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Identical twins form from the same egg and obtain the same genetic material from their parents – but that doesn’t mean they’re genetically identical at the time of their birth.
This is because the so-called identical twins detect genetic mutations in the womb, as their cells weave new strands of DNA and then divide into more and more cells. On average, twin pairs have genomes that differ on average by 5.2 mutations that occur early in development, according to a new study.
“A particularly surprising observation is that in many pairs of twins, certain mutations are carried by almost all cells of one twin but completely absent from the other,” Ziyue Gao, assistant professor of genetics at the University of Pennsylvania, who was not involved in the research, said in an e-mail.
Related: Seeing Double: 8 Fascinating Facts About Twins
The study authors estimate that in about 15% of identical twin pairs, one twin carries a “substantial” number of mutations that the other does not share.
“It can represent up to 10 to 15 mutations,” said lead author Kari Stefansson, CEO of deCODE genetics, a subsidiary of biopharmaceutical company Amgen that studies the human genome. The study did not specify where in the genome these mutations occur, or whether they mainly appear in genes encoding specific types of proteins; this could be an area for future research, Stefansson said.
Twinning occurs when a single fertilized egg, called a zygote, divides and gives rise to two separate embryos; this usually happens between one and seven days after fertilization, Stefansson said, although in rarer cases, the pairing can take place between the 8th and 13th day. The later the split occurs, the more cells will have accumulated when the twins separate.
So when one twin has many mutations that the other does not, it is possible that the siblings separated very early. development, shortly after their shared egg first split into two separate cells, Gao said.
Alternatively, the egg may have divided after a dozen cells accumulated, but this group of cells did not divide evenly among themselves. Instead, one twin (i.e. half of the zygote) may have caught a group of cells that mainly came from a parent cell and therefore carried common mutations, while the other twin got a group of cells lacking these mutations, she said.
“Some of them are probably harmless… and some of them can lead to disease,” Stefansson said. In twin studies, which scientists use to explore whether genetics or environmental factors have more influence on a given trait, “we’ll have to account for the influence of these mutations,” he said.
The new study, published January 7 in the journal Genetics of nature, offered this unique snapshot of early development because the authors did clever detective work using DNA from three generations of people.
These people included 387 pairs of identical twins and two sets of triplets, as well as their parents, spouses and children. (The triplets were also monozygous, that is, separated from the same egg.) By sequencing entire genomes of all of these family members, the team could track which mutations appeared in which twins and which of these mutations were then passed on to the offspring of the twins. .
If a mutation is passed on over several generations, it indicates that it is a germline mutation – a mutation that appears in eggs, sperm and their precursors. If this same mutation also appears in the somatic (non-reproductive) cells of the parent, this mutation probably appeared during their early development, note the authors.
This is because in the first few weeks after fertilization, no cells have yet been scheduled to become germ cells, so all cells can inherit the same mutations. Once the cells differentiate into germ cells and somatic cells, new mutations that appear in the somatic cells will not be passed on to the person’s children, while new mutations in the germ cells would, according to the National Cancer Institute.
So “if such a mutation is found both in the blood of a twin and passed on to their offspring, this mutation occurred during their early development,” when all the cells were closely related, Gao said. .
In addition to tracking mutations between generations, the authors looked for mutations that were shared within a set of twins but were not present uniformly in all of their cells. This phenomenon, known as mosaicism, indicates that the mutation occurred after the egg was fertilized but before the egg separated, since both siblings carry the oddity, Gao said.
Using these two methods, the team was able to identify which mutations occurred in this narrow window of early development and how often one twin had a mutation the other did not. In short, one cannot assume that identical twins share identical DNA, they found.
One of the limitations of the study is that the authors collected DNA from cheek swabs and blood samples, but they did not collect any DNA, for example, from sperm or eggs, Gao said. If more tissues are sequenced, the authors will likely identify more embryonic mutations and be better able to localize these mutations at a certain stage of development based on their frequencies in different tissues, she noted.
Additionally, the authors noted that they did not know which twins shared an amniotic sac, placenta, or chorion – the membrane that gives rise to the fetal part of the placenta. With this information, they could determine whether the sharing of these structures is related to the number or timing of genetic mutations early in development.
For now, the takeaway from the current study is that scientists should not assume that identical twins share 100% identical DNA; such assumptions could lead them to overestimate the influence of the environment, when in reality a genetic mutation may be the source of a given disease or trait, Stefansson said.
However, “such genomic differences between identical twins are still very rare, on the order of a few differences in 6 billion base pairs,” with base pairs being the building blocks of DNA, Gao said. It is not known how many of these small mutations would cause a functional change that alters the functioning of the cell, and in general, “I doubt these differences have any appreciable contribution to the phenotypic. [or observable] differences in twin studies, ”she added.
Originally posted on Live Science.
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