Identical in appearance, but genetically different
Twins can have many genetic differences
Kirill Stasevich, CompulentaIt is generally believed that identical twins have identical genomes.
On the one hand, it really is: they are born from a single fertilized egg, divided in two. All alleles (gene variants) are identical. On the other hand, when cells multiply, DNA is doubled, and so-called copying errors are inevitable, which are not always corrected. These mutations often lead to death even during embryonic development. However, there are also those that do not cause much harm to the multiplying cells of the embryo and spread through the tissues. These mutations are called somatic, they are not transmitted to descendants, since they do not capture germ cells, but they persist in humans throughout life.
Researchers from McGill University (Canada) decided to check whether the genomes of twins will differ, taking into account such errors obtained during individual development. It should be clarified here that the very existence of such mutations, of course, has long been no secret, but until now it was believed that they are rare, and if we talk about twins, they cannot differ significantly. However, this time scientists have come to a different conclusion. They analyzed the genomes of 92 pairs of identical twins in search of single–nucleotide substitutions in the genetic code (that is, when one of the twins had one nucleotide in a certain place, the other had another in the same place). All mutations, according to the authors of the work, could occur at the early stages of development, and then spread to tissue cells.
The probability of such mutations was determined: it turned out that the average pair of twins carries 359 genetic differences that arose during embryonic development. That is, from a genetic point of view, they are not such twins. This may explain, in particular, why, despite the amazing similarity, they may have a different medical history: one may have, for example, cancer, and the second will never encounter a tumor at all.
The results of the work were reported by scientists at the congress of the American Association of Researchers of Human Genetics.
Here, however, it is necessary to emphasize two things that the researchers are going to take into account in further experiments. For their analysis, the authors took blood samples, but we have cells that divide both faster and slower than blood cells. Accordingly, the level of accumulated mutations in them may be higher or lower, which again cannot but affect the likelihood of certain diseases. On the other hand, there are also epigenetic differences that also develop in the early stages of development, persist throughout life, but are not related to the genetic code itself, representing modifications of DNA and histones serving it.
Prepared based on the materials of LiveScience Identical Twins Are Genetically Different, Research Suggests.
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