Why genetic testing is doubly important for twins

“Are you identical or not?” is a question constantly faced by twins throughout their lives and can cause considerable angst, a new study shows.

While it may seem a benign question to non-twins, for twins who are not sure if they are identical or not, or have been misled – and surprisingly there are many – it can have considerable significance for their physical and mental wellbeing.

Such knowledge can have implications for the bonding of twins, tissue compatibility in organ transplantation, assessing disease risk, the personal right to know your identity, legal and educational reasons, estimating the likelihood of the mother or close relatives giving birth to further sets of twins and to avoid embarrassment when asked by family, friends and strangers.

A joint study by the Australian Twin Registry, based at the University of Melbourne and the Murdoch Childrens Research Institute found 32 per cent of parents of identical junior twins were unsure or incorrect about their twins’ genetic identity or zygosity (DNA comparison). When it came to non-identical pairs, 29 per cent of parents were incorrect or unsure. When combining adult twin responses with junior twins, 27 per cent of all twins in the study were incorrect or unsure of their zygosity.

“We found a substantial proportion of parents and twins had been misinformed by their own parents or medical professionals,” says Associate Professor Jeff Craig of the Australian Twin Registry and Senior Research Fellow, Early Life Epigenetics Research Group, at the Murdoch Childrens Research Institute.

The study investigated the accuracy of genetic identity of junior and adult twins, and whether knowing zygosity was important to twins.

The study has provided such strong evidence that knowing their true genetic identity gave twins peace of mind and positive emotions, Associate Professor Craig has suggested medical professionals universally recommend genetic testing of same-sex twins as early in life as possible. He also believes medical professionals, twins and their families should receive increased education about zygosity issues.

Sue Sukkel, the mother of eight-year-old twins, participated in the study and has first-hand experience of the impact of twin identity confusion.

She had always believed her twins, Lilly and Abbie, were non-identical after being told so at their birth by the doctor and midwife – although a lingering doubt persisted because the girls looked so similar. It wasn’t until she attended a twins’ festival many years later where free genetic testing was on offer that she decided it was time to address the uncertainty. 

“When the results arrived in the mail advising they were identical, I was so overwhelmed that I burst into tears,” Sue says. “I was so pleased to have my real ‘inner gut’ feelings confirmed and pleased the girls will grow up with this knowledge. As an adopted child myself, I know what it is like to be unsure about your genetic heritage and how that can affect you.”

Associate Professor Craig said the confusion often arose due to wrong assumptions that identical twins always share a placenta in the womb and always look and behave identically. But nearly one-third of identical twins (as in the case of the Sukkel twins) and all same-sex non-identical twins have separate placentas.

To add to the confusion, twins with separate placentas can be implanted so closely together in the womb that the placentas appear to “fuse”. To the naked eye they appear to be a single placenta.

Identical twins can also look and behave differently, particularly as they age. The only way to know whether same-sex twins are identical or fraternal is to have a DNA comparison

 test done. Genetic testing is done by providing cheek swabs, which are then returned by mail. The ATR provides this service at a special discounted rate of $120 per twin pair.

“It is interesting how that initial wrong advice played out in our lives,” Sue Sukkel says. “I used to tell my family and friends, ‘You are crazy to think the girls are identical’, as I was so convinced they weren’t. The girls were teased and embarrassed at school because they would argue they weren’t identical and their classmates didn’t believe them.

“In the end, I think it is important to know the truth and to have certainty for the girls and our family. As they say, knowledge is power.”

www.twins.org.au

www.mdhs.unimelb.edu.au