![]() when you’re trying to understand whatever differences there may be between monozygotic twins, you should take a step or two away from the term ‘identical.’ KS: I think that identical twins, all of them, are always going to be sufficiently similar to each other that it shouldn’t offend anyone to call them identical. TS : So should we stop referring to monozygotic twins as “identical”? We can begin to use the mutations to develop understanding of how cells are allocated from the early embryo to develop the various organs in the body. This is not just a study that has relevance when it comes to understanding of the genetics, but also human development: How do we probe early human development in an ethical manner, a non-interventional manner? This is one way of doing that. So this certainly places a new kind of burden on those who use identical twins to establish the separation between nature and nurture. But before you can make that interpretation, you’d better make sure that one of them does not have a de novo mutation in an important gene that the other one does not. The classic interpretation of whatever differences would be that the difference between them would be due to different environments. Take, for example, studies looking at autism in identical twins who are reared separately or apart. KS: It the assumption that the differences between identical twins is always due to the environment. How do your findings affect that kind of research? TS : Twin studies have played an important role in a lot of genetics research. So they are rare these very early mutations, because methylation hasn’t taken place, or a very small amount of the methylation has taken place. We can show that the number of those mutations increases with development. That is the CpG to TpG mutation that is considered to be dependent on demethylation of methylated CpGs. There’s one, for example, specific type of mutation that is dependent on methylation. ![]() This certainly places a new kind of burden on those who use identical twins to establish the separation between nature and nurture. One of the things we did is to look at the kind of mutations that happen during development, and we were able to show that the type of mutations that happen early on differ from the mutations that happen later-which is not a surprise to anyone, really. KS: Most of those mutations, like most mutations in general, happen during the replication of cells-they’re replication errors. TS : Were you able to establish what caused these mutations? it looks like coincidence is an extraordinarily important factor in which cells in the inner cell mass go into making a person. What’s interesting about it is that when you look at all the combinations that we have found. This is the first time that this is demonstrated. So one of the twins has just formed from the descendant of this one cell where the mutation happened, and the other is formed in part by descendants of that cells and in part by something else. Then we have found twins when the mutation is found in all cells in the body of one of the twins, and in 20 percent of the cells in the body of the other twin. ![]() That means basically that one of the twins is formed solely from the descendant of the cell where the mutation took place. We have found a twin pair where one of the twins has mutations in all cells of his body, and they are not found in any cell in the body of the other twin. During this stage of development, this inner cell mass can split to form two separately developing embryos. The new study focuses specifically on mutations that occur as or before embryos form from the mass of cells inside the blastocyst, a structure that implants in the uterine wall. Rather than having exactly the same DNA sequences, twins start accumulating genetic variation from the earliest stages of development, researchers at Iceland-based company deCODE genetics found, meaning that one twin harbors variants that aren’t present in the other.Īlso known as monozygotic twins because they develop from a single fertilized egg, identical twins have long been central to research on the relative effects of genes and environment-aka “nature versus nurture.” Although everyone accumulates some genetic mutations during their lifetime, the differences in identical twins were assumed to be minimal, particularly when twins are young, allowing researchers to study how different environments influence the development of people with the same genotype. ![]() Identical twins are not as identical as previously assumed, according to a study published today (January 7) in Nature Genetics. ![]()
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