Why the question ‘fraternal or identical’ is important to all twins?

Twins come in two types: identical and fraternal. Identical twins result from the splitting of a very early embryo and share the same genetic sequence. Fraternal twins are as genetically different as any two siblings; they just happen to share a womb.

When twins are born, the first thing a parent asks is “are they identical?” If one’s a boy and one’s a girl, it’s easy – they are fraternal. With all the other twins, this is where things get complicated.

If we know for sure that the twins shared a placenta, there is a high chance they are identical. This leaves almost half of all twins who have a separate placenta and are the same sex and the only way to know for sure whether they are identical of fraternal is to get a DNA fingerprint (‘zygosity’) test done.

To complicate things even further, everyone, including strangers, family and friends seem to have an opinion about ‘identical or fraternal’ one way or another. Furthermore, many wrongly assume that identical twins always look and behave identical or that identical twins always share a placenta. These false assumptions have led to confusion, yet little research has been conducted with the twins community about understandings and assumptions.

So, at the 2012 Australian TwinsPlus Festival, we asked for twins and parents of twins who were in any way unsure of their genetic identity. In one day we got over 100 pairs!

We asked for their best guess on ‘identical or fraternal’; we asked about the reasons for their assumptions and we asked about the importance of accurate knowledge. Responses were compared with twins’ true genetic identity determined from cheek cell DNA they provided on the day.

We found that many parents and twins had been misinformed in the past but that knowledge of their true genetic status provided peace of mind and made them and their families happy, if a little surprised in some cases.

The main reasons given for the importance of such knowledge related to feelings of certainty, a sense of identity and concerns about health implications. For these reasons we propose that all same-sex twins and their parents should be advised to seek the certainty of a genetic test to minimise confusion and to provide peace of mind.

In the meantime, we have gathered more evidence in favour of universal zygosity testing for same-sex twins and we have a manuscript under review. Watch this space!

If you would like more details, see our publication which is also available for personal use on request.

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