We generally think that monozygotic twins are natural clones, which are identical twins. At the beginning of development, the embryo divides, separates and produces two groups of cells whose genes are identical, each of which will grow into a different but equal baby.
The results of a new study published in Nature indicate that cell allocation during development shapes genomic differences between monozygotic twins. But How relevant is this to determining that twins are not identical?
Monozygotic twins
The monozygotes are so similar that, even in 2009, in Germany, the arrest warrant for two of them was lifted when it could not be proven who had been the author of a jewelry theft in Berlin given their extraordinary resemblance (DNA included). But perhaps a small variation in the environment can produce very large effects on the phenotype (or the opposite), like the butterfly that flaps its wings at one end of the world and, through the art of birlibirloque, ends up generating hurricanes at the other end.
But before the birth of twins, we can take into account other important aspects that could undermine the common assumption that The genome sequences of monozygotic twins are almost identical. In fact, there is a paucity of studies characterizing the genomic differences between these twins.
Thus, the average number of differences between the genomes of monozygotic twins is unknown. Furthermore, the types of mutations that lead to these differences and their timing are unknown. That is what the cited study has tried to elucidate..
To estimate the number and timing of mutations that differ between monozygotic twins, Postzygotic mutations present in the somatic tissue of one of the twins were looked for., but not the other, and were timed by comparing whole genome sequencing (WGS) data from monozygotic twins, their offspring, spouses, and parents.
It was then shown that monozygotic twins differ on average in** 5.2 early developmental mutations** and that approximately 15% of monozygotic twins have a substantial number of these early developmental mutations specific to one of them.
Las mutaciones CpG> TpG aumentaron en frecuencia con el desarrollo embrionario, coincidiendo con un aumento en la metilación del ADN. Así pues, los resultados indican que la asignación de células durante el desarrollo da forma a las diferencias genómicas entre gemelos monocigóticos.
With everything, Some clarifications must be made regarding these findings..
Precisions about these results
This study is one of hundreds of articles from deCODE (DEcentralized Citizen Owned Data Ecosystem), which have repeatedly demonstrated how much deeper we can still delve into human biology when you have the genomic sequence of an entire country.
The authors studied a subset of de novo mutations (a de novo mutation is a mutation that appears for the first time in a family; neither parents nor grandparents have this genetic alteration) that arise after the formation of the zygote (that is, after fusion of sperm and egg).
Most de novo mutations are pre-zygotic and are present in the sperm or egg. The authors report 5.2 post-zygotic mutations in the 15% of the twins. This is ~0.1% of the total de novo mutation rate, which is 74 per individual.
We must keep in mind that a human being has six billion base pairs (considering the diploid genome). Consequently, the post-zygotic mutation rate is 5.2 / 6e09 \u003d 8.6e-10, which amounts to a simple drop in the ocean of the genome.
Pre-zygotic mutations are less random as they are associated with the sex and age of the parents. But, post-zygotic mutations are likely to occur by chance.
Therefore, most of the large effect associations reported for de novo mutations are likely due to prezygotic mutations. Nature study shows nothing about pathogenicity or phenotypic effects of post-zygotic mutations.
Given this background, claiming that the findings of this study call into question the validity of twin-based heritability studies is unwarranted. Identical twins remain essentially identical in their genomic sequence.
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The news
Are the genomes of identical twins really identical?
was originally published in
Xataka Science
by
Sergio Parra
.