What effect does the grey gene have on Appaloosa patterning?

The grey gene (G) is epistatic to all coat colours. This means it gradually and completely obscures Appaloosa patterning, including dark body spots and varnish marks.

This happens because over time the action of the grey gene causes a stop in the ability of pigment produced in the follicle to move up into coat hairs. As this process shuts down, spots fade and the horse eventually become white.