The silver gene is a unique gene that only dilutes black pigment.  It is also called the " Z " gene or silver dapple.  The term silver dapple can be misleading because not all horses carrying the gene are silver in color or have dapples.  

The silver gene is dominant.  When present it dilutes the black pigmentation on the body and points of the horse.  The dilution can vary greatly within a breed and quite differently from breed to breed.  The body color can range from a light silver in color to a dark chocolate brown color.  The silver gene also dilutes the points of the horse.  These points can also vary greatly from a very light flaxen looking mane and tail to a mane and tail that are the same color as the horse's body color.

Thanks to Monique for giving us permission to use this.