In 2000, the Human Genome Project unveiled a road map of the six billion chemical bases, or alphabet molecules, that make up the body's genetic structure called DNA.
The DNA encodes for 30,000 genes or proteins which are responsible for every physical characteristic in the body, including eye and hair color.
At the time, scientists said all humans could be 99.9 %genetically identical. But as they peered more deeply into the DNA of unrelated individuals, researchers made a startling discovery -- large segments of their DNA, from thousands to millions of units, varied greatly, a phenomenon called copy number variations, or CNVs.
The discovery means that the genes of any given individual are at least 10 to 12 % different from those of another human.
"This was really quite startling and shocking. So, it adds a whole new type of variation for us to consider in disease mapping studies."
That's Stephen Scherer, a University of Toronto and Howard Hughes Medical Institute researcher. Each person inherits 46 chromosomes, 23 from each parent, which means that genes usually come in pairs. But not always, according to Scherer.
"What we found in this study of 270 individuals worldwide is that up to 10 percent of the genes are actually present varying from the general two that we usually see. So, in some cases there's one. In some cases there's three. In some cases there's five. In some cases, there's actually none."
The findings, published in Nature, are part of the international HapMap project to identify and catalog similarities and differences among human beings of different racial and ethnic groups.
Investigators sampled the DNA of people in Africa, Asia and Europe. In a separate paper published in journal Nature Genetics, Scherer's group compared the International Human Genome Project map and one by the private company, Celera Genomics.
Scherer says the overlay produced 30-million chemical variations that had not been seen before. He says the discovery could have a major impact on medical research. Instead of looking for an abnormality in a particular gene, as researchers now do, Scherer says investigators could soon begin focusing on whole regions where genes are repeated and deleted.
"There actually are already some examples where new disease mutations have been found using this approach and using the technologies that we describe."
Copy number variations have already led scientists to list 17 nervous system conditions -- including Alzheimer's and Parkinson's disease -- in a database for further study.
