Identical twins grow apart, genetically, as the years pass, a team of European and U.S. researchers reported on Tuesday.
Their study of identical twins show the genetic code itself does not change, but rather chemical changes after birth alter the way the gene is expressed, a process known as epigenetics.
The study, which involved researchers and twins in Spain, Denmark, Britain, Sweden and the United States, can help shed light on how environment and genes interact to produce disease and ordinary differences between people.
They studied 80 twins from Spain, and found significant epigenetic differences in 35 percent of them. The younger pairs of twins were identical, while the older pairs were more likely to differ from one another.
“Most importantly, we found a direct association between the remarkable epigenetic differences observed and the age of the monozygotic (identical) twins: the youngest pairs were epigenetically similar, whereas the oldest pairs were clearly distinct,” the researchers wrote in this week’s edition of the Proceedings of the National Academy of Sciences.
“Our study reveals that the patterns of epigenetic modifications in (identical) twin pairs diverge as they become older.”
Identical twins occur in one out of every 250 births around the world. But although their genetic codes are virtually identical, there are clear differences that are obvious to more than just their mothers.
For instance, psychiatric diseases such as schizophrenia and bipolar disease do not occur uniformly among identical twins. And there are often physical differences.
“There are several possible explanations for these observations, but one is the existence of epigenetic differences,” Manel Esteller of the Spanish National Cancer Center in Madrid and colleagues wrote.
This supports theories that environmental factors, such as smoking, diet and exercise, affect DNA directly, the researchers said.
It is also possible that, just as DNA mutations occur with simple aging, the epigenetic effects on genes also “drift” with age, the researchers said.