Examples of scatter plots from the gene expression levels of two hypothetical genes illustrating the concept of synergy. There is an equal number of red and green dots representing cancerous and healthy samples, respectively, therefore H(C) = 1 and the synergy is bounded by -1 and +1, where C is the symbol for the presence of cancer (see Methods). Also shown are the projections of the scatter plots to each of the two axes, thus allowing visualization of the relationship of each gene to cancer. Panels d, e, f show scatter plots of binarized expression levels corresponding to panels a, b, c, respectively, depicting circles for the multiple point at the vertices. (A) Each of the two genes ("oncogenes" in this case) is by itself sufficient to determine the presence or absence of cancer and there is negative synergy (redundancy). (B) In combination, the two genes are sufficient to determine the presence or absence of cancer, but each of them individually is uncorrelated with cancer and the synergy is positive. (C) Both genes, including their combination, are uncorrelated with cancer, and the synergy is approximately zero. (D) Illustration for the scatter plot from the binarized expression levels in panel a. The synergy is equal to -1. (E) Illustration for the scatter plot from the binarized expression levels of panel b. The synergy is equal to +1. (f) Illustration for the scatter plot from the binarized expression levels of panel c. Split bi-colored circles indicate the simultaneous presence of an equal number of healthy and cancerous samples. The synergy is equal to 0.
Watkinson et al. BMC Systems Biology 2008 2:10 doi:10.1186/1752-0509-2-10