from the article:
Definition: A type of twinning identified by scientists in a report in the Journal of Human Genetics in 2007. Described as identical on the mother's side but sharing only half their father's genes, the rare twins developed when two sperm fertilized a single egg, forming a triploid, which then split.
The twins, born in 2003 or 2004 in the United States, are described as somewhere between identical and fraternal. One twin is a hermaphrodite being raised as a female, with both testicular and ovarian structures, while the other is anatomically male.
Semi-identical twinning is distinguished from polar body twinning because the egg was fertilized by two sperm before splitting.
Also Known As: sesquizygotic, a term postulated by biologist Michael Golubovsky