Humans

Some identical twins don’t have the exact same DNA

Genetic differences may come from mutations that arise early in development

twin girls

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

Kumacore/Getty Images Plus

By Tina Hesman Saey

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.

Power Words

More About Power Words

average: (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

cell: The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

develop: To emerge or to make come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size, mental maturity, size or sometimes even shape.

development: (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

DNA: (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

egg: The unfertilized reproductive cell made by females.

embryo: The early stages of a developing organism, or animal with a backbone, consisting only one or a few cells. As an adjective, the term would be embryonic — and could be used to refer to the early stages or life of a system or technology.

fraternal: Of our relating to brothers, or others with whom people develop close friendships and affection. (in genetics) The term for a type of twin birth where each baby comes from a separate fertilized egg. This is in contrast to identical twins, which result from a single fertilized egg (creating two separate but nearly identical babies).

genetic: Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genome: The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.

Iceland: A largely arctic nation in the North Atlantic, sitting between Greenland and the western edge of Northern Europe. Its volcanic island was settled between the late 800s and 1100 by immigrants from Norway and Celtic lands (ones governed by the Scots and Irish). It is currently home to roughly a third of a million people.

mutation: (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

replicate: (in biology) To copy something. When viruses make new copies of themselves — essentially reproducing — this process is called replication.

trait: A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

A version of this article appears in the October 1, 2022 issue of Science News Explores.

Citations

Journal:​ H. Jonsson et al. Differences between germline genomes of monozygotic twins. Nature Genetics. Published online January 7, 2021. doi: 10.1038/s41588-020-00755-1.

About Tina Hesman Saey

Tina Hesman Saey is a senior staff writer and reports on molecular biology at Science News. 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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