The title of cohen is paternally inherited. Most Jewish men surnamed Cohen are also cohanim, but so are many men with other surnames. Tradition traces the lineage back to Aaron, the first high priest and Moses' brother, who is said to have lived about 3,300 years ago.
Now a genetic study of modern-day cohanim has provided the first scientific evidence supporting the oral tradition of an ancient priestly lineage. Karl Skorecki, a physician who studies the genetics of kidney disease at the Technion in Haifa, Israel, and colleagues in London and the United States realized that they could study the lineage of priests by looking at the Y chromosome, which only men carry. Unlike all other chromosomes in our cells, the Y chromosome, which bears the male sex-determining gene, is passed essentially unchanged from father to son, barring rare mutations.
The researchers extracted dna from cells scraped from the inner cheeks of 188 unrelated, self-identified cohanim from Israel, North America, and Britain. They then looked for the presence or absence of a well-studied small stretch of dna on the Y chromosome called yap. They found that only 1.5 percent of the cohanim in their study carried the yap stretch. In contrast, the yap occurred in 18.5 percent of a group of lay males in the study. Furthermore, the cohanim were much more likely to carry a specific variant of another length of dna found elsewhere on the Y chromosome than their lay counterparts, suggesting that they share a common ancestor who had this genetic signature.
The study also suggests that the ancestor of these cohanim predated the split of the Jewish community into Sephardic Jews, of Spanish and North African descent, and Ashkenazim, who are of German and Eastern European descent. This division occurred between one and two thousand years ago. Only a small percentage of the priests in each group carried the yap dna. By further study of these genetic markers, Skorecki, who is himself a cohen, hopes to construct a biological timetable that will both lead back to the ancestor of the priests and tell us when he lived. "It's like an archeological discovery where you find some manuscript that confirms an ancient oral tradition," he says, "except instead of digging in the dirt, we dug into the genome."
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