Dec. 14, 2005 -- In a discovery that begins to shed light on what makes one person brown and another white, scientists have identified a gene that appears to be a key player in human pigmentation.
People share 99.9 percent of the same genes, yet pinpointing the very minor genetic variations that cause skin-color differences long has been a mystery to scientists. This discovery, published in the journal Science, marks a significant step toward understanding what's behind the panoply of human skin tones.
"The gene we found seems to modulate the number, size and density of cellular packets that contain brown pigment," said Keith Cheng, a geneticist at Penn State College of Medicine in Hershey, Pa.
Cheng's team found that people with the normal form of the gene SLC24A5 had brown skin, while fair people of European descent carried a modified form of the gene that led to having fewer and smaller pigment packets, known as melanosomes.
Skin-Tone Genes May Help in Understanding Obesity, Blood Pressure
Understanding what causes differences in skin color may seem like a straightforward task, but in fact it's much more complex than understanding variations such as eye color. This is because skin pigmentation is a continuous trait -- people aren't simply brown or white, but many shades in between.
Finding the genes behind skin variations could help scientists find new cures for skin cancers and possibly even identify safer ways of tanning than lying in the sun, but more importantly it might help in understanding other critical health conditions. Health factors such as blood pressure, obesity and dementia are all considered to be continuous traits and may be caused by a similar concert of genes.
"Something like gestational diabetes -- it's not whether you have it or not -- it's how serious it is," explained Gregory Barsh, a geneticist at Howard Hughes Medical Institute at Stanford University School of Medicine in Stanford, Calif. "Part of understanding pigmentation is understanding how genes interact to create a trait. Pigmentation is one trait where we can begin to understand how this works."
The Hunt: From Fish to People
Cheng and his colleagues stumbled upon SLC24A5 by accident -- while searching for a cancer-causing gene within a common aquarium pet, the zebrafish.
The researchers were looking at the genetics of different versions of the fish to locate genes possibly involved in cancer. It turns out people and zebrafish share many genes, including those that code for pigment. Cheng's team found that a kind of zebrafish called "golden" had fewer, smaller, and less heavily pigmented melanosomes than normal fish. This suggested the same gene may be at play among golden zebrafish and fair-skinned people.
To locate a similar gene in people, Cheng turned to colleague Mark Shriver of Pennsylvania State University to scout through new resource the HapMap. This free database lists genetic variations in the human genome as they're discovered. Shriver zeroed in on SLC24A5 when he found that the same slight variation in the gene was carried by fair people in European populations.
While this gene may cause skin-color differences between West Africans and Europeans, it doesn't seem to play a role in determining the fairer skin tones of Asian people. Cheng, who is ethnic Chinese, says he's now scouting for that gene.
"I have a personal interest," he said, since it would explain his own skin color.
'Race Is Not Skin Color'
Shriver argues that because this newly found gene doesn't explain Asians' fairer skin color, there is a lot yet to learn about skin-color genetics.
"The fact that Europeans and East Asians are similar [in] skin color for different reasons tells us that we still don't know much," he said. "There are more genes to locate."
Finding a gene behind skin pigmentation may be a big step in science, but researchers caution it has no implications for understanding race. Skin color may lie near the root of much controversy and unrest -- such as the recent violence in Sydney, Australia, where white youths attacked people they believed to be of Middle Eastern descent. Still, Barsh of Stanford cautions this work is just about deciphering pigmentation.
"Skin color is not race," Barsh said. "Race is a much more complicated concept that involves culture, religion and where your parents are from. It's an important part of society, but it's not about pigment alone."