Answer to Question 1
Answer: Phenotypes produced by a single gene produce characteristics that are categorically different. Polygenic traits (caused by more than one gene) are continuously distributed, and thus polygenic abnormal traits are on a continuum with normal traits. This point may make it difficult to determine the threshold of abnormality, which may be a question of degree rather than qualitative differences. What's more, when behavior geneticists find that a given mental disorder is genetic, our interpretation must be cautious. This finding does not mean the disorder is caused by the presence or absence of one or two genes, nor does it confirm that the abnormal behavior is in a different category from normal behavior.
Answer to Question 2
Answer: In interpreting any correlation, there are always two alternative explanations. Reverse causality means that if X and Y are correlated, X could cause Y but Y might also cause X. The third variable problem means that the correlation between X and Y could be explained by their joint relation with some unmeasured factor. Thus, even if there is a correlation between depression and the depletion of neurotransmitters, it is possible that the depletion causes the depression, but it is also possible that the depression causes the depletion. Moreover, both the depression and the depletion could come about as the result of stress.
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A polygenic model for a threshold trait