In general, it is important to distinguish monozygotic twins from dizygotic twins, because there is a big difference here. Whereas in identical twins, a single egg cell is fertilized by a sperm and then divides randomly into two separate groups of cells during its development, which then develop into two organisms independently of each other, dizygotic twins arise from two fertilized egg cells. From a genetic point of view, this simply means that two normal siblings are growing in the womb at the same time.
Are twins clones?
Thus, dizygotic twins only share about 50 percent of their DNA, just like any other pair of siblings. On the other hand, identical twins are created from the same fertilized egg. So they must in fact be genetically identical – according to the popular view. Almost identical looks, often the same way of doing certain things, and similar tastes and interests underscore this.
But if you take a closer look, you will notice the differences here as well: while one is left-handed, the other can be right-handed, and traits and character can also vary greatly. Just like identical twins, they are never equal in size, and they also get moles in completely different places. In addition, twins rarely live in the same time period and also develop different diseases. Fingerprint or iris scans are very clear: the matches are different.
Differences in gene activity
For a long time, science explained this fact exclusively with the so-called epigenetics. Epigenetics is a mechanism that can cause different properties in the same genetic material. This works by preventing or facilitating the reading of individual genes by way of attachments to DNA. As a result, depending on the epigenome, some genes are active and others are permanently or temporarily turned off – with a corresponding DNA code.
Because of these epigenetic connections, twins can also develop different characteristics despite having the same DNA. The development of diseases in one twin can also be explained in this way: even if both twins carry a mutation that increases breast cancer risk, their activity can be inactivated in one of the epigenetic genes—and thus that twin is unlikely to develop the disease.
These differences are mainly due to environmental influences that affect the binding of epigenetic molecules to our DNA. As a result, identical twins also develop larger and further apart during their lifetime, the more different their lives will be.
At the level of the DNA code, identical twins possess exactly the same genetic material – according to conventional wisdom. But is this actually the case? In fact, identical twins are not genetically identical either. In 2021, a research team from the Icelandic genetics company DeCODE discovered that their genomes can vary greatly. The researchers compared the basic DNA sequences of identical twins and found distinct differences. On average, they found about 5.2 of these mutations, and 15 percent of the twins differed more: the scientists identified up to 100 mutations.
Accordingly, identical twins do not match 100, but only about 99.99 percent of their genome. In addition to environmental influences and the impact of them on the epigenome, DNA differences could also explain why one twin sometimes develops a genetic disease while the other does not.
Results by different genetics
This finding is important, among other things, in the so-called twin research. Because this actually assumes that identical twins are completely genetically identical and examines the influence of environmental factors on this basis. We now know that during evaluation it must be ensured that genetic influences can also play a role.
In addition, these genetic differences now make it possible to distinguish identical twins based on their genetic fingerprint. Where criminal and paternity issues previously remained unresolved, the sequence of bases in DNA can now be precisely analyzed so that single mutations can be detected and identical twins clearly distinguished.