They are and they aren't. Modern jews are spiritual descendants of biblical Israel (look at how they behaved back then vs now. practically identical). In terms of genealogy though, there has been so much admixture that any claim of tracing a pure line back to one of the 12 tribes is ridiculous. Not to mention God eradicated 10 of them when the northern kingdom fell, and then eradicated the sadducees when Rome destroyed Jerusalem.