We know that twins are of two types. Some twins come from two different eggs fecundated by two different sperms (this means they can have different genders); they are just ordinary brothers
born at the same moment. This are called dizygotic or non-indentical twins and, like any brothers, they share usually 40-60 % of their DNA.
The second category of twins are identical or monozygotic twins, when two embryos develop from one egg fecundated by one sperm (thus, they have always the same sex). Common knowledge says that they have 100 % the same DNA. Is it so? A new research published in the American Journal of Human Genetics and lead by an international team from the University of Alabama, Leiden University Medical Center and VU University (The Netherlands), Uppsala University and Karolinska Institutet, Sweden shows that 100 % is not quite correct, and explains why, in pairs of identical twins, one can be plumper or a little taller than the other or why one might develop a disease and the other does not.
It has been blamed so far on the environmental factors, but the new study made on 19 pairs of monozygotic twins detected differences in copy number variations (CNVs) of the DNA. These differences occur when a set of coding nucleotide bases in DNA are missing or when extra copies appear. In the moment of the conception, the identical twins may have 100 % the same DNA, but during subsequent DNA replications during the cell divisions, mutations may occur; they add or steal one to several nucleotide bases in the DNA copies of a particular bit. Most of these mutations may not impact health or development, but some can increase the risk of cancer or other diseases.
"The presumption has always been that identical twins are identical down to their DNA. That's mostly true, but our findings suggest that there are small, subtle differences due to CNV. Those differences may point the way to better understanding of genetic diseases when we study so-called discordant monozygotic twins….a pair of twins where one twin has a disorder and the other does not. One twin might develop a particular disease… Parkinson's, for example… while the other does not," said lead authors Dr. Carl Bruder and Dr. Jan Dumanski of UAB's Department of Genetics.
"More importantly, changes in CNV may tell us if a missing gene, or multiple copies of a gene, are implicated in the onset of disease. If twin A develops Parkinson's and twin B does not, the region of their genome where they show differences is a target for further investigation to discover the basic genetic underpinnings of the disease," said Bruder.
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