Board Thread:Boruto Discussions/@comment-33758328-20171201171033/@comment-25122992-20171201190638

Even Wood Release is not an exception, if Hashirama is the only known original user, it still doesn't mean that WR isn't passed genetically, Hashirama is just the only person who manifested WR through those genes.

P.S. If combined transformations didn't require special genes, then the shinobi skilled in nature transformations like Hiruzen or Kakashi could use then with ease, which they didn't.