Some identical twins don’t have the exact same DNA

Genetic differences may come from mutations that arise early in development

Identical twins were thought to be genetically the same. However, a new study finds that’s not always the case.

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Identical twins may not be exact copies at the DNA level after all. On average, identical twins differ by 5.2 genetic changes. Researchers shared their new finding January 7 in Nature Genetics.

Identical twins are also called monozygotic (MON-oh-zy-GAH-tik) twins. They come from a single fertilized egg, or zygote. That egg replicates and forms an embryo. Then, it splits into two clusters of cells. Each of these embryos will go on to become a baby. (Fraternal twins develop from two separate fertilized eggs. That’s why they’re known as dizygotic twins.)

Identical twins have long been thought to have the exact same genes. That’s why some scientific studies recruit these twins. Such studies try to determine whether particular traits, diseases or conditions are due to genes or instead reflect environmental influences. Any differences in the health of identical twins were thought to be due to different environmental exposures. The new findings, though, suggest that genetic changes might also explain why identical twins aren’t truly identical.

Researchers in Iceland recruited 381 pairs of identical twins. Then the scientists deciphered the all the genes in each person. Only 38 pairs of twins had all the same genes. Most twin pairs differed somewhat in their DNA. Those differences likely arose long before birth. It could have happened either just before one embryo split to form two. Or it could have happened shortly after the split. Some of the twins had many genetic differences. Thirty-nine pairs had more than 100 differences in their genes.

Gene changes, or mutations, seen in the newly studied twins suggest that embryos don’t split neatly in half when twins form. Some twins may arise when a single cell or a small group of cells splits off from an embryo. How many cells each twin originates from may determine how genetically different they are. More uneven splits of the embryo may lead to a greater number of gene differences in the twins.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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