I am confused on the interpretations of DNA testing and Jewish ancestry. I have read here, in this forum, that DNA tests indicating Cohen blood are meaningless. Why would such a statement be made, what justification can one have for ignoring ancestral, proof by DNA testing, evidence of specific Jewish bloodlines? I have found two distinct and contradicting Rabbinical opinions on your site regarding this subject. In opinion 1, Rabbi Blass states that DNA testing proves nothing, by stating " The test you suggest doesn’t make one Jewish either halachically or in the eyes of the State of Israel." In opinion 2, Rabbi Lewis plainly states, " Judaism is not defined by practice. If you were born to a Jewish mother you are Jewish. Even if you don’t practice. Even if you don’t believe in G-d. Even if you declare yourself non Jewish – you still are." Which is correct?! One can prove through DNA testing that one’s mother is Jewish, i.e. "born to a Jewish mother", even though she may refuse to admit that. By the statement of Rabbi Lewis, then, DNA testing would be proof that one is Jewish if that test showed that one’s mother was of Jewish bloodlines. Yet, Rabbi Blass seems to believe that bloodlines prove nothing and that one’s mother must be "practicing" to be considered to be Jewish. My own studies of the Torah indicate to me that Rabbi Lewis is correct in his statements, which then indicate that Rabbi Blass is mistaken. What is your opinion?

It is common for two rabbis to have different and even contradicting opinions; the Mishna, the Talmud and rabbinic literature throughout the generations are filled with this phenomenon. However, in your question you missed out the point and there is no dispute or contradiction between the two answers. It is agreed by all, that Judaism is defined by ancestral proof and does not have to do with practice of Mitzvot. Similarly it is agreed by all that to be a Cohen you have to be a child of one. The question is HOW to prove that. Classically in Halacha these are to be proved through witnesses and Chazakah. The questions are: Can DNA testing be a halachacly valid proof of descent and therefore of your Jewishness? Is there really a DNA unquestionable proof for being a Cohen? The answer to the fist question is rather complex and there are more than one valid opinion on this issue The second question is simpler and agreed that the DNA Cohen test it is no valid proof for Halachic requirements and how much more so not a proof of Jewishness in the case of non Jewish parents, which was what the referred answer is dealing with.