The huge geographic variation in diapause suggests it has an underlying genetic basis, a feature further revealed by selection experiments that are successful in both increasing and decreasing the diapause incidence as well as modifying other attributes of diapause. Genetic crosses between lines with different diapause characteristics reveal diverse inheritance patterns. While some such crosses reflect a simple Mendelian-type pattern, polygenic inheritance patterns are more commonly revealed. Examples of dominance and incomplete dominance abound and a few examples can be found showing diapause traits linked to recessive alleles. Likewise, both sex-linkage and autosomal inheritance are noted in some examples. Mapping quantitative trait loci is a powerful technique for identifying single nucleotide polymorphisms associated with diapause. Results of such analyses elegantly demonstrate the complexity of the diapause response as well as the large number of chromosomal sites that have both major and minor impacts on diapause.

Much of the focus of personality disorder genetics has been aimed at identifying putative genes. Most of the research has been accomplished using a classical monozygotic and dizygotic twin design or adoption study methodologies. This vast body of research has repeatedly shown that the observed variability in personality disorder measures are directly attributable to the action of genes, but few, if any genetic loci have been reliably identified. The chapter explores the reasons for this failure such as the impact of a grand unitary theory of personality function that has created concepts and measures that are poorly suited for genetic studies. Moreover, there are inherent problems in genetic methodology, despite the faith in the latest gene hunting methods that have not been able to overcome psychometric and conceptual issues with personality constructs and measures. Indeed, attempts to revise measures and diagnostic entities have likely exacerbated the problems. Suggestions to overcome these issues, from shifting focus from descriptive personality constructs to active personality constructs such as behavioral choice, the development of personality endophenotypes, and new methods to incorporate data from unrelated individuals in gene hunting studies to increase power and detect small effects of multiple genes are discussed.