We don't know Wirth's opinion about this. In order for Wirth to think that all efforts to identify a single gene or set of genes responsible for manic-depression have failed, Wirth needs to believe that all efforts at identifying a single gene have failed and that all efforts at identifying a set of genes have failed. Although we know Wirth thinks all efforts to identify a single gene responsible have failed, we don't know what he thinks about efforts to identify multiple genes that, together, might influence manic-depression. Wirth might not even be aware of those efforts.

Neither speaker explicitly speculates about the future change of finding a single “manic-depression gene.” If anything, both speakers likely disagree with this claim: each seems to take for granted that no single “manic-depression gene” exists.

Like (A), the speakers both agree with this claim. This is one of the facts Wirth uses as support, and Chang accepts that Wirth’s support is true. The disagreement is about the conclusion, not the evidence.

Wirth agrees with this claim but Chang disagrees, making it the point at issue. Wirth believes that the absence of a single “manic-depression gene” means there can be no predisposition; Chang thinks a predisposition can still exist due to the interaction between multiple genes.

Neither speaker explicitly discusses the thoroughness of researchers’ attempts to find a “manic-depression gene,” but both seem to accept that no such gene exists. The rigor of scientists’ approach is not an issue between Wirth and Chang.