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 has implications for the bonding, tissue compatibility in organ transplantation, for the assessment of disease risk in the co-twin of an affected individual or for estimating the likelihood of the mother or close relatives giving birth to further sets of twins, amongst other reasons
A joint study by the Murdoch Children's Research Institute and Australian Twin Registry found that 32 percent of parents of identical junior twins were unsure or incorrect about their twins’ genetic identity or zygosity. When it came to non-identical pairs, 29 percent of parents were incorrect or unsure. When combining adult twin responses with junior twins, 27 percent 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,” according to A/Prof Jeff Craig Senior Research Fellow at Murdoch Children's.
The study investigated the accuracy of genetic identity of junior and adult twins, and whether knowing zygosity was important to twins.
Findings show strong evidence that knowing their true genetic identity provided twins with peace of mind and positive emotions, Dr Craig has suggested that medical professionals universally recommend genetic testing of same-sex twins as early in life as possible. He also believes that medical professionals, twins and their families should receive increased education about zygosity issues.
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 had always believed her twins, Lily and Abbie, were non-identical after being told so at their birth in hospital by the doctor and midwife - although a lingering doubt persisted because the girls looked so similar. It wasn’t until she attended a twins’ event 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 said. “I was so pleased to have my real ‘inner gut’ feelings confirmed and pleased that the girls will grow up with this knowledge. 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”
Identical twins can also look and behave differently, particularly as they age. 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.