We are talking about GFI, yes? If so, there are a number of possible answers, though the past tense ("...he thought he was...") narrows it down some. The character must be dead by the end of the movie.

Sollozo gives Tom a pretty long speech about the Don's outdated old world sensibilities and the likely order of succession (when he thinks the old man is dead) amongst the sons, based on his notions of who got what genes from the old man...

Course, Sonny makes a few comments to and about Michael, implying some missing gene, some "white sheep" anomoly-- Mike will never truly be his father's son. He also offends Tom by pointing out he's not Sicilian, can't be a wartime consiglieri...

But I'd have to go with Vito, based on his heart to heart talk with Michael in the garden. The Don passes the reins reluctantly, commenting on the fitness of each of his sons (one now dead) to inherit the crown, so to speak...to carry on the bloodline.

So Vito thinks he's a genealogist.

MY TURN.

This guy knows Latin better than any other in the film, and is probably a virgin.