Identical twins are identical, right? After all, they derive from just one fertilized egg, which contains one set of genetic instructions, or genome, formed from combining the chromosomes of mother and father.

But experience shows that identical twins are rarely completely the same. Until recently, any differences between twins had largely been attributed to environmental influences (otherwise known as "nurture"), but a recent study contradicts that belief.

Geneticist Carl Bruder of the University of Alabama at Birmingham, and his colleagues closely compared the genomes of 19 sets of adult identical twins. In some cases, one twin's DNA differed from the other's at various points on their genomes. At these sites of genetic divergence, one bore a different number of copies of the same gene, a genetic state called copy number variants.

Normally people carry two copies of every gene, one inherited from each parent. "There are, however, regions in the genome that deviate from that two-copy rule, and that's where you have copy number variants," Bruder explains. These regions can carry anywhere from zero to over 14 copies of a gene.

Scientists have long used twins to study the roles of nature and nurture in human genetics and how each affects disease, behavior, and conditions, such as obesity. But Bruder's findings suggest a new way to study the genetic and environmental roots of disease.

For example, one twin in Bruder's study was missing some genes on particular chromosomes that indicated a risk of leukemia, which he indeed suffered. The other twin did not.

Bruder therefore believes that the differences in identical twins can be used to identify specific genetic regions that coincide with specific diseases. Next, he plans to examine blood samples from twin pairs in which only one suffers from asthma or psoriasis to see whether he can find gene copy number changes that relate to either of these illnesses.

The result might also call into question the many findings of previous twin studies that assumed identical twins were indeed identical, Bruder notes. "It's pretty unlikely they're going to significantly change any of the results found so far," counters Kerry Jang, a psychologist at the University of British Columbia in Vancouver, who runs Canada's largest twin study. "We can adjust our models to take [genetic differences] into account in the same way we've adjusted for different environments."

The discovery of this genetic variation gives hope for an obscure but pressing issue in the case of a criminal suspect who is an identical twin. "If one twin is a suspect and the whereabouts of the other twin cannot be determined, then the jury is often left without the ability to find guilt beyond a reasonable doubt" in cases that rely on DNA evidence, says Frederick Bieber, a pathologist at Harvard Medical School.

"If the twin issue comes up in a criminal investigation it's possible that if there are [copy number variants] that differ between the two twins that might help sort that out," Bieber says.

Given that there are 80 pairs of identical twins in Virginia's convicted offender database alone, this might not be as small an issue as it may sound. And such genetic variation also matters to the population at large.

Bruder speculates that such variation is a natural occurrence that accumulates with age in everyone. "I believe that the genome that you're born with is not the genome that you die with—at least not for all the cells in your body," he says.

Charles Lee, a geneticist at Brigham and Women’s Hospital in Boston, agrees. Genetic variations can arise after a double strand of DNA breaks when exposed to ionizing radiation or carcinogens. "It reminds us to be careful about our environment because our environment can help to change our genome," he says.

Plus, these variations may predict age-related diseases. Lee adds: "As you age … your chances for having a genomic rearrangement that causes a certain disease increases all the time."

The differences between identical twins increase as they age, because environmentally triggered changes accumulate. But twins can also begin their lives with differences, according to Bruder's study, and that calls into question their very name.

"Maybe we shouldn't call them identical twins," Harvard's Bieber says. "We should call them 'one-egg twins.'"