The genetic differences between individual human beings are much greater than was previously thought, according to research that offers a fresh explanation for the unique physical and psychological characteristics of every person.

A revolutionary map that compared the genomes of 270 people has shown that humans are not 99.9 per cent genetically identical as assumed, but that much more of our DNA varies between one person and the next.

Whereas previous analysis of the human genome had suggested that no more than 0.1 per cent of it underlaid all the genetic differences between individuals, the new findings indicate that at least three times more DNA actually varies.

The results reveal a new category of genetic diversity that promises to explain certain inherited disorders, and why some people are more susceptible to diseases or respond badly to particular drugs.

The work could be especially significant for HIV/Aids, Parkinson’s disease, Alzheimer’s disease and developmental disorders similar to Down’s syndrome, which can all be affected by the newly identified disparities. The extra variation also expands the ways in which a person’s DNA profile can affect temperament and behaviour, and points to previously unknown genetic differences between Homo sapiens and our closest animal relative, the chimpanzee.

Just as humans seem to be 0.3 per cent genetically different from one another, and not 0.1 per cent, so we may be 3 per cent and not 1 per cent different from chimps, scientists behind the study say.

Charles Lee, of Harvard Medical School, a leader of the research, said: “The evidence shows that we are more genetically unique, compared to one another. That is gratifying in a way. We are all physically different, and we all react differently to environmental stimuli and to drugs.

“We are also finding evidence that could help to explain why humans are not chimps. We can safely say that there’s a lot more genetic variation between the human genome and the chimp genome than was appreciated.”

The first studies of the human genome, which was mapped five years ago, reached the figure of 0.1 per cent variation between individuals by looking at changes in single DNA “letters” known as single nucleotide polymorphisms, or SNPs. Similar studies put the genetic difference between humans and chimps at about 1 per cent of DNA.

This surprisingly low genetic variation led many scientists to wonder how so little DNA could be responsible for such significant differences as those between individual humans, and between humans and other animals.

The new research offers an answer: the old estimates were incomplete. It has expanded the range of variation by looking also at much larger segments of genetic code, which can be repeated many times or deleted entirely from the genome.

While most people have two copies of each gene, one inherited from each parent, some genes or fragments of genes can be absent altogether, or repeated five or ten times over in certain individuals. These changes, known as copy number variation (CNV), sometimes have no obvious effects, but they can influence disease or other aspects of human development.

An international research team has now mapped the genome for CNV, encompassing genetic changes of 500 or more DNA letters.

Their results, published today in the journal Nature, show that 12 per cent of the human genome is susceptible to such variation. Not all these possible changes will separate any two individuals, but scientists think that their genomes will usually vary by about 0.3 per cent.

Matthew Hurles, of the Wellcome Trust Sanger Institute, in Hinxton, Cambridgeshire, a leader of the research, said: “The variation that researchers had seen before was simply the tip of the iceberg, while the bulk lay submerged, undetected.”

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