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RandomFactor writes:

A paper published wednesday in The New England Journal of Medicine details a pair of semi-identical twins born in Australia that occupy an extremely rare middle ground between Fraternal and Maternal twins.

In maternal or identical twins, a normally fertilized egg splits into two 100% genetically identical embryos.

In fraternal twins, two sperm fertilize two different eggs leading to two ~50% genetically similar twins.

These sibling are semi-identical or "sesquizygotic" twins however, and

the authors theorize, two sperm cells simultaneously fertilized the same egg. Ordinarily, that sort of mistake quickly results in a miscarriage, since humans usually can’t develop with three different sets of chromosomes. Somehow, though, the resulting zygote incorporated an equal split of DNA from all three sets, with three groups of cells forming afterward: 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. Over time, the third group of sperm-only cells was effectively crowded out by the cells containing DNA from both parents. Then, even more unexpectedly, the bundle of cells divided into two embryos, creating the twins.

The resulting twins have 100% identical genetic material from the mother and in this case 78% identical from the father, which makes them genetically ~89% identical.

The rare occurrence was discovered because

“The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” [co-author Nicholas Fisk, an obstetrician and deputy vice-chancellor of research at the University of New South Wales] said in a statement. “However, an ultrasound at 14 weeks showed the twins were male and female, which is not possible for identical twins.”

The twins are now 4 years old. The sister ran into complications resulting in the loss of an arm and her ovaries but both the male and female chimeric twins are now developing normally.

Original Submission