When identical twins are born, they are the same genetically—they share all of their DNA. But as they grow and develop, each twin will acquire his or her own set of mutations. Hundreds of them, in fact, all turning what were once carbon copies into vague lookalikes.
Life Science explains a study done by Rui Li at McGill University, who wanted to study how many somatic mutations—mutations that arise when cells copy themselves—occur in the early lives of twins:
To find out how often these mutations occur in early development, Li and her colleagues studied the genomes of 92 pairs of identical twins and searched hundreds of thousands of sites in their genomes for differences between twins in base pairs, which are represented by letters that make up DNA. For instance, one twin may carry an A at one point while another carries a C. The researchers could only detect differences that would occur very early in fetal development and would show up in most cells in the body.
What they found was that an average twin pair has 359 mutations in their genome where they differ, all of which happened during early development.
Identical twins differ in other genetic ways too. Scientific American writes:
Geneticist Carl Bruder of the University of Alabama at Birmingham, and his colleagues closely compared the genomes of 19 sets of adult identical twins. In some cases, one twin’s DNA differed from the other’s at various points on their genomes. At these sites of genetic divergence, one bore a different number of copies of the same gene, a genetic state called copy number variants.
This can make a big difference, too. Scientific American again:
For example, one twin in Bruder’s study was missing some genes on particular chromosomes that indicated a risk of leukemia, which he indeed suffered. The other twin did not.
Of course, the idea that identical twins are genetically identical is the keystone to many genetic studies. Twins provide a unique opportunity to look at, for example, just how much nature influences a child’s personality or development. Because twins were always thought to have the exact same genes, they can essentially be their own control group.
Here’s the University of Utah on twin studies:
For example, when only one identical twin in a pair gets a disease, researchers look for elements in the twins’ environments that are different. Data is collected and compared for large numbers of affected twins and coupled with DNA and gene product analysis. These types of twin studies can help pinpoint the exact molecular mechanism of a disease and determine the extent of environmental influence. Having this information can lead to the prevention and treatment of complex diseases.
To illustrate, for twin pairs where schizophrenia occurs, in 50% of cases both identical twins in a pair develop the disease, while only 10-15% of cases in fraternal twins show this pattern. This is evidence for a strong genetic component in susceptibility to schizophrenia. However, the fact that both identical twins in a pair don’t develop the disease 100% of the time indicates that there are other factors involved.
But if they’re not actually identical, then perhaps these studies aren’t quite as well controlled as we once thought.
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