The Twins That Are Neither Identical Nor Fraternal
Gabbett eventually found a report on “sesquizygotic twins”: not identical, but not fraternal either. Biologists had first proposed their existence in 1984, but it wasn’t until 2007 that doctors documented the first case of sesquizygotic twins. The sesquizygotic twins are likely the result of three separate events in the womb, each one rare by itself.
Source: www.theatlantic.com