The scientific world is shocked by the world's only known case of "semi-identical" twins.
Signaled by journals Nature and Human Genetics, this American case (whose exact location has not been revealed) is of a twin pair identical on their mother's side, but sharing only half their genes on their father's side.
They emerged as two sperm cells fertilized a single ovule (egg), which divided to produce two embryos in which each sperm came with genes to each child.
Moreover, one sperm was X (the type that gives female gender) and one Y (for male).
All this is highly unlikely, that's why the case could be unique.
Normal twins develop either from the same egg fecundated
by a sole sperm that later splits to generate identical twins (they are the genetic clone of each other, possessing 100 % the same genes) or from two separate eggs which are fertilized by two separate sperm (one sperm for one egg), resulting non-identical twins, which theoretically share between 1 - 99 % of their genome (in most cases 40-60 %).
In rare cases, two sperms can fertilize just one ovule, but this occurs in just roughly 1% of human conceptions and most of these embryos die.
This case, in which the babies were born without problems, captures the scientists' attention just because one was born with hermaphroditism (sexually ambiguous genitalia): he/she displayed both ovarian and testicular tissue, while the other twin is anatomically male.
Genetic analysis proved both are "chimeras": their bodies are made by male cells (with sex chromosome set XY) and female cells (XX), due to the two different type sperm cells.
The twins are now toddlers and they have a good health. "Their similarity is somewhere between identical and fraternal twins. It makes me wonder whether the current classification of twins is an oversimplification", said Vivienne Souter, a geneticist at the Banner Good Samaritan Medical Center in Phoenix, Arizona, involved in the research.
"There's value in understanding that this can happen, but it's extremely unlikely that we'll ever see another case", said Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina.
"The number of these cases is very small, but before they were reported, most people would have said this could never happen. Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say. A lot of what we know about fertilization is deductive, because we can't observe these events in humans", said David Bonthron, a geneticist at the University of Leeds.
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