The Mystery Around The Rare 'Semi-Identical' QLD Twins

A Queensland brother and sister have been identified as the world's second-ever set of "semi-identical" twins and, well, we're slightly confused.

The now four-year-old brother and sister are identical on their mother's side but share only part of their dad's DNA -- likely the result of two sperm fertilising the same egg.

They're an exceptionally rare case, and the first to be identified during pregnancy by researchers from the University of New South Wales (UNSW) and the Queensland University of Technology (QUT).

A paper published on Thursday was years in the making.

Fetal medicine specialist and UNSW deputy vice-chancellor Professor Nicholas Fisk was working in Brisbane when he noticed an abnormality in the then-pregnant mother's ultrasound.

"At six weeks, (it) showed a single placenta and positioning of amniotic sacs that indicated she was expecting twins," he said.

"However, an ultrasound at 14 weeks showed that the twins were male and female, which is not possible for identical twins."

READ MORE: Motherhood Is Double When You're Expecting Twins

Twins are normally either "identical", meaning a single fertilised egg divides into two foetuses, while "fraternal" twins grow from two separate eggs.

Scientists refer to the third rare category as "sesquizygotic" twins.

"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing," Fisk said.

The case took extensive genetic testing to rule out chromosomal abnormalities.

When one egg is fertilised by two sperm, this typically results in three sets of chromosomes -- one from the mother and two from the father -- which tend to be incompatible, according to clinical scientist Dr Gabbet, who worked alongside Fisk.

"In the case of the Queensland twins, the fertilised egg appears to have equally divided up the three sets of chromosomes into groups of cells which then split into two, creating the twins," he said.

This also explains why the twins only share a proportion of their dad's DNA.

The twins are healthy and developing -- an outcome Fisk said is a rarity.

The only other reported case of sesquizygotic was identified in the U.S. in 2007, when the boy and girl were infants.

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