The transmission disequilibrium test (TDT), designed as a test of linkage in the presence of
association (i.e. linkage disequilibrium), has received considerable attention in the recent statistical
genetics literature due to its advantages over other within-family analytic methods. One limitation
of the conventional TDT is its application solely to linkage disequilibrium between a genetic marker
and a single categorical trait (e.g. presence or absence of a disease). In this paper, we present an
extension of the TDT using logistic regression to examine the relation between a candidate gene or
genetic marker and one or more continuous or categorical explanatory variables. This logistic
regression extension of the TDT possesses all of the desirable features of the conventional TDT, as
well as many advantages associated with traditional regression analysis. We describe the model and
its properties, as well as a number of its possible applications, and apply it to examine linkage
disequilibrium between the dopamine receptor D2 gene (DRD2) and symptoms of childhood
attention deficit hyperactivity disorder (ADHD). We also briefly compare the logistic regression
TDT to other quantitative TDTs that have been proposed in the literature, and highlight the
advantages of a regression-based approach for examining the relation between a candidate gene and
one or more continuous or categorical traits. Given its features, we regard the logistic regression
extension of the TDT as a flexible new data analytic method with extensive potential applications
to problems in medical, psychiatric, and behavioral genetics.