A new type of twinning was identified by scientists in 2007. They used the term Semi-Identical Twinning in a report published in the Journal of Human Genetics in 2007. The study was based on an unidentified set of twins, described as somewhere between identical and fraternal (also known as monozygotic or dizygotic). They determined that the twins were identical on the mother's side but sharing only half their father's genes.﻿﻿

These rare twins are believed to have developed when two sperm fertilized a single egg, forming a triploid, which then split. In contrast, identical (monozygotic) twins form when a single fertilized egg splits into two; fraternal (dizygotic) twins form from two separate eggs fertilized by two different sperm.

In this case of semi-identical twinning, two sperm fertilized a single egg that split into two. Genetically, the twins have the same maternal genes, but share only about 50% of their paternal genes, the same as dizygotic twins or siblings.

Details about the twins' identity were not revealed, other than that they were born in the United States, probably in the mid-2000s. They were conceived without reproductive assistance and both twins appeared developmentally normal.﻿﻿

The researchers noted that this type of twinning was extremely rare. One twinning expert said that it was extremely unlikely that another set of semi-identical twins would ever be discovered. In this case, the twins came to the researcher's attention when Twin A was identified as a true hermaphrodite with ambiguous genitalia, having both ovarian and testicular tissue. A hermaphrodite is defined as an individual in which both male and female reproductive organs are present in the body. However, Twin B is anatomically male.﻿﻿

Causes

Researchers weren't exactly sure what caused this type of twinning, much as monozygotic twinning remains somewhat of a mystery. One theory suggested that an egg cell divided, but before separating, each cell was fertilized by different sperm, muddling the genes before fully separating.﻿﻿

Biologist Michael Golubovsky identified the concept of this type of twinning in a 2002 study. He suggested that the term sesquizygotic could describe twins resulting from the "involvement of two male pronuclei in the fertilization of two female meiotic products," an "exceptional intermediate" between monozygotic and dizygotic twins.﻿﻿