Brenton Gurney was experiencing chronic headaches but an MRI showed nothing malicious. When his twin brother, Craig, who showed no symptoms, went in for a scan - it revealed a rare brain tumour the size of a lemon.
Angela Ralph and Marlene Crockett are mirror identical twins. They developed breast cancer - at exactly the same time.
Reigning winners of Australias Most Identical Male Twins competition, Phillip and Douglas Griffiths are (unsurprisingly) visually inseparable. But Phillip is gay, and Douglas is straight.
Their crown of similarity is something twins Anna and Lucy Decinque aspire to: already identical, they eat the same things, exercise the same amount, dress in the same clothes and even share a boyfriend.
Theyre both convinced theyll be pregnant at the same time - one way or another.
For social and scientific researchers, twins are the perfect natural experiment.
Identical twins share 99.9 percent of the same DNA, and fraternal twins share half.
Changes between twins (or lack thereof) when they grow up - whether in health, sexuality, intellect or behaviour - can tell us much about the impact of genetic versus environmental factors in these areas.
Insight finds out in this special, double episode feature.
Part 1
Part 2
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