And now it starts to get interesting: Gene-gene interactions

In the beginning, there was the OGOD model: one-gene one-disease.1 Even before the age of molecular genetics, the study of patterns of inheritance made it clear that this model was not applicable to the study of personality traits. When the first replicated reports were published of positive associations between a candidate gene and a personality trait-dopamine D4 receptor gene (D4DR) and novelty seeking (NS)2,3-the authors speculated that that there might be 10 or so different genes, each of small effect size, responsible for the trait.3 The assumption was that the effects of these genes would be additive, each one responsible for about 10% of the observed genetic variance. However, as Richard Ebstein notes in Chapter 18 of this book, finding these genes has proven frustratingly difficult and results are often inconsistent. More recently, evidence has begun to accumulate, which suggests that part of the answer to this conundrum may be found in gene-gene interactions.