For a century or so, the image of the Jewish mother has been a significant icon in the Jewish consciousness. The sentimental standard "My Yiddishe Momme" - composed in 1925 by two Jewish immigrants to the United States, Jack Yellin and Lew Pollack - has attained such classic status that it was recently described on a blog on TheMarker as "an old, old song of unknown origin." Its appreciative, nostalgic yearning for the self-sacrificing and all-loving (Jewish) mother made it a cross-over hit: It was sung by risque entertainers like Sophie Tucker as well as pious cantors like Yossele Rosenblatt, by Jewish entertainers like the Barry Sisters and by non-Jewish stars like Tom Jones. The same blog even reports that there's an Arabic version.

This image is guaranteed to evoke tears among many, and therefore is an easy target for parody and dismissal by others. Either way, the image of the Jewish mother is a symbol that triggers one's deepest feelings about Jewish identity, for good and for ill.

The nexus between this image and various attitudes toward Jewish identity might seem to make sense, since after all, for at least the last 1,800 years, a person is automatically defined as Jewish if he or she has been born of a Jewish mother.

Absent that condition, a person needs to "become" Jewish through conversion.

Yet, while the Jewish mother has been tradition's significant definer of Jewish identity, she has been largely absent as a significant player from that tradition. It is no coincidence that "My Yiddishe Momme" was written and became a hit precisely at a time when the Jewish world was experiencing severe dislocation and fragmentation - generational, geographic, religious and otherwise. It was only when determination of identity became a problem for Jews that the image of the Jewish mother became compelling.

What was it about the Jewish mother that let her define Jewish identity? Was there a biological component to Jewishness? Tradition seemed to give a mixed message. If biology truly defined identity, how could anyone join the people of Israel by conversion? Furthermore, what was one to make of the tradition's rejection of the father's genetic contribution to the child's Jewishness? The wisdom of the tradition was in adopting these initially perplexing rules - that is, to found Jewish identity on a biological basis that was still open-ended and not genetically determined. Jewish identity was to be founded on the experience of being born into the Jewish people.

One becomes a Jew by emerging from a Jewish womb. The womb can be either the physical womb of a Jewish woman or the symbolic womb (or mikveh) designated as such.

Sadly, this concept has proven too elusive to uphold. Genetic theories of Jewishness were adopted by mystics and other traditionalists. But the racist associations that such an approach could evoke were repugnant to many (though not all) modernists.

What was the alternative? Sentimental warhorse that it is, "My Yiddishe Momme" was one attempt to offer a modern answer. One's Jewish identity, says the song, is a gift bestowed through the unstinting love and dedication of one's mother. It was not her genes that mattered, nor the fact that she gave birth to you, but rather the physical, emotional and spiritual generosity she provided afterward.

One's Jewishness was, then, the memory of that caring relationship. The problem was that such a definition was valid for only a fleeting moment in history. As the atomizing process of modernity has continued apace, the Jewishness of those memories has faded away.

The genetic concept of Jewishness has captured the field. On the left, a post-Zionist academic publishes a book that purports to destroy Israel's legitimacy by denying its racial purity. On the right, we have witnessed the Orthodox rabbinate's nearly total destruction of the avenue of conversion as a legitimate entry into Jewish peoplehood. And the American Reform Movement is also guilty of misconstruing the basis of Jewish identity. Under the impression that it was creating a more modern and egalitarian definition of "who is a Jew," and after already abandoning the requirement of conversion in a mikveh, the movement has adopted a policy recognizing "patrilineal descent." Ironically, by rejecting the mikveh and accepting patrilineality, Reform Judaism has de facto (its official requirement of "raising the child Jewishly" is not enforced) embraced a genetic criterion for Jewishness.

Now we have news of one more step toward a genetically based definition of Jewishness. Rabbi Dr. Mordechai Halperin, a prominent figure connected to Haredi rabbis and the government in Israel, has announced that rabbinic authorities, whom he refuses to name and whose legal reasoning is unavailable for study, have overturned hitherto accepted definitions of Jewishness such that the religion of the birthing mother will not determine whether her baby is Jewish. The criterion will now be whether the source of the fertilized egg is a Jewish woman. Apparently the Knesset plans to offer a bill, unquestioningly accepting this announcement.

What is left but to put on Billy Holiday's rendition of "My Yiddishe Momme" and cry?

David Greenstein is rabbi of Congregation Shomrei Emunah, in Montclair, New Jersey.