Assumimg you're a male, you can have a child who is "100% pure Jewish": if you define Jewish as being fully accepted as a Jew in a minyan (prayer service) or for any other reason by other Jews.
The issue here, again, is not who is Jewish. The issue is whether I, or other Jews,are direct descendants of the Jews you read about in the Bible.
I think the Lemba prove that the Jewish populations are not 100% pure. However the genetic evidence also shows, in my view, that most Jewish populations have some ancestry from the Jews of the Bible. But they are not pure descendants.
Whether that is meaningful or not is a matter of opinion. I'm sure the Gentile populations of Europe and the Middle East also have some ancestry from the ancient Jews, due to conversions by Jews into Christianity or Islam. Should European Gentiles and Arabs claim to be descended from the ancient Israelites? One could just as well claim that black Americans should be considered Anglo-Saxon because they have some English ancestry (Genetic studies show that black Americans have about a 28% admixture rate). A genetic study of Englishmen showed that 1% of English genes are of African origin. Should the English be considered "African" because of this?
What we make of all these facts is a matter of opinion. Surely the Zionists will be prone to claim the modern Jews are 100% pure so as to claim the land of Palestine while the Arabs will be prone to say the Ashkenazim have 0% ancestry from the ancient Jews so as to deny the Zionist claim on Palestine.
So I think the issue is not whether modern Jews are descended from the Biblical Jews but rather to what extent they are descended from the Biblical Jews. Surely modern Jews have some ancestry from the ancient Jews. But the question is exactly how many of the ancestors of modern Jews were the Biblical Jews. It's not a matter of whether modern Jews are descended from ancient Jews or not, it is a matter of to what degree. It is a matter of degree not of "if".