Fig. 1

Correlated variables may be functionally unrelated. Here, \(X_1, X_2, Y\) are random variables characterizing the expression of three genes. A \(X_2\) is directly and causally linked to Y and \(X_1\). Here, \(X_1\) and Y might be highly correlated, but \(X_1\) does not causally affect Y. B \(X_1\) is directly and causally linked to \(X_2\), and similarly, \(X_2\) is connected to Y. Here, \(X_1\) and Y might be highly correlated, but their association is indirect