I do not think "mental illness" is caused by defective genes. I do not think there is a single gene, or a small group of genes, that "cause" mental illness. I think mental illness is a response of an organism (us) mostly with genetic inheritance within the normal range -- a response to experiences that stress the organism, or certain systems within the organism, beyond its capacity to cope with the stress successfully. This normally happens in childhood, where the organism (person) has undeveloped resources, and the stress put upon it is too great for it to cope without long-term damage or changes to the system. So I see "mental illness" as a stress-response, one that reflects the genetic sensitivities of the organism (us) but not something caused by wildly abnormal genetic material.