. Rules of Consanguinity are also used to determine heirs of an estate according to statutes that govern intestate succession, which vary from jurisdiction to jurisdiction.

The degree of relative consanguinity can be illustrated with a consanguinity table, in which each level of lineal consanguinity (i.e., generation or meiosis) appears as a row, and individuals with a collaterally consanguineous relationship share the same row. The Knot System is a numerical notation that defines consanguinity.

Issues of consanguinity arise in several aspects of the law.

Laws prohibiting incest govern the degree of kinship within which marriage or sexual intercourse is permitted. These are almost universally prohibited within the second degree of consanguinity. Some jurisdictions forbid marriage between first cousins, while others do not. Marriage with aunts and uncles (avunculate marriage) is legal in several countries.

Consanguinity is also relevant to inheritance, particularly with regard to intestate succession. In general, the law favors inheritance by persons closely related to the deceased.

Some jurisdictions bar citizens from service on a jury on the basis of consanguinity with persons involved in the case.

In many countries, laws prohibiting nepotism bar employment of, or certain kinds of contracts with, the near relations of public officers or employees.

Under Roman civil law, which early canon law of the Catholic Church followed, couples were forbidden to marry if they were within four degrees of consanguinity. In the ninth century the church raised the number of prohibited degrees to seven and changed the method by which they were calculated. Eventually the nobility became too interrelated to marry as the pool of non-related prospective spouses became smaller. They had to either defy the church's position or look elsewhere for eligible marriage candidates. In 1215 the Fourth Lateran Council made what they believed was a necessary change to canon law reducing the number of prohibited degrees of consanguinity from seven back to four. The method of calculating prohibited degrees was changed also. Instead of the former practice of counting up to the common ancestor then down to the proposed spouse, the new law computed consanguinity by counting back to the common ancestor. In the Roman Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for a declaration of nullity, but during the eleventh and twelfth centuries dispensations were granted with increasing frequency due to the thousands of persons encompassed in the prohibition at seven degrees and the hardships this posed for finding potential spouses.

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