Is Judaism defined by DNA?

Mark Lawson, in the Guardian, finds it sinister that someone would edit his Wikipedia entry to say that he is of Jewish descent and he's probably right. He also objects to being on the influential Catholic list of the Tablet.

Judaism is defined by DNA, Catholicism is conferred through practice. So it is provocative for the Tablet list to include the novelist David Lodge (an "agnostic Catholic"), Clare Short (a "cultural Catholic") and Bob Geldof (an atheist), all of whom would cause incense to come out of Papa Ratzinger's ears, while it cannot acknowledge Tony Blair, who is a Roman Catholic in everything except baptism.

But the magazine is right to be so catholic, as it were, in its definition of Catholicism. The novels of Lodge would not have the style or subjects that they do if he had been raised as a Methodist; nor would the screenplays of Jimmy McGovern (Cracker, Priest), another agnostic listed. The politics of Geldof and Short are also clearly shaped by the charitable and pacifist aspects of Catholic social teaching.

Ok, a bit garbled. It's wrong to suggest that being of Jewish descent might drive ones outlook because then you're making an issue of someone's DNA. But it's also wrong to call people Catholics if they don't practice Catholicism. Until Mark Lawson decides that it is ok.

But Jewish DNA? What DNA do I share with, say, Ethiopian Jews that I don't share with British gentiles? Also, ignoring the fact that Judaism is a religion and is defined by belief and practice, contrary to what Mark Lawson says, Jews are clearly a descent group more than a religious group. But is it simply the being born Jewish that makes one be Jewish or are there social, cultural, even political considerations? All this and much much more, I just don't know the answer to.