Identical twins are formed from a single fertilised egg, which went on to split into separate embryos. This means their DNA is exactly the same and they are known as monozygotic. However, a new study is forcing scientists to refine their thinking on the genetics of twins.
Scientists in Iceland sequenced DNA from 387 pairs of identical twins. They found ‘early mutations’ that “separate identical twins”, according to geneticist Kári Stefánsson from the University of Iceland.
A mutation occurs when a cell divides and an error is made while the DNA is being duplicated. This alteration in a sequence of DNA can affect physical features. This could, for example, make one twin taller than the other. It can also affect susceptibility to certain diseases, possibly putting one sibling at greater risk of some cancers. The researchers found that, on average, identical twins have 5.2 mutations, but Dr Stefánsson said some pairs have up to 100.
In fact, his team found a mutation present in all cells of the body of one twin, but none in the other twin at all. The study is reported in the journal Nature Genetics .
In the past, it was believed that physical differences between twins may have been the result of environmental factors, including nutrition and lifestyle. This new research will force scientists to rethink the influences of genetics and environment.
Zygosity twin testing
It’s not always clear in utero whether twins are identical as they don’t always share the same placenta and sac: it all depends on when the eggs splits.
By around 2-years-old, it may become clearer from their physical features. For example, hair and eye colour, ear shape, teeth formation, hand and feet appearance can indicate whether the children are monozygotic or dizygotic. Some identical twins, however, don’t look exactly the same: there can be minor differences between siblings that lead their parents to believe they are non-identical.
If changes in physical features aren’t apparent, zygosity twin testing can help determine whether twins are identical or not.
Zygosity twin testing involves analysing tiny amounts of DNA from inside each sibling’s mouth. Specific markers present in repeat sections of DNA are examined. These 35 Short Tandem Repeat (STR) loci include the Amelogenin (gender) locus. The DNA seen at each of these STR loci is compared between the tested individuals; identical twins will share the same DNA profile whereas non-identical siblings will have different DNA profiles. Mutations as reported in the Icelandic study will not affect these results.
The Twin DNA test costs just £99 for next day results and includes a keepsake certificate.
An increasing and novel use of a DNA zygosity profile is as a unique gift idea for multiple birth parents and/or their children.
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 Differences between germline genomes of monozygotic twins. https://doi.org/10.1038/s41588-020-00755-1″>https://doi.org/10.1038/s41588-020-00755-1