A previously unreported form of twinning in humans has been discovered in America. The twins - dubbed 'semi-identical' - are thought to be the result of a single egg being fertilised with two sperm, and then splitting. Double fertilisations are thought to account for around one per cent of all conceptions, but rarely survive. The twins came to the attention of scientists as one was born with ambiguous genitalia - chromosome analysis showed that both twins were chimeras, with both having some 'male cells' and some 'female' cells. While one twin is said to be a true hermaphrodite with both ovarian and testicular tissue, the other is anatomically male. A single egg being fertilised with one 'male' Y chromosome sperm and one 'female' X chromosome sperm and then splitting would account for the mix of male and female tissue in the twins.
The children, who are now toddlers, were conceived naturally. Their identity and location are being kept secret but they are said to be progressing well. Genetically, the twins are identical on the maternal side but varied on the paternal side, making them more related than fraternal twins - who share around 50 per cent of their genes - but less related than identical twins, who share all of their genetic material. Geneticist Vivienne Souter and her team at the Banner Good Samaritan Medical Center in Phoenix, Arizona discovered the case, which they report in the journal Human Genetics. Biologist Michael Golubovsky of the University of California, Berkeley had speculated in 2003 that this type of twinning was possible. His theory is now confirmed. Commenting on the case Golubovsky said, 'There are a lot of unclear situations in the genetics of twins and twinning. We need to keep our eyes open for other unusual scenarios'.