A brother and sister in Australia have become only the second case of semi-identical twins ever recorded, according to doctors.
The four-year-old siblings, from Brisbane, share all of their mother's DNA but only a proportion of their father's, The New England Journal of Medicine reported.
They were first identified as semi-identical, or sesquizygotic, using genetic testing while still in the womb.
Professor Nicholas Fisk, who led the team that cared for the mother and twins at the Royal Brisbane and Women's Hospital in 2014, said it was an "exceptional case".
"It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing, he said.
"The mother's ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins.
"However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins."
Identical twins develop when cells from a single egg, fertilised by a single sperm, divide into two.
This means they share identical DNA and are the same sex.
Non-identical twins occur when the two babies develop at the same time from separate eggs and separate sperms.
Semi-identical twins come from one egg, fertilised by two sperms, which then split.
This type of fertilisation results in three sets of chromosomes.
Dr Michael Gabbett, from Queensland University of Technology, said: "Three sets of chromosomes are typically incompatible with life and embryos do not usually survive.
"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.
"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% of the same paternal DNA."
Semi-identical twins were first reported in the US in 2007, but only came to the attention of doctors during their infancy.