Telling apart twins

04 December 2018

Question

How come some identical twins look kind of different?

Answer

Chris Smith put Steph's question to geneticist Patrick Short...

Patrick - Yeah it's a great question. So twins have had a very special role in the history of genetics. They've helped define this concept of what we call heritability which is basically how much does variation and genetics explain variation in a trait? They've helped define that for a huge number of disorders, in the early days it was often identical twins that were compared to non identical twins which are basically like siblings but that happen to be born at the same time.

And so when you compare those two groups you can basically say OK the identical twins have the exact same DNA with the exception of maybe a small number of mutations that one or the other might have, whereas the non identical twins are basically like brothers and sisters so they share half their DNA and that allows you to calculate if a trait varies about the same amount and identical and not identical twins then it's probably not genetic at all and if the identical twins are very close it has more of a genetic basis.

But what is often forgotten is almost every trait has an environmental component. Know you could anything we can think of whether it's cancer, cardiovascular disease, even our personality traits are molded by our environment. But there's a third factor which is just randomness or what we might call stochasticity. So even if you're studying a trait that you think has a huge genetic component to it, maybe face structure for example, and what people might think of as something identical twins should be very similar in, there is always an element of randomness in development. That means even if something is supposedly perfectly genetically tuned which which probably even facial development isn't then there's an element of randomness that just means you're going to end up with differences over such a long process of many years of development.

Carolin - So does this include something like freckles or moles on a face? Because in my experience that's been a way to identify them?

Chris - I was thinking exactly that. Yeah yeah. We used to tell two kids apart at school because one had a mole. Is that an example of what you mean by this randomness?

Patrick - Yeah that's right. It could also so could be something like freckles or moles that might represent a mutation in an individual skin cell. So that's what we call somatic mutations. So yeah I'm not sure exactly how how a mole arises. But freckles for instance have to do with the mutations that change the way pigment accumulates in the cell.

But there are other beyond just genetic changes on a cellular level. Simply the morphology of your face, how every cell you know at some point you're a single cell and then you're two and then you're four and then you're eight and the way those cells move around and arrange and become your body in your face, it's impossible to program perfectly.

There's there's a lot of great photos on the internet. If you take Obama's face and you kind of slice a line down the middle and copy the left side of the face across to the right or if you do the same and copy that right across the left, they look like completely different people so even your left side and right side of your own face if you were to duplicate it to the other side you'd look completely different.

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