Identical twins aren't perfect clones, research shows

FILE - In this Saturday, June 18, 2005 file photo, identical twins Alf, left, and Sven Fehnhanhn, left background, 79, from Kassel, pose along with seven-month-old Luis Carl, right, und Albert Frank Millgramm, right background, during a twins' meeting in Berlin. According to research published on Thursday, Jan. 7, 2021, identical twins are not exactly genetically the same. (AP Photo/Jockel Finck) (Copyright 2021 The Associated Press. All rights reserved.)

WASHINGTON – If you’re an identical twin who’s always resisted being called a clone of your sibling, scientists say you have a point.

Identical twins are not exactly genetically the same, new research shows.

Scientists in Iceland sequenced DNA from 387 pairs of identical twins — those derived from a single fertilized egg — as well as from their parents, children and spouses. That allowed them to find "early mutations that separate identical twins,” said Kari Stefansson, a geneticist at the University of Iceland and the company deCODE genetics, and co-author of the paper published Thursday in the journal Nature Genetics.

A mutation means an alteration in a sequence of DNA — a tiny change that is not inherently good or bad, but can influence physical features or susceptibility to certain diseases. They can occur when a cell divides and makes a slight error in replicating DNA.

On average, identical twins have 5.2 of these early genetic differences, the researchers found. But about 15% of identical twin pairs have more genetic differences, some of them up to 100, said Stefansson.

These differences represent a tiny portion of each twin’s genetic code, but they could influence why one twin is taller or why one twin is at greater risk for certain cancers.

Previously, many researchers believed that physical differences between identical twins were related mostly to environmental factors, such as nutrition or lifestyle.

Jan Dumanski, a geneticist at Uppsala University in Sweden, who was not involved in the new paper, praised it as “a clear and important contribution” to medical research.