An Australian brother and sister are the second pair of siblings to be identified as “semi-identical” twins, their doctors say.
The siblings’ case study was peer-reviewed and published in the New England Journal of Medicine on Wednesday. The twins are what’s known as “sesquizygotic,” and their genetic makeup lies somewhere between fraternal and identical. The twins share 100% of their mother’s DNA, but only 78% of their father’s DNA.
Gizmodo explained the difference between fraternal and identical twins:
Fraternal twins happen when two different sperm cells fertilize two different eggs, creating two different zygotes, both of which end up implanted in the uterus as developing embryos. So they’re as genetically similar (roughly 50 percent) as any two average siblings would be—they’re just born at the same time. Identical twins, on the other hand, are made when the same sperm fertilizes the same egg, but then that zygote splits into two embryos, each sharing the same genetic mix of DNA from mom and dad.
The authors of the recently published paper theorize that in the case of semi-identical twins, two sperm cells fertilized the same egg at the same time. Gizmodo further reported that this would ordinarily cause a miscarriage, hence the rarity. Dr. Michael Gabbett, one of the authors of the case study, said in a press release that “[t]hree sets of chromosomes are typically incompatible with life and embryos do not usually survive.”
In this instance, however, it appears the twins did receive three groups of cells – “Cells containing the mother’s DNA and DNA from sperm 1; cells with the mother’s DNA and DNA from sperm 2; and cells containing DNA from only sperm 1 and 2,” according to Gizmodo.
“It is likely the mother’s egg was fertilised simultaneously by two of the father’s sperm before dividing,” said Professor Fisk in a press release. Fisk led the team that cared for the twins when they were born in 2014. They are now four years old.
“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,” Fisk added.
USA Today reports the first set of semi-identical twins were reported in the U.S. in 2007. That just the first reported instance, meaning there certainly could be more cases out there. The U.S. twins were only discovered to be semi-identical when they were identified as having “ambiguous genitalia,” the Queensland University of Technology’s Institute of Health and Biomedical Innovation reported. A resulting chromosomal investigation found the twins to share 100% of their mother’s DNA but only half of their father’s.
Fisk cautioned that the rarity of semi-identical twins meant there was no need for “routine genetic testing.”