Melanism in jaguars is the result of a mutation in the MC1R gene (Melanocortin 1 Receptor).
"Melanistic animals were found to carry at least one copy of a mutant MC1R sequence allele, bearing a 15-base pair inframe deletion."
http://en.wikipedia.org/wiki/MC1R"MC1R is one of the key proteins involved in regulating mammalian skin and hair color. It is located on the plasma membrane of specialized cells known as melanocytes, which produce the pigment melanin through a process referred to as melanogenesis. ...Mutations of the MC1R gene can either create a receptor that constantly signals, even when not stimulated, or can lower the receptor's activity. Alleles for constitutively active MC1R are inherited dominantly and result in a black coat colour,..."
http://en.wikipedia.org/wiki/MelanismThat means the mutation of the Jaguar's MC1R gene has resulted in a receptor that constantly signals the body's pigment cells to produce melanin. Such a mutation is both dominant and it also results in a black coat. Other mutations of the MC1R gene can result in light skin tones in humans, especially in Northern Europeans and East Asians. Many individuals of both of these populations have defective MC1R genes, and they cannot tan easily.
The gene that controls melanism in the leopard is less well known. Since the lion, but not the leopard, is the closest relative of the jaguar, melanistic leopards and jaguars probably evolved their melanism independently of one another, in much the same way that East Asians and Northern Europeans have independently evolved light skin tones by having different mutations in their MC1R gene. Perhaps the mutation in the MC1R gene in the melanistic leopard (if indeed that is how leopards achieve melanism) can be suppressed by the presence of a normal copy of the gene.
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