Fig. 1. (redrawn after Lebreton et al., 1995) Four models for the possible association between two traits whose expression is influenced by the same chromosome region. A, Linkage. Traits 1 and 2 controlled independently by two distinct QTLs which are closely linked but not separated by the statistical tests adopted. B, Pleiotropy. Two traits causally associated and regulated by a QTL acting directly on one trait only. C, Pleiotropy. Traits 1 and 2 controlled independently by a single gene. D, Linkage and pleiotropy. Traits 1 and 2 controlled independently by two distinct QTLs which also have pleiotropic effects (see text for an example).