According to the original view, genes were necessarily the cause of inheritable phenotypes because that is how they were defined: as whatever in the organism is the cause of that phenotype. Johanssen even left the answer on what a gene might be vague: ‘The gene was something very uncertain, “ein Etwas” [‘anything’], with no connection to the chromosomes’ (Wanscher, 1975). Dawkins (Dawkins, 1982) also uses this ‘catch-all’ definition as ‘an inheritable unit’. It would not matter whether that was DNA or something else or any combination of factors. No experiment could disprove a ‘catch-all’ concept as anything new discovered to be included would also be welcomed as a gene. The idea becomes unfalsifiable.This is, I think, a criticism that is very easy to refute. Science consists of, among other things, hypotheses, axioms and terminology. Hypotheses are what needs to be falsifiable. Axioms and terminology, not so much. You can criticize an axiom by saying it is false, but saying it is always true isn't much of a criticism. A similar situation applies to terminology. You can say terminology is unhelpful, unproductive, misleading and so on. However, the criticism that it is unfalsifiable cuts no ice. In science, it is hypotheses that need to be falsifiable. Unfalsifiable terminology is fine. It I say an atom is a stable mixture of protons, neutrons and electrons that can't be subdivided without changing its chemical properties, nobody can say that that's a bad definition because it is unfalsifiable. That's just a misunderstanding of how science works.