Genetic genealogyThe fact that the Y chromosome (Y-DNA) is paternally inherited enables patrilines, and agnatic kinships, of men to be traced through genetic analysis. Y-chromosomal Adam (Y-mrca) is the patrilineal human most recent common ancestor, from whom all Y-DNA in living men is descended. Y-chromosomal Adam probably lived between 60,000 and 90,000 years ago, judging from molecular clock and genetic marker studies. Early medical theoriesIn ancient medicine there was a dispute between the one-seed theory, expounded by Aristotle, and the two-seed theory of Galen. By the one-seed theory, the germ of every embryo is contained entirely in the male seed, and the role of the mother is simply as an incubator and provider of food: on this view only a patrilineal relative is genetically related. By the two-seed theory, the embryo is not conceived unless the male and female seed meet: this implies a bilineal, or cognatic, theory of relationship. It may be significant that Galen lived at about the same time that Roman law changed from the agnate to the cognate system of relationships. Common to both theories was the mistaken belief that the female emits seed only when she comes to orgasm. Given that assumption, the evidence for the one-seed theory is the fact that a woman can conceive without coming to orgasm (though this was still a matter of dispute in the ancient world and the Middle Ages[2]). The evidence for the two-seed theory is the fact that a person can look like his or her maternal relatives. These two facts could not be reconciled until the discovery of ovulation in the early 1800s, confirming the two-seed theory as biological fact and dissociating the production of female seed from the occurrence of the orgasm. In early Greek and Roman history, a few philosophers claimed that although every child has one absolute mother, it did not follow that every child had one absolute father. They suggested that a child's genetic character could be influenced by the seed of two or more men if they had inseminated the same mother. This was considered a fringe theory even in its time, however, and was never widely accepted. Traces of such a theory appear to underline various myths of a hero (such as Heracles) with both a human and a divine father. Roman lawThe terms "agnate" (for patrilineal relatives) and "cognate" (for all relatives equally) are taken from Roman law. In Roman times, all citizens were divided by gens (clan) and familia (sept), determined on a purely patrilineal basis, in the same way as the modern inheritance of surnames. (The gens was the larger unit, and was divided into several familiae: a person called "Gaius Iulius Caesar" belonged to the Julian gens and the Caesar family.) In the early Republic, inheritance could only occur within the family, and was therefore purely agnatic. In Imperial times, this was changed by the Praetorian edict, giving paternal and maternal relatives equal rights. In the BibleThe line of descent for monarchs and main personalities is almost exclusively through males. Tribal descent, such as whether one is a kohen or a Levite, is still inherited patrilineally in Judaism, as is communal identity as a Sephardi or Ashkenazi Jew.[3] This contrasts with the rule for inheritance of Jewish status in Orthodox Judaism, which is matrilineal. See Davidic line and Matrilineality in Judaism.[4][5] See also
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