Twins have been a fascinating field of study for the researchers. Parents who are blessed with twins are really lucky , as their happiness doubles. Children are bundles of joy and when you have twins the same joy doubles. Scientifically, twins are either identical (monozygotic) or fraternal(dizygotic).
Monozygotic or identical means that these twins arise from the same egg and are genetically and phenotypically identical. So do identical twins have identical genomes?
Well, that was what we thought , or what was taught to us in schools and colleges. It also seems to be the possibility as both the babies arise from a single zygote. So, they have to be similar.
But according to the studies conducted by Geneticist Carl Bruder of the University of Alabama at Birmingham and his colleagues monozygotic twins are not genomically identical. They conducted studies on 19 sets of identical twins and found differences in their DNA at certain points. One twin carried a different number of copies genes than the other . This condition is known as copy number variants.
Identical twins look identical and even though some may have slight differences in their appearance ,it is attributed to environmental effects. For example, if one twin takes up better nutrition than the other, they end up having different physique.
Normally, a child shares half of its genome with the father and other half with its mother. That is what makes us all differ from our parents.
In case of identical twins, fertilized egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information. Until recently we where all sure that identical twins were all the same.
But studies have proved that even identical twins differ with the copy number of a particular gene. A genes copy number also known as the copy number variants (CNV) can be defined as the number of copies of a particular gene in the genotype of the individual.
Normally every individual has two copies of a gene (two copy rule), one from father and one from mother. According to the current researches, their are regions in the genome with more copy number for a particular gene and that’s where copy number variants (CNV) are seen.
If we take an example according to the studies conducted by Bruder on identical twins, one such pair showed difference in their copy number for a gene. One twin was missing some genes in a particular chromosome that indicated the risk of leukemia, which he indeed suffered and the other twin didn’t.
Furthermore it was suggested by Bruder that CNV increase with our age. The genome we are born with is not the one we die die with. Also the chances of genomic rearrangement increase with our age. Thereby increasing the risks of diseases.
The reason why these studies could be useful is that it could be easy to identify the genetically. Now, why is it so important ?
What if one twin commits crime? How are we going to identify which twin did it?
This is where Copy number analysis is going to be useful. Copy Number analysis scan help us differentiate even between identical twins.
The reason behind the splitting of the zygote is no known. The are is still under research. scientists have tried to repeat the conditions invitro and have failed. Till now it is speculated that it may be the environment in the womb that triggers the splitting that results in monozygotic identical twins. Otherwise, the gene for identical twins doesn’t generally run in families.
Certain theories also suggest hat he sperm holds an enzyme that can trigger(splitting) the formation of identical twins.