CENTURIES OF HUMAN BREEDING CREATED THIS DAPPLED STEED’S STRIKING LOOK, but its beauty comes at a cost. Research by Swedish geneticists on eight breeds of horses (including Icelandic and Arabian) has shown that the mutation in a gene group that causes horses to become gray, and eventually white, also leaves them vulnerable to melanoma, which may lead to premature death. Because humans and horses share this gene group, Australian scientists looked into whether common variants in the group cause susceptibility to melanoma in humans and found no risk.