In this article I examine whether an influential theory of mechanisms proposed by Peter Machamer, Lindley Darden and Carl Craver can accommodate polygenic effects. This theory is both interesting and problematic, I will argue, because it ascribes a central role to activities. In it, activities are needed not only to constitute mechanisms but also to perform their causal role. These putative functions of activities become problematic in certain situations where several causes or elements of a mechanism contribute simultaneously, i.e. with certain forms of polygenic causation. The problematic form of polygeny, polygeny 2, occurs when the polygenic contribution concerns one and the same property or aspect of the affected object. When the result of such causation is that nothing happens, the theory suggested by Machamer and his colleagues cannot be applied. More generally, it seems that, whenever polygeny 2 is involved, the Machamer approach leads to an impoverished conception of mechanism.