A challenge in genetics is to understand the molecular basis of genetic and allelic heterogeneity. Divergent phenotypes caused by different variants of the same gene determine allelic heterogeneity. In the past few years, we have been studying an allelic series of mutations in the gamma-subunit of the cGMP phosphodiesterase gene (Pdeg) that resulted in visual defects ranging from stationary night blindness to progressive retinal degeneration. Here we describe the morphology and physiology of the retina in mice carrying four different Pdeg alleles: Pdeg(tm), Del 7C, Y84G, and W70A and the effect that these mutations of PDE gamma have on components of the activation and deactivation phases of phototransduction.