Blood kinship charts decide who may marry and who inherits
Consanguinity means shared descent from a common ancestor—Latin for blood relationship. Roughly 20% of people live where cousin marriage is preferred, yet most laws bar close-kin sex and marriage, almost universally to the second degree. Tables and the Knot System number how near two relatives sit.
Roman civil law, followed by early church canon law, barred marriage within four degrees. Around the ninth century the church raised the limit to seven and began counting only the generations back to the shared ancestor instead of up and down again. The Fourth Lateran Council of 1215 returned the limit to four while keeping the newer count, so fourth cousins could marry without dispensation. The earlier, stricter ban provoked a schism in the Croatian church in the eleventh century, and some European nobles later invoked close kinship as convenient grounds to end marriages.
Among the Christian Habesha of Ethiopia and Eritrea, children learn their paternal line at least seven generations deep, because a shared patrilineal ancestor within that span rules out marriage; the maternal limit is about four generations. The Quran's list of forbidden partners omits first cousins, and Muhammad married his cousin Zaynab bint Jahsh. The UAE since 2004 and Qatar since 2009 require premarital screening for inherited blood disorders, withholding the marriage grant from couples who test positive.
Genetically, fewer distinct ancestors raise the chance that a child inherits two copies of a harmful recessive allele. Wright's 1922 coefficient of relationship gives 0.5 for parent and child or for siblings, 0.25 for an aunt or uncle and 0.125 for first cousins, and unions of second cousins or closer count as consanguineous. At least 8.5 percent of children worldwide have related parents. Rates run high in parts of the Arab world: around 40 percent of Egyptians marry a cousin, and 67 percent of Saudi marriages are between close relatives.
Source: Consanguinity
