Hello,
Many posters on this thread may be very interested in the new book Legacy: A Genetic History of the Jewish People, by Harry Ostrer (Oxford, May 2012).
According to genetic testing that has been done, Levites and Cohanim each represent approximately 4% of Ashkenazi Jewish males (the percent of Levites is somewhat lower among Sephardic Jewish males). This statistic comes from page 100 of this book and references a 1999 publication — Bradman, Thomas, and Goldstein’s The Genetic Origins of Old Testament Priests, found in Population Specific Polymorphisms, pages 31-44 (Cambridge University Press).
Legacy contains results of myriad genetic testing studies done up through 2011, and Dr. Ostrer has been involved in much of the research — he is Director of Genetic and Genomic Testing at Montefiore Medical Center and Professor of Pathology and Genetics at Albert Einstein College of Medicine. The research that he discusses in Legacy goes into much depth regarding many of the issues debated in this thread, especially the idea that many Jews (in particular, Ashkenazim) have Jewish heritage primarily as a result of conversions of local populations in the distant past — an idea that is not born out by the research results covered in this book.
I found the book fascinating and could hardly put it down!