ABSTRACT
Although the primary objective of case-control studies is to assess the effects of genetic variants between cases and controls, secondary phenotypes are often collected in such studies without much extra cost. For example, in the Diabetes Genetics Initiative (DGI) study, there were 1,464 patients with type 2 diabetes and 1,467 controls from Finland and Sweden, while at the same time, a variety of secondary phenotype traits were available for these patients, including anthropometric measures, glucose tolerance and insulin secretion, lips and apoliporoteins and blood pressure. These secondary phenotypes are typically the exposures/risk-factors of interest for the main outcome. In the Wellcome Trust Case Control Consortium (WTCCC), a case-control study consisting of 1,924 U.K. type-2 diabetes patients and 2,938 U.K. population controls, body mass index (BMI) and adult height were also measured as secondary traits in the study. With the availability of second phenotype information, it is cost-effective to study the association between genetic variants and these additional traits without need to conduct new studies. Indeed, the DGI study identified association of a particular single nucleotide polymorphism (SNP) in an intron of glucokinase regulatory protein with serum triglycerides in both case and control groups.
