An Australian brother and sister have been identified as only the second-ever known set of semi-identical twins in history.
And according to the Queensland University of Technology, they're the first ever to be identified as such whilst still in the womb.
The pair have identical DNA from their mother, but on their father's side are more like conventional siblings. The medical term for it is sesquizygotic.
"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins," said Prof Fisk, who cared for the Brisbane mother and twins following the birth.
"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins dividing."
That's because identical twins come from the same zygote, so have almost identical DNA. In contrast, fraternal twins happen when two eggs are fertilised - they're ordinary siblings, just born at the same time, and can be either male or female.
"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing," said Prof Fisk.
Normally this would result in an embryo with three sets of chromosomes instead of two, and death.
"In the case of the Brisbane sesquizygotic 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."
One of the sets contained genetic material from the two sperm, and died. But the other two kept dividing, forming the twins from two sperm, but a single egg.
"Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion, rather 100 percent, of the same paternal DNA."
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The only previous case of semi-identical twins happened in 2007. Prof Fisk says they looked for other cases in the literature, and even analysed genetic data from nearly 1000 other cases of fraternal twins, but came up empty.
"We know this is an exceptional case of semi-identical twins. While doctors may keep this in mind in apparently identical twins, its rarity means there is no case for routine genetic testing."
While the birth went well, the Guardian reports the girl had to have her arm amputated at four weeks because of a blood clot, and her ovaries removed at age three because they hadn't fully formed, increasing her risk of cancer.
In the 2007 case, one of the children was male and the other a "true hermaphrodite with both ovarian and testicular tissue", journal Nature said.
The latest case was detailed in the New England Journal of Medicine.