Identical? Not really. Twins are not DNA clones
And why are there so many stories about an 'evil twin'?
Beloved Fido bites the dust after a long loving life, and your overwhelming grief drives you to buy ‘cloning services’ to recreate another young version of your pet. Same genes, so that fun Fido personality will be generated too, right? You never know, you could end up, instead, with Fido’s evil twin…
Epigenics
Twins studies measure what is the same between identical monozygotic (single egg origin) twins, and compare them to fraternal dizygotic (two egg origin) twins, attributing the difference in sameness to genetics. Animal Husbandry depends on these genetic traits ‘breeding true’ as they build cattle herds or racehorses. But are genes really replicated similarly from one clone to the next?
New research focuses on what’s discordant (different) between twins, in order to determine which traits (phenotypes) are susceptible to ‘epigenetics’ - modifications of chromosomal DNA that regulate expression of genes in each cell. Commonly seen are DNA methylation and histone acetylation.
The figure above shows pairs of chromosome 3 from three year (top) and 50 year (bottom) monozygotic twins. They used red tags for one twin's epigenetic tags (methyls or histone acetylations) and green tags for the other twin. If they are at the same place in both twins in the overlaid image, the color will be yellow. If they don’t overlap (tagged on different genes), you see the red and green. The older twins’ chromes differences light up like a Christmas tree.
Such are the marks left by life. It starts as early as in utero, as soon as the embryo is split. Sometimes the cleave is across an axis, causing mirror image babies. Perhaps even their brain hemispheres are flipped? Blood flow, amniotic fluid movement, placental placement, nutrition - all contribute to the unique exposure of that individual fetus. Stressful events such as famine or war can also significantly alter the epigenetic landscape.
GENETICS
But at least the DNA is identical right? Ahh nope. New research just came out from Kari Stefansson, a geneticist at the University of Iceland:
Scientists in Iceland sequenced DNA from 387 pairs of identical twins … as well as from their parents, children and spouses. That allowed them to find early mutations that separate identical twins, … On average, identical twins have 5.2 of these early genetic differences, … But about 15% of identical twin pairs have more genetic differences, some of them up to 100 … Stefansson said that his team had found pairs of twins where a mutation is present in all cells of the body of one twin, but not found in the other twin at all. However, sometimes the second twin may show the mutation in some cells, but not all cells. (a chimera)
So, the genes are NOT completely the same with identical twins, and a person could even have a chimeric mix of different genes in their body makeup.
Certainly complicates that Nature versus Nurture debate. And we still don’t know how to tell which twin is the evil one.
REFERENCES
https://en.wikipedia.org/wiki/Evil_twin
https://www.latimes.com/local/lanow/la-me-evil-twin-20171128-htmlstory.html
http://biologicalexceptions.blogspot.com/2015/08/epigenetics-and-evil-twin.html
https://www.theatlantic.com/science/archive/2018/05/twin-epigenetics/560189/
https://www.scientificamerican.com/article/identical-twins-genes-are-not-identical/
https://medicalxpress.com/news/2021-01-identical-twins-clones.html
https://www.nature.com/articles/s41588-020-00755-1
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