By Knight Ridder Newspapers

PHILADELPHIA – The merest fragment of one gene plays a major role in the differing skin colors of white and black people, scientists have found, capping an 11-year effort that began with the study of similar color variations in a common pet-store critter, the zebrafish.

The team of 25 geneticists, molecular biologists and anthropologists, most of them from Pennsylvania State University, says the work could have implications for skin cancer treatment, crime-scene analysis, and even cosmetics.

For those bent on altering their skin color, the gene could lead to pharmaceutical products that would be safer than tanning salons or the chemical skin-lightening creams popular in India, said project coordinator Keith C. Cheng, a cancer geneticist at the Penn State College of Medicine in Hershey, Pa.

The research, published Friday in the journal Science, also challenges a common assumption about when the various races branched off after leaving the scene of their common beginnings in Africa, more than 50,000 years ago. The people who would become Asians and northern Europeans were thought by some to have evolved their light skin together, before migrating their separate ways.

The new research indicates that the two groups developed lighter skin after the separation – giving just a taste of the secrets of history that can be unlocked with the human genome, said Penn State anthropologist Mark Shriver.

“There’s a lot left to be learned,” Shriver said. “We’re basically explorers right now.”

Team members say the genetic variation they discovered accounts for about one-third of the difference in melanin, or pigment, between the skin of black and white people. Other skin-color genes have been found previously, but none plays such a significant role. Lighter skin may have evolved to help people absorb more light in climates with less sun so they can maintain vitamin D levels.

The underlying genetic code for the one-third variation seems, at a glance, to be almost inconsequentially small: a change in just one of the 3 billion base-pairs in the human genome. So a significant part of the difference we perceive between the races is caused by just one rung on the twisted ladder of our DNA. Otherwise, as geneticists have been discovering more and more as they study the human genome, we are all pretty much alike.

“There’s more variation within (racial) populations than there is between,” Cheng said.

Theodore Schurr, a University of Pennsylvania anthropologist who was not part of the team, said the skin-color findings were significant.

“I think it will lead to other sorts of investigations,” Schurr said, “not only into the migration history of our species but also into the underlying basis for our differences.”

The research began as a by-product of Cheng’s work on cancer, in which his team was using zebrafish as an animal model to study the disease.

Researchers studied the “golden” type of zebrafish, whose stripes are lighter than the fish’s usual dark-brown.

On the microscopic level, the different pigments are the result of different quantity, size and density of melanosomes, packets that contain melanin. The different shades of black and white skin in humans are caused in much the same way.

The team spent years turning different zebrafish genes on and off in the lab, looking for the one that caused the skin-color variation. They then used a variety of techniques to establish that the corresponding gene in humans performs a similar function.

First, researchers used data from the HapMap – an international effort that is cataloging genetic differences in 270 people from four ethnic groups: Japanese, Chinese, African and northern European.

Researchers found the genetic variation in every person of northern European descent in the sample. Almost all of the Chinese, Japanese and Africans did not have it.

Because Asians did not have the variation, other genes must account for their lighter skin, the researchers said.

The scientists then implanted the light-skinned human variation in zebrafish eggs and created light-striped fish, generating a further layer of proof.

They also used reflectometers to measure skin color in 400 people of African descent, and found that relative darkness was correlated to whether they had the variation. When one parent had the variation and the other did not, the effect on offspring lay in between.

Shriver, the anthropologist, said the discovery of such genetic markers could one day lead to better forensic analysis.

Currently, police must have DNA from a suspect in order to compare it with samples from a crime scene. But the skin-color gene could help paint a picture of a suspect when none has been identified, he said.

The discovery might also help identify a treatment for skin cancer, a condition associated with abnormalities in melanin, study authors said.