For readers of a more biological bent, it might help to illustrate one possible scenario that would be described by Figure 4a. As seen in Figure 4b, imagine we had a shared pathophysiological pathway from A to D, with each step having its own genetic influences. Then one etiological pathway goes in one direction through steps E and F and influences EP. The other direction, which includes sets G and H, leads to PD. In this case, EP and PD would share genetic influences on A, B, C and D, but have unique genetic influences that affect stages E, F, G and H.
