Jews are actually a race. I was born from a Jewish father and mother, so I remain a Jew until I die.
I have to admit, I was going to refute this, but when I went to get the article that I planned to refute it with, I found that I was wrong.
A male receives his y-chromosome unchanged from his father. Barring mutation, this chromosome should be identical if traced back through the patrilineal line. Studies of y-chromosomes have been used to chart human migration, the male equivalent of the more famous mitochondrial DNA studies that were popularized in books like The Seven Daughters of Eve (which totally ignores the fact that there are an additional 12 mitochondrial DNA lines in Africa, but Nineteen Daughters just doesn't scan).
Anywho, if the Kohens are descended from Aaron, they should all have the same y-chromosome. And in a study of 188 Jews from Israel, England, and North America, of 106-self described Kohens, 97 had what is now called the Cohan Modal Haplotype.http://www.cohen-levi.org/jewish_genes_and_genealogy/the_dna_chain_of_tradition.htm
I was aware of the CMH prevalence, but I was going to argue that non-Kohens are not necessarily part of that race. After all, my family is not descended from the Hebrews, but from the Kuzar tribe, which once lived in what is now Turkey, but which migrated into what is now the Ukraine about 1500 years ago and converted to Judaism about 300 years later. I read this in a book on patterns in human migration, and it was confirmed by my cousin, who is a Talmudic scholar.
However, based on genetic studies, "This genetic research has clearly refuted the once-current libel that the Ashkenazi Jews are not related to the ancient Hebrews, but are descendants of the Kuzar tribe--a pre-10th century Turko-Asian empire which reportedly converted en masse to Judaism. Researchers compared the DNA signature of the Ashkenazi Jews against those of Turkish-derived people, and found no correspondence."