Why are some twins not identical

From now on, identical twins are no longer genetically identical

Scientists at the Eurofins laboratories have developed the first genetic test to differentiate for forensic purposes and to prove paternity

Until now, one had to believe that identical twins could not be distinguished genetically. Conventional genetic testing has not been able to determine which of the twins has committed a crime or is the father of a child, for example. Twins could therefore let the identification run nowhere, which also means that no one could be held responsible. Identical twins are rare, but at least 6 out of 1,000 men do. Doubts about the genetic identity have been expressed, but the differences have not yet been proven.

In a judgment of the Higher Regional Court of Celle in January of this year, the problem for the proof of paternity was negotiated and it was determined that there was "no tried and tested procedure" so far, which is why the defendant identical twins refuse to give a sperm sample because of the right to informational self-determination can: "In contrast to the 'pure' genetic fingerprint, the coded components of the DNA allow significant conclusions to be drawn about character or disease-related personality traits. The defendant and his twin brother would not have this even with regard to the plaintiff's right to know his parentage to tolerate and accept. Especially since the analysis of these genetic material components would be carried out by way of a procedure with an experimental character, which does not promise any reliable and verified results.

However, identical twins are known to be by no means physically and psychologically identical, although they have the same genes. This was explained by epigenetic influences. Scientists at the Eurofins gene laboratory in Ebersberg near Munich have now developed the first genetic test to differentiate between identical twins for forensic and paternity tests, according to a message that speaks of a "milestone". There is no real identity in the biological world.

Theoretically, it had already been predicted that there would have to be at least one mutation that was present in the father and child but not in the other twin. With an "ultra-deep next-generation sequencing technique," as reported in the journal Forensic Science International: Genetics, the scientists have now solved the problem of genetic differentiation. This allows extremely rare mutations (single nucleotide polymorphisms SNPs) to be recorded. In conventional genetic tests, only short tandem repeats (STR) or microsatellites are tested in a DNA strand, but these do not differ in identical twins.

For the test, sperm, blood and saliva samples from two identical twins and one blood and saliva sample each from the child from one of the twins and its mother were sequenced. The scientists in the laboratory didn't know who the child's father was. Sequencing revealed five mutations that the father and child had in common, but not the twin. The genetic differences found in the sperm and saliva samples but not in the blood samples are seen as evidence that rare mutations arise shortly after the division of the blastocytes, which usually occurs no later than the ninth day after fertilization, and in both and sperm cells are maintained. If they were also present in the blood samples, then, according to the scientists, they would have arisen before the division.

Since the SNPs of father and child were also found in the saliva samples, a sperm sample is not necessary to identify a twin; saliva samples are sufficient. Collecting these is ethically and legally easier. The genetic test can be used not only for proof of paternity, but for forensic purposes, since the differences can also be detected in skin, hair or sperm found at crime scenes. Of course, Eurofins has not only developed a new test method, but also offers the test to authorities, courts and private individuals. (Florian Rötzer)

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