While I understand the basic grammar rules of the verb used in restrictive clauses, I came across many sentences like the example below, supposedly written by a native speaker (?).
Disease A is due to mutations in the X gene that result in ...
I thought the verb "result" should be "results", as the noun immediately in front of the relative pronoun "that" is gene, which is singular. Understandably, the clause is actually used to describe the "mutations", so it also seems correct to use "result", not "results".
Which verb is correct?