Passage Summary: Wirth argues that since scientists can't find a specific gene for manic-depression, the condition isn't genetic; Chang counters that it could be caused by multiple genes working together rather than just one.

Conclusion: Claims of a genetic predisposition to manic-depression are incorrect because no single gene for the condition has been identified.

Reasoning: Researchers have failed to find a specific 'manic-depression gene,' and most experts now agree such a single gene does not exist.

Analysis: To find the point at issue, we apply the Agree/Disagree Test to the central claim. Wirth concludes that the lack of a single gene proves there is no genetic predisposition at all. Chang explicitly disputes this conclusion, pointing out that a predisposition could arise from a complex interaction of several genes. Therefore, the disagreement centers on whether the absence of a single gene justifies the claim that no genetic predisposition exists. Look for an answer choice that focuses on whether a genetic predisposition to manic-depression actually exists.