Extremely rare semi-identical twins identified for second time

The birth of extremely rare semi-identical, or sesquizygotic, twins has been reported in Brisbane, Australia. 

Twins are typically either monozygotic or dizygotic. In a monozygotic case, a single sperm fertilises a single egg, which divides to create 'identical' twins of the same gender, who share all of their DNA. In dizygotic, or 'fraternal' twins, two separate eggs are fertilised by two separate sperm and the embryos develop at the same time. They share 50 percent of their DNA, the same as full siblings who are not twins.

Sesquizygotic twins, on the other hand, share between 50 and 100 percent of their DNA. They are very rare, with the only other case reported in 2007.

'It is likely the mother's egg was fertilised simultaneously by two of the father's sperm before dividing,' said Professor Nicholas Fisk from the University of New South Wales, who led the team that documented the case. The twins were discovered through a routine pregnancy scan. At six weeks, the embryos shared a placenta, suggesting that they were monozygotic. However, the 14-week scan showed that they were different genders, which can only happen if they are dizygotic.

'I was a bit cheeky and accused Professor Fisk of having inadequate ultrasound skills, but turns out they were perfectly good,' co-author Dr Michael Gabbett told the Australian Broadcasting Corporation.

It is particularly unusual for sesquizygotic twins to be born because the egg employs blocking mechanisms to prevent more than one sperm from fertilising it. Furthermore, if two sperms were to penetrate the egg, the resulting embryo should not be viable. 

'An embryo with three rather than the normal two sets of chromosomes won't survive as a fetus,' said Dr Gabbett.

However, in this case, the authors hypothesise that a mixture of cells was created, each with two of the three available sets of chromosomes. The ones with genetic material from both sperms but not from the egg died off or were outcompeted. This left two sets of cells, all containing the egg genome, and the chromosomes from one or other sperm. 

When, in a separate unlikely event, the blastocyst divided into two – creating twins – more of the cells with genetic material from one sperm (which had an X chromosome) ended up in one embryo, while the other had a greater proportion of cells formed by the other, Y chromosome bearing sperm. This is why one fetus developed female characteristics and the other male, although both twins have cells from both sperm in their bodies, making them chimaeras. 

The twins, whose identities have not been made public, are now said to be healthy four-year-olds.

The case was published in the New England Journal of Medicine.