A young boy and girl from Brisbane, Australia are the first "semi-identical" twins to be detected while still in their mother's womb.
According to a press release from the Queensland University of Technology, the pair of twins are identical on their mother's side, sharing 100 percent of her DNA. On their father's side, they are regular siblings that only share a portion of his DNA.
The unnamed brother and sister is only the second pair of twins in the world to be identified as "semi-identical," also called sesquizygotic, but the first to be detected during pregnancy.
"It is likely the mother's egg was fertilized simultaneously by two of the father's sperm before dividing," explained Nicholas Fisk, Deputy Vice-Chancellor (Research) at University of South Wales. He was part of the team that cared for the mother and twins. "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."
In normal cases, twins are either identical or fraternal. They are identical when the resulting cells of a single egg that is fertilized by a single sperm split into two. They, however, are fraternal when each twin develops from two eggs, each fertilized by separate sperms.
A Rare Set Of Twins
The only other documented semi-identical twins were born in the United States. They were discovered back in 2007 after one of the children was born with ambiguous genitalia.
A global analysis of databases further highlighted the incredible rarity of semi-identical twins.
"However we found no other sesquizygotic twins in these data, nor any case of semi-identical twins in large global twin studies," reported Fisk.
Researchers involved in the study of the twins published a paper titled "Molecular Support for Heterogenesis Resulting in Sesquizygotic Twinning" in The New England Journal of Medicine" on Thursday, Feb. 28.