One of the most detailed studies ever done on the DNA of identical twins has quantified the genetic differences that occur between them at the start of pregnancy, shortly after they become separate embryos.
Scientists estimate that on average, such twins can be differentiated by about five mutations (changes in DNA) that occurred early in embryonic development. It is of course relatively little when one takes into account the enormous genetic similarity between the siblings created in this way, but nonetheless the effect can be significant in terms of diseases and traits that are present in different ways in each twin. what can be influenced by variations in the genome.
The research responsible for making so-called monozygous twins look less identical than you think has just been published in the journal Nature Genetics. The study, coordinated by Hakon Jonsson of Icelandic company deCODE genetics (the company’s name is written in very small letters), analyzed the DNA of 381 pairs of twins and two groups of triplets, and also combed the genetic material of the parents, spouses, and children of each brother as this enables us to keep track of when and in whom possible mutations have occurred.
Monozygous twins usually appear between four and seven days after a single egg is fertilized by the sperm, during the first rounds of cell proliferation that make up the embryo. For unclear reasons, this single embryo can separate once and form two new embryos – theoretically with the same DNA.
However, as development continues, the genetic material in the cells of future babies will continue to be copied (since the vast majority of human cells must carry their own DNA).
This leads to copying errors that can occur in a wide variety of cell types. Since this is a (largely) random process, there is no reason to believe that the same mutations will occur in both twins.
The point is that most of these mutations will only be somatic (from the Greek term “soma”, body) and are limited to the person’s organism. However, mutations can also affect germ cells – eggs and sperm – and thus be transmitted to offspring. If the same mutation is somatic and germinating, it is strong evidence that it occurred when the person’s organism was at the beginning of its development and spreading over much of it.
To understand how often this happened in monozygous twins, the team obtained blood and oral cell samples from hundreds of volunteers and sequenced (ie “spelled”) their DNA with a so-called large cover-in more than 150 times in most cases. Roughly speaking, it would be like reading the same book over and over again so as not to miss a single comma or letter, and then doing the same number of readings on a very similar edition of that book to see if there were any errors in the output.
As expected, the initial analysis found dozens of different mutations between members of each couple, and this number tended to be higher with the age of the volunteers (as somatic mutations tended to accumulate over a lifetime). However, some of these mutations were present in most of the same person’s cells, suggesting that they were likely “descendants” of an older cell that had undergone the original mutation at the beginning of the person’s development – that is, still in the Uterus.
What confirmed this was the comparative analysis that also involved the DNA of the twins’ parents, children, and spouses, as the transmission of a mutation to the offspring shows it is germinating and its presence in non-reproductive cells confirms that it does too is somatic – this again indicates that it originally appeared at the beginning of embryonic development.
Of course, there is variability – some twins are actually very close to being genetically identical, while others differ by an above-average number of mutations. In addition, the changes may include replacing a single chemical “letter” in DNA, as well as insertions or deletions, that is, sections of DNA that have been added or deleted from the brothers’ genome.
Double myths
What’s real and what’s just folklore in what people say about the twins
Identical twins have exactly the same DNA
No it is not. Subtle mutations that occur after the fertilized egg from which the twins emerged have been split into two parts result in two slightly different genomes
Twin twins’ fingerprints are the same for both
In fact, while similar, their impressions are so different that a twin who commits a crime (say) cannot blame his brother
There is some kind of telepathic connection between them
Incredibly, this idea has been tested in experiments – for example, identical images for twins separated. It does not work
They tend to be closer together than “normal” siblings
Fact – In childhood, for example, identical twins would rather interact with each other than fraternal twins (not identical) or siblings who did not share the womb
It’s better to put everyone in their own class in school
Bullshit. Each twin’s academic performance is usually worse when this separation occurs
If you dress them up in the same way, the same personalities will emerge
Another cascade: the similarity in behavior of twins wearing the same clothes does not increase compared to those who dress differently
Identical twins are clones of one another
Yes, which indicates the degree of genetic similarity between them
Clones are identical twins
None of this, because a crucial factor would be missing – the sharing of the uterine environment – between a person and their clone, created decades later (when one day it obviously happens to humans).
Univiteline twins raised separately look like fraternal brothers, not identical
In fact, the resemblance is high and similar to what it would have been if they were created together
They have the same deformities (if any) and health problems
No, this does not happen because of random events like the location of the cells that give birth to babies in the womb and differences in behavior, large or small, in postpartum behavior
Genetically speaking, children of twins are brothers to one another
Yes. If the other members of the pair are not twins, the babies are genetically half-brothers. On the rare occasion that identical twins marry identical twins, babies are even genetically siblings
In families, the birth of twins tends to skip a generation.
It’s a myth. It seems that there are genetic factors that affect the birth of identical twins as well as fraternal twins, but it is not yet clear how this works, and it has nothing to do with the transmission of generations