Identical twins are brothers who originated from the same egg and sperm. That is, when these two gametes joined, there was also the formation of a cell. This cell, in turn, ended up separating at some early stage of the embryo and gave rise to two embryos. Thus, theoretically the brothers have the exact same genetic material, and the difference between the two happens byenvironmental factors of each sibling. At least that's what was thought until then.

It turns out that researchers in Iceland have discovered that identical twins actually don't have exactly the same DNA.

Image: ParallelVision/Pixabay

The study, available in the journal Nature Genetics, had the participation of 381 pairs of identical twins, besides two trios of identical triplets. To determine mutations and differences in DNA of the brothers, researchers analyzed collections made from individuals, but also from family members, such as parents and children. From this it was possible to register that identical twins have an average of 5.2mutations different from their siblings. In some cases, however, this number has passed 100 mutations. Ziyue Gao, assistant professor of genetics at the University of Pennsylvania tells Live Science that the number of mutations could be even higher if the researchers had studied sperm and eggs of the brothers.

The results are some of the first to show that monozygotic twins are not actually clones. Until then it was believed that all differences in organisms happened due to environmental factors leading to mutations or physiological reactions. However, the research shows that these differences can happen from the very first days of the embryos.

How identical twins arise

As stated before, identical twins arise from a single zygote that has separated into two or three - or more - at a later date. Thus, the egg cell that forms has a single genetic material that is shared by the twins. Dizygotic twins, on the other hand, are born when fertilization of two different eggs by two different sperm happens as well.

Image: Lisa Runnels/Pixabay

The research, therefore, shows that some mutations occur before zygote separation. For this reason, they are inherited by both siblings. However, some mutations can occur after zygote separation (around 7 days after fertilization). This causes substantial differences to occur in the genetic material of the siblings. According to the authors, the earlier the zygote separation occurszygote, the greater the tendency for one sibling to possess genetic characteristics different from the other.

With these results, the research represents a milestone in the study of developmental genetics. This is because currently, identical twins are models for genetic studies because they have theoretically identical DNA. However, this type of assumption should change in the scientific discussion involving Nature and Nurture. In other words, this is one more step towards understanding how our DNA directs our livesto the detriment of the environment.

The article is available in the journal Nature Genetics.