12 December 2013

Not exactly identical twins

Now a story like the one described in the long-standing article "Father's Conception with a finger", when one of the two twins who slept with a common girlfriend in turn demanded a genetic examination to establish paternity, may not be as absurd as in 2007. It is not the same as a wound: with the help of a thorough genetic analysis, it is possible to determine quite accurately which of the identical twins is the father of the child, and who is the uncle. Or to find out from the DNA traces which of the twins has stamped the hat and cocked the grandmother (C).

Geneticists have learned to distinguish identical twins

<url>The German company Eurofins has developed the first genetic test that is able to distinguish identical twins.

Previously, this was only possible in a few special cases. A description of the technology of searching for a needle in a haystack is published in the journal Forensic Science International (Weber-Lehmann et al., Finding the needle in the haystack: Differentiating “identical” twins in paternity testing and forensics by ultra-deep next generation sequencing), briefly about it can be read in the press release of the company Genetic Differences Between “Identical” Twins Discovered.

The work was carried out with the participation of two identical twins and the son of one of them. Geneticists have managed to detect differences in the genome of men, which make it possible to unambiguously determine which of the two is the father.

The essence of the method consists in ultra-deep (i.e. multiple) sequencing of DNA isolated from male sperm and subsequent search for single-nucleotide variations (SNP) in two read sequences. Multiple sequencing of the same genome is required in order to reduce the error rate to a value that allows you to find single substitutions in completely identical sequences. During the experiment, geneticists had to read the genome of each of the three participants an average of 94 "standard" times.

For the father and son who participated in the experiment, the scientists managed to find five SNPs that were common to the two of them, but differed in the twin uncle.

One of these snips (illustration from the Eurofins – VM website).

As previously predicted in a theoretical experiment, at least one such difference should exist in 80 percent of identical twins. However, until now, only epigenetic differences have been experimentally demonstrated, that is, differences in the chemical modification of DNA and histones, and not in the nucleic acid sequence itself. These modifications, however, are not always inherited, which generally makes it impossible to determine paternity. In criminology, epigenetic differences have not yet been applied.

Variations in the DNA of twins occur in the course of individual development after the beginning of division of the zygote. Such differences are inherited, however, only if mutant cells fall into the sexual line. Thus, the time window for the appearance of inherited mutations in identical twins turns out to be very narrow.

The inability to distinguish identical twins using standard DNA tests has caused several legal incidents. For example, in Malaysia in 2009, the judge was forced to acquit both twins in order not to punish the innocent, although she was sure of the guilt of one of them.

Portal "Eternal youth" http://vechnayamolodost.ru12.12.2013

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