A young boy and girl have been identified as semi-identical twins, only the second set ever reported in the world, say Australian doctors.
The twins, now four years old, share 100 percent of their mother’s DNA, but only a portion of their father’s DNA, said a peer-reviewed case report published in the New England Journal of Medicine on Thursday.
Identical twins happen when a single egg is fertilized by a single sperm, then splits into two, sharing 100 percent of the DNA from both parents. For fraternal twins, each child develops from a single egg fertilized by its own sperm, with each sharing half their parents DNA.
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“It is likely the mother’s egg was fertilized simultaneously by two of the father’s sperm before dividing,” said Nicholas Fisk, a fetal medicine specialist and deputy vice-chancellor of research at the University of New South Wales, in a statement. Fisk assisted in the case and led the team who cared for the mother and twins while they were hospitalized in 2014.
Semi-identical, or “Sesquizygotic” twins, were first reported in the U.S. in 2007.
Australian doctors say this second set is the first to have been discovered during pregnancy. Fisk said the mother’s ultrasound at six weeks indicated she was expecting identical twins, but a second ultrasound completed at 14 weeks identified them as male and female, which is not possible with identical twins.
Dr. Michael Gabbett, a clinical geneticist with the Queensland University of Technology and lead author of the study, said “Sesquizygotic” embryos usually don’t survive because the three sets of chromosomes – one set from the mother and two sets from the father – created when one egg is fertilized by two sperm are often “incompatible with life.”
In this case, the egg equally divided the sets of chromosomes, then split. “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 (than) 100 percent of the same paternal DNA,” Gabbett said in a statement.
Follow Brett Molina on Twitter: @brettmolina23.