That's a good question. There are a lot of conditions where twin and adoption studies have established a genetic predispositions decades ago, but the exact caus

That's a good question. There are a lot of conditions where twin and adoption studies have established a genetic predispositions decades ago, but the exact cause of these disorders is unknown, and understood to be more stochastic in nature. Rather then single gene mutations, such complex disease predispositions could be combinations of single base variations in the binding sites of transcription factors and/or slight variations in proteins, which elevate disease risk, but may still require environmental factors for an actual outbreak. Our understanding here is improving rapidly, while we are learning about the regulatory elements in our genome (see the ENCODE project), and the influence of the 3D arrangement of the genome into what is known as chromatin. However, teasing apart these variations and causal relationships is extremely complex, as you can imagine. Bottom line, there is still so much we don't know or understand, but progress is being made.