Identical or not?

Genetic testing doubly important for twins

Written by the Australian Twin Registry and published in the AMBA Magazine
September 2015

A recent study found that nearly a third of twins were incorrect when asked to identify if they were identical or not. Why does it matter and how can you find out for sure? “Are they identical or not?” is usually the first question asked of new parents about their twins. While it may be a matter of curiosity for most parents, for others it has become lifesaving knowledge.

Mother of two-year-old twins, Kylie Tyrell, discovered the importance of knowing her twins’ genetic identity when one was diagnosed with a rare immune disorder. Kylie had only recently found out for sure her twins were identical and this led to the other twin, who wasn’t displaying any symptoms, to be tested and the discovery both twins had the disorder. “I hope that some parents hear my story and realise that if they don’t get their twins tested for their own curiosity, at least get them tested as a medical precaution”, Kylie said. A joint study by the Australian Twin Registry and the Murdoch Children’s Research Institute involved twins who were uncertain of their zygosity and undertook a DNA test to find out for sure. It found that nearly one-third of the twin pairs were incorrect when asked to identify their zygosity prior to the test.

“We found a substantial proportion of parents and twins had been misinformed by their own parents or medical professionals”, said the ATR’s Deputy Director, Associate Professor, Dr Jeff Craig. The study provided such strong evidence that knowing their true genetic identity provided twins with peace of mind and positive emotions, that Dr Craig has suggested for medical professionals to universally recommend genetic testing of same-sex twins as early in life as possible.

He says the knowledge can have implications for the bonding of twins, tissue compatibility in organ transplantation, assessing disease risk, promoting the personal right to know one’s identity, legal and educational reasons, and estimating the likelihood of the mother or close relatives giving birth to further sets of twins.

Mother of eight-year-old twins Sue Sukkel participated in the study and has first-hand experience of the impact of twin identity confusion. She always believed her twins, Lilly and Abbie, were non-identical. “When the results arrived in the mail advising they were identical, I was so overwhelmed that I burst into tears. I was so pleased that 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.”

Assoc Prof Craig said the confusion often arose from the wrong assumption 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 it looks as if there is a single placenta. 

According to Dr Craig, the only way to know for sure whether same-sex twins are identical or fraternal is to have a DNA fingerprint (zygosity) test done. Genetic testing is usually done by collecting a mailed kit with a cheek swab. The ATR provides this service at a special discounted rate per twin pair. Find details at