Identical or Non-Identical?

Stories from twins who discover their true zygosity in adulthood

Over the 24 years of studying twins we have learnt that non-identical twins can look surprisingly similar to each other and that about 10% of identical twins have in fact significant differences in appearance or health. This has changed forever how we conduct our research, and goes some way to explaining how about 7% of members of TwinsUK report a zygosity (whether they are identical or non-identical) that is different to what their genes have revealed to us.

The success of our research depends on having accurate zygosity information on all our twins. So how do we determine this? Firstly, we ask all new members of TwinsUK to complete the “Peas in the Pod Questionnaire” which asks about similarity during childhood with questions such as “At school, did people have trouble telling you apart?” The results of the PPQ (which is accurate 95% of the time) is used to determine zygosity. However, sometimes the PPQ does not match the zygosity reported by twins. In which we case we probe further, asking for example how many placentas there were at birth, as there is a misconception that two placentas at birth occurs only in non-identical twins. In fact, approximately a third of identical twins have two placentas. Similarly, the presence of a single placenta can be misleading as some placentas fuse to give the appearance of a single placenta. We may also ask whether there are marked physical or asymmetric differences. Sometimes we even ask for photos.

If we are still none the wiser, then we arrange DNA zygosity testing. This test looks at 16 variable gene markers for similarities or differences between the twins and gives a 99.9% level of accuracy. Many of our genetic studies have involved ‘genotyping’ which also examines genetic similarities and differences and we have also used this information, when available, to confirm zygosity.

A dedicated member of staff, Raj Gill, our valued project co-ordinator, coordinates all our zygosity work, including helping to confirm zygosity in cases that are ambiguous. Below, two sets of twins explain how they learnt of their true zygosity, after a life-time of thinking they were non-identical. Wendy and Gaye, and Anne and Emma tell their stories of discovery and the difficulties that occurred as a result of growing up the “wrong” zygosity and their emotions on discovering the truth.

Wendy and Gaye

‘The doctor who delivered us told my parents we were non-identical as we had two placentas. We were, however, Gaye_and_wendy2very similar as children and were constantly mixed up by our teachers and friends (and we often swapped our name As_childrenbadges in school to further the confusion). Our father however insisted we were identical. Our mother stuck by the doctor’s word that we were non-identical, but we now think consciously, or sub-consciously, she wanted us to grow up independently of each other, and she may have felt that being non-identical would make that easier to achieve. She was herself a non-identical twin and she clearly wanted us to be too.

And then we came to the Department of Twin Research for a twin visit in March 2015. The researchers compared our facial features -which suggested to them that we were identical- with our responses from the “Peas in the Pod” questionnaire – which implied we were non-identical. With such a discrepancy, the DTR always seek to resolve the matter, as their research depends on correctly classifying identical and non-identical twins. They therefore sent off our sample for zygosity testing which compared our DNA at 16 variable sites. The twin visit was special for a number of reasons, not only because it started us on the process of confirming our zygosity, but because we were the 3000th twin visit since the NHS – research funded visits started in 2012, and we had a special lunch with the team and received flowers from Professor Tim Spector and Professor Graham Lord (Director of the BRC).

A few months later, Raj Gill, who coordinates the DTR’s zygosity work, contacted us to tell us that we were in fact identical twins! We were very happy with the news as we had secretly hoped this to be the case. We also felt it was important for health reasons to resolve this matter and to complete our family medical history. It was even more wendy_gaye_Adultpoignant as this news had sadly arrived just prior to our father’s funeral. He would have loved to hear how right he had been all along, but we made sure to let him know as we bid our farewells’ at his funeral, and the news did much to lift the atmosphere of the day.

Looking back we are happy it turned out this way. We had different personalities and in Gaye’s words “Thinking we were non-identical during our youth helped me during the years where I felt I didn’t achieve as much as Wendy – it made it ok to be different as we were non-identical”. But now we have discovered we are in fact identical twins, at the age of 53, we are both thrilled, and the whole family loves it too. We were always very close, but now we feel even closer. Thank you Department of Twin Research for all your wonderful work and for solving our family mystery – we will always remember the day we got our results.’

Emma and Anne

‘The doctor told our mother we were non-identical twins because we had separate placentas. However we were constantly mixed up by teachers and acquaintances. Only people who we were very close to us were able to tell us apart.Emma_AnneI_think  The result, we believe, negatively impacted on our relationship and led to a lot of conflict and competition.  Perhaps if we had known we were identical it might have made it easier for us to share things with each other, to value the relationship more and to forgive each other our mistakes more easily.

At school one of us was labelled “the clever one” and the other “the musical one”. Due to the school system in Northern Ireland we ended up in different classes and ultimately in different years. We both ended up going our separate ways, in law and music, and were constantly defining ourselves by the ways we were different. Interestingly, we both ended up studying music, and we believe now that we always had more in common than we allowed ourselves to believe. Our mother now says that had she known we were identical, she would have fought more to keep us together through our schooling and handled things differently. Perhaps we would have not fought so hard to be different and we would have allowed our similarities to determine our choices in life.

Photo credit - Dagmara Shannon at Venus Images

Photo credit – Dagmara Shannon at Venus Images

At the age of 34, we were happy for the Department of Twin Research, who needed to know for their records, to determine for once and for all our zygosity. Deep down we knew (and hoped) that we would be identical twins. It had always seemed so wrong to describe ourselves as non-identical. When we were told that we were in fact “identical”, in one sense it came as a bit of a shock, but in another, it came as a big relief; it all made so much more sense. It has definitely made us feel closer to each other. Several years have gone by since we found out and we feel we are more demonstrative now with each other and that we have been able to resolve our issues and become the people we really are. We also both now have children. We love the fact that they are closer than cousins, as genetically-related as half-siblings. Because of this we try very hard to keep communication alive between our children, despite their growing up in different countries.

Had we not taken part in twin research, we would never have known the answer to a really fundamental question about who we are as individuals and as a couple. It doesn’t mean we will never fight, but it does mean that we now treasure our absolutely unique bond and see ourselves much more in the other than we did before.’