Skin color is one of the most obvious traits that humans inherit from their parents, but the genes responsible for it have never been nearly so obvious.

More than 100 genes are known to affect the fur color of mice, for example, but aside from unusual cases of pigmentation in humans, such as the extremely light-skinned condition known as albinism, the genetics of human skin color has remained mysterious.

While studying zebrafish, however, a cancer geneticist at Penn State University College of Medicine in Hershey appears to have stumbled upon a gene that explains much of the difference between light-skinned people of European descent and dark-skinned people of African descent.

The discovery could help in finding treatments for a dangerous skin cancer, known as malignant melanoma, or perhaps lead to a safe way to darken the skin without exposure to harmful ultraviolet rays. It also may help geneticists improve their methods of studying complex genetic conditions, such as hypertension and diabetes.

Keith C. Cheng and his colleagues, who are reporting their discovery today in the journal Science, say one of the gene's mutations -- which might be called a "depigmentation" gene -- is found in the vast majority of light-skinned Europeans and is uncommon in Africans and Asians.

"We believe it accounts for the bulk of the differentiation of European populations from African populations," Dr. Cheng said. It doesn't play much of a role, however, in differentiating African populations from lighter-skinned Asians; both groups tend to carry the same version of the gene.

Gregory S. Barsh, a geneticist at Stanford University School of Medicine who studies skin pigmentation, said he wouldn't go so far as to say the gene plays a major role in explaining light-skinned Europeans, but he said it "has a very significant effect." But more than one gene clearly is at work, he emphasized. "If one gene explained much or most of the difference between Africans and Europeans, you wouldn't see the range of pigmentation that exists, particularly among African-Americans," he said.

A skin color gene is not the same as finding a gene that accounts for race. Dr. Cheng said the new gene, known as SLC24A5, seems to affect skin pigmentation and nothing else. It plays no role in other characteristics associated with race, such as body shapes, face shapes and hair types.

Dr. Cheng didn't set out to study pigmentation genes; rather, for the past decade he has been looking for genetic mutations that occur rapidly in zebrafish. Cancer occurs, he explained, because of genetic mutations, so he hoped to gain insights by finding genetic instabilities in the common aquarium fish, which also is a popular laboratory animal for genetic studies.

In particular, he would look into the eyes of zebrafish, checking for dark cells that have turned light. A mutated form of zebrafish, known as golden, proved particularly good for these studies. Unlike most members of the species, which have long, dark stripes, the golden variety has lighter stripes.

Dr. Cheng was intrigued when he noticed that the difference in pigmentation between the two forms of the fish were similar to the differences in pigmentation between Africans and Europeans. As in light-skinned Europeans, the pigment granules, called melanosomes, in the golden variety were fewer in number, smaller in size and lighter in color than the regular fish.

He and his lab workers isolated the responsible gene mutation, SLC24A5. Working with pharmacologist Victor Canfield they found a similar in humans.

When the protein produced by the normal zebrafish gene was injected into the golden mutants, they developed normal zebrafish coloring. And when the human version of the protein was injected into the golden fish, it too caused the fish to develop the darker coloring.

The researchers, including Penn State anthropologist Mark D. Shriver, searched for variants of the gene using a publicly available database of human genetic diversity, called the HapMap. They found two versions of the gene, which vary by a single amino acid. Almost all Africans and Asians have one version, while 98 percent of Europeans have the other.

Dr. Shriver calculated that the two versions of the gene account for 25 to 38 percent of the skin-color differences between Europeans and Africans.

Dr. Cheng continues to study the genetic basis of cancer, but now he also is trying to figure out the mechanism by which the gene operates.

"I can't help it now," he said. "The Pandora's box is open."