This is an extremely rare case. The second in the world, and the first to have been detected during pregnancy: these ‘identical twins’ have turned out to be of different sexes. A little girl and a little boy, who at the beginning of their life as embryos were believed to be identical.
‘Their mother's ultrasound at six weeks [of pregnancy] showed a single placenta, and a positioning of the amniotic sacs that indicated that she was expecting identical twins. But an ultrasound performed at 14 weeks showed that the twins were of the genders male and female, which is not possible for identical twins,’ says Professor Fisk, director of the service where the mother of the twins was monitored during her pregnancy, at Royal Brisbane and Women's Hospital in Australia.
‘Two small seeds’ from dad
To explain this unique case, the doctor put forward a hypothesis: ‘It is likely that the egg from the mother was fertilised simultaneously by two of the father's spermatozoa before division,’ said Professor Fisk. In identical twins, only one egg is fertilised by a single spermatozoon and it is only at the time of division of the egg-cell that the separation occurs. In non-identical twins, however, it is two very distinct eggs that are fertilised by two equally independent spermatozoa. Nothing to do, however, in one case or the other, with the phenomenon that occurred with this Australian mother.
For these ‘semi-identical twins’ a single egg appears to have been fertilised by two sperm cells. A tripartite meeting which is unusual, and which is not inconsequential on the genetic material of future babies.
Viability usually compromised
‘If an egg is fertilised by two spermatozoa, it results in three sets of chromosomes, one from the mother and two from the father. [Possessing] three sets of chromosomes is normally incompatible with life, and the embryos do not usually survive’ says another doctor in the department, Dr. Gabbett.
‘In the case of the Brisbane semi-monozygotic twins, the fertilised egg seems to have equitably distributed the three sets of chromosomes into groups of cells that then split into two, giving birth to the twins. Certain cells then contain the chromosomes of the first spermatozoon, while the remaining cells contain the chromosomes of the second spermatozoon, with the result that the twins share only a portion, rather than 100% of an identical paternal DNA,’ Dr. Gabbett says.
This biological prodigy makes these Australian twins the second in history to have lived such a genetic adventure; their only predecessors were born in 2007 in the United States. ‘We know that this is an exceptional case of semi-monozygotic twins, and although doctors must keep this in mind in the apparent cases of identical twins, the rarity [of the phenomenon] does not justify the achievement of a systematic genetic test,’ concludes Professor Fisk. This is an extremely rare case, and it would seem that it could remain so for a long time yet.