Merle is a genetic pattern that can be in a dog's coat. Merle comes in different colors and patterns and can affect all coat colors. The merle gene creates mottled patches of color in a solid or piebald coat, blue or odd-colored eyes, and can affect skin pigment as well. There are two general types of colored patches that will appear in a merle coat: red and blue. Some breeds that can be affected by this genetic trait are Australian Shepherds, Catahoula Leopard Dogs, and American Pit Bull Terriers. This genetic trait should be taken seriously when breeding merles. Health issues are more typical and more severe when two merles are bred together, so it is recommended that a merle be bred to a dog with a solid coat color only.