How Much Genetic Makeup Do Identical Twins Share
Identical twins don't share 100% of their DNA
Identical twins course from the same egg and become the aforementioned genetic material from their parents — but that doesn't mean they're genetically identical by the time they're born.
That's because and then-called identical twins option up genetic mutations in the womb, equally their cells weave new strands of Dna and so separate into more than and more cells. On average, pairs of twins have genomes that differ by an average of 5.2 mutations that occur early in evolution, according to a new report.
"One particularly surprising observation is that in many twin pairs, some mutations are carried by about all cells in ane twin just completely absent in the other," Ziyue Gao, an assistant professor of genetics at the University of Pennsylvania, who was non involved in the research, said in an email.
Related: Seeing double: 8 fascinating facts about twins
The written report authors estimate that, in near 15% of identical twin pairs, one twin carries a "substantial" number of mutations that the other does not share.
"That can be upwardly to well-nigh 10 to xv mutations," said senior author Kari Stefansson, CEO of deCODE genetics, a subsidiary of the biopharmaceutical company Amgen that studies the human genome. The study did not specify where in the genome these mutations occur, or if they mostly crop up in genes that code for specific kinds of proteins; this could be an area of future inquiry, Stefansson said.
Twinning occurs when a single fertilized egg, called a zygote, splits and gives ascent to two separate embryos; this typically occurs between 1 and seven days afterwards fertilization, Stefansson said, although in rarer cases, twinning can occur between Day viii and 13. The later the split occurs, the more cells will accept accumulated when the twins separate.
And then when one twin has many mutations that the other doesn't, it's possible that the siblings may take carve up from each other extremely early on in development, soon after their shared egg first divided into 2 separate cells, Gao said.
Alternatively, the egg may have split after a dozen or so cells accumulated, but that cluster of cells didn't divide evenly between them. Instead, one twin (i.e. one half of the zygote) may take snagged a clump of cells that mostly stemmed from one parent jail cell and therefore carried common mutations, while the other twin got a clump of cells lacking those mutations, she said.
"Some of them are probably inconsequential … and some of them may atomic number 82 to diseases," Stefansson said. In twin studies, which scientists employ to explore whether genetics or the environmental factors hold more influence over a given trait, "we volition take to account for the influence of these mutations," he said.
The new report, published Jan. 7 in the journal Nature Genetics (opens in new tab), offered this unique snapshot into early development because the authors did some clever detective work using DNA from three generations of people.
These individuals included 387 pairs of identical twins and two sets of triplets, every bit well as their parents, spouses and children. (The triplets were likewise monozygotic, meaning split from the same egg.) By sequencing whole genomes from all these family members, the team could rails which mutations appeared in which twins, and which of those mutations were then passed downwards to the twins' offspring.
If a mutation gets passed down through multiple generations, that indicates that information technology'south a germline mutation — ane that appears in the eggs, sperm and their precursors. If that same mutation also appears in the somatic (non-reproductive) cells of the parent, that mutation likely appeared during their early on development, the authors annotation.
That's because in the first few weeks after fertilization, no cells take been slated to go germ cells all the same, so all cells can inherit the same mutations. After cells differentiate into germ and somatic cells, new mutations that appear in the somatic cells won't get passed to the person'southward children, while new mutations in the germ cells would, co-ordinate to the National Cancer Institute.
So "if such a mutation is plant in both a twin'due south blood and transmitted to their offspring, this mutation occurred during their early development," when all cells were closely related, Gao said.
In addition to tracking mutations between generations, the authors looked for mutations that were shared inside a gear up of twins but not present uniformly in all their cells. This phenomenon, known as mosaicism, indicates that the mutation occurred subsequently fertilization of the egg only before the egg split apart, since both siblings carry the quirk, Gao said.
Using both these methods, the team was able to pinpoint which mutations cropped up in this narrow window of early development, and how often one twin had a mutation that the other didn't. In short, one can't assume that identical twins share identical Deoxyribonucleic acid, they constitute.
One limitation of the written report is that the authors collected DNA from cheek swabs and blood samples, merely they gathered no DNA from, say, sperm or eggs, Gao said. If more tissues are sequenced, the authors volition likely identify more than embryonic mutations and be meliorate able to pinpoint those mutations to a certain developmental stage based on their frequencies in dissimilar tissues, she noted.
Additionally, the authors noted that they didn't know which twins shared a amniotic sac, placenta or chorion — the membrane that gives rise to the fetal office of the placenta. With that information, they could determine whether sharing these structures is at all related to the number or timing of genetic mutations in early development.
For now, the takeaway from the current study is that scientists should not presume that identical twins share 100% identical Dna; such assumptions could lead them to overestimate the influence of the surround, when in reality, a genetic mutation may exist the source of a given disease or trait, Stefansson said.
However, "such genomic differences between identical twins are nevertheless very rare, on the order of a few differences in six billion base pairs," with base pairs existence the edifice blocks of DNA, Gao said. It'southward unclear how many of these small mutations would issue in a functional change that alters how the cell works, and in general, "I doubt these differences will have appreciable contribution to phenotypic [or observable] differences in twin studies," she added.
Originally published on Alive Science.
Source: https://www.livescience.com/identical-twins-dont-share-all-dna.html#:~:text=That's%20because%20so%2Dcalled%20identical,according%20to%20a%20new%20study.
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