Until recently, scientists believed that there were only two types of twinning: identical or monozygotic twinning (which results when a single sperm fertilizes a single egg, which splits in half into two embryos) and fraternal or dizygotic twinning (which occurs when two separate eggs are fertilized by two separate sperm). As such, identical twins are always the same sex and share the same placenta, while fraternal twins can either be the same or the opposite sex and each have their own placenta.

In March 2008, doctors at the Children’s Hospital and Regional Medical Center in Seattle shocked the medical community when they announced the first known case of semi-identical, or half twins. Semi-identical twinning appears to occur when both twins inherit exactly the same genes from their mother, but different genes from their father. While the exact fertilization mechanisms are not completely understood, semi-twinning can occur when two sperm cells fertilize one egg—an event assumed to be very rare—which then splits into two embryos. Thus, each sperm contributed a different set of genes to each twin.

Thus, semi-identical twins are more genetically similar than typical siblings or fraternal twins (who are only as similar as other siblings), but they do not share 100% of their DNA like identical twins do. It is estimated that semi-identical twins share roughly 75% of their DNA with one another. Before this discovery, scientists were aware that it was possible for two sperm to fertilize a single egg, but it is only estimated to occur in roughly 1% of human conceptions. Moreover, most embryos created this way do not become viable.

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