What might be the basis for the claim that "the Old Testament always defines Israel as genetic Israel"?

I was speaking with a Dispensationalist pastor and he made the claim that "the Old Testament always defines Israel as genetic Israel". I mentioned Josh. 8:33 as a counterexample, but he said that that verse is talking about the "assembly of Israel" which should be distinguished from Israel proper, which he claims must be genetically defined. Notes from a friend of mine who attends his church similarly say that "Every time ‘Israel’ is read in the OT, it only ever means a genetic descendant of Jacob (and sometimes it means a subset of this group—northern 10 tribes) – ethnic Jews." This strikes me as particularly radical--even for Dispensationalists--since it would exclude converts to Judaism from Biblically defined "Israel". Is there any basis for these assertions? Unfortunately, I was unable to ask the pastor about where he got this idea from. It strikes me as incompatible with Josh. 8:33-35. Exodus 12:42-49 and Ruth 1:16 are also relevant. I know that Dispensationalists don't all emphasize *genetic* Israel the way this pastor did, since some of my dispy friends also were surprised by this claim. **Are there other Dispensationalist authorities that would make the claim that "Israel" in the Bible is always be defined genetically?** If so, what reasoning is used to justify it? If not, then what Dispensationalist doctrine might this pastor have been misinterpreting? ***Update:*** This article by Thomas Ice seems to also take this view, especially the quotations from Fruchtenbaum's book *Israelology: The Missing Link In Systematic Theology* and Ruthven's *The Prophecy That Is Shaping History: New Research on Ezekiel’s Vision of the End* seem promising. However, I'm not sure sure if these works emphasize the *genetic* aspect (i.e., excluding converts to Judaism). Do either of these works actually define "Israel" this way, and if so, how do they justify it?