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Techly Explains: Are twins genetic?

Imagine the moment, you’ve just been told you’re expecting twins. You are trying not to think about The Shining.

You are wondering why you, specifically, have somehow ended up doubly pregnant. Allow Techly to shed some light on the subject.

Now while that clip from the late ’80s buddy comedy Twins isn’t the most scientific thing you’ll see today, it’s always fun to see Arnie acting in the rare scenes when he isn’t mowing down foot soldiers and it does raise a significant point. There is a large difference between identical (or monozygotic) twins and fraternal (dizygotic) twins, here demonstrated ably by DeVito and Schwarzenegger.

In the case of identical twins, as the medical term monozygotic may suggest, they occur when one zygote (essentially a fertilized egg) splits into two halves during early development, meaning both embryos have identical genetic information. Fraternal twins, on the other hand, develop from two separate zygotes and are therefore made up of differing, while similar, genetic information.

So, is there a genetic reason for the occurrence of twins? Could there be some genetic predisposition to carrying twins? Well according to this post on The Stanford Tech forum it’s kind of ‘yes and no’ territory. To be more specific, the post states ‘identical twins do not run in families and a history of fraternal twins only helps if it comes in on the mother’s side’. Furthermore, it says that a female fraternal twin is 2.5 times more likely to give birth to a further set of twins and that goes up to 3-4 times when the woman already has already given birth to a set of fraternal twins.

According to the Sciencemag site ‘scientists from eight countries found two genes that increase a woman’s chance of having twins’. A team of researchers in Amsterdam, where the Nederlands Twin Register which currently contains 75,000 cases, started in 1987 collated data from databases in the Nederlands, USA and good ol’ Australia.

The researchers, working on a sample of over 2000 births, examined the genetic information of the mothers to see if there was a common link between the mothers of fraternal twins. They eventually narrowed it down to two SNP’s (essentially single stretches of DNA that signpost genetic differences between people) and subsequently reported in the American Journal of Human Genetics that having a copy of each of them will increase that person’s chances of giving birth to fraternal twins by a huge 29%. The first SNP relates to the production of the follicle stimulating hormone (FSH), which, if the levels remain quite high while the ovaries mature, can lead to the production of more than one egg. The other SNP is a little more mysterious, SMAD3 has been noted to ‘change how ovaries respond to FSH, at least in mice’ but in terms of its role in human fertilization, research is ongoing.

So there you have it, of course, a full genetic analysis is not necessarily available to everyone, so whether or not you are genetically predisposed to have your own DeVito/Schwarzenegger caper may have to remain a surprise for now. Having said that, you’re family history can, of course, be a handy indicator when considering your own genetic make-up, so Auntie Jane should be able to give you some idea!

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