Doctors have discovered the world's first known case of "semi-identical" twins. The two young children, conceived normally in the US are identical on their mother's side but share only half the genes of their father's side. They are the result of two sperm cells, one X and one Y, fertilising a single egg, which then divided to form two embryos - and each sperm contributed genes to each child. Each stage is unlikely, and scientists believe the twins may be unique (although if both of the sperm had been X or both had been Y, the condition may never have come to anyone's attention).
Normally twins develop from one egg which splits early in development, creating identical twins who share all of their genetic material, or they are the product of two separate eggs fertilised by two different sperm, creating non-identical (fraternal) twins - who share on average 50% of their genetic material. Sometimes, two sperm can fertilise a single egg, but this is only thought to happen in about 1% of human conceptions. Most embryos created this way do not survive. (Molar pregnancies can also occur if an ovum is fertilised by two sperm, and this condition brings with it a small - 2% - chance of cancer developing in the mother's uterus.)
This case came to light because one of the twins, now toddlers, had an abnormality in sexual development with both ovarian and testicular tissue (sexually ambiguous genitalia). This child is being raised as a girl. The other twin is anatomically male, but genetic tests show both are "chimæras", and have some male cells - with an X and Y chromosome - and somr female cells - with two X chromosomes.
Writing in the Journal of Human Genetics, researchers said the "semi-identical" twins are more genetically similar than fraternal twins, but less similar than identical twins. "This shows that our understanding of how twinning arises is probably something of a simplification, and that there are some very probably rare variations on how this can arise," lead author Dr Vivienne Souter, a geneticist at the Banner Good Samaritan Medical Center in Phoenix, Arizona who investigated the case, said.
The two children are progressing well according to co-author Dr Melissa Parisi: "I can tell you that in my last contact with the family, the twins were doing very well - healthy, growing well, developing normally."
Charles Boklage, an expert on twinning who works at Eastern Carolina University in Greenville, North Carolina, said: "There's value in understanding that this can happen, but it's extremely unlikely that we'll ever see another case."
David Bonthron, a geneticist at the University of Leeds, said: "The number of these cases is very small, but before they were reported, most people would have said this could never happen." He added: "Whether these things are academic curiosities, or whether we've overlooked something significant is hard to say. A lot of what we know about fertilisation is deductive, because we can't observe these events in humans."
Semi-Identical Twins source: http://news.bbc.co.uk/2/hi/health/6498215.stm
A Baby with Two Brains
A Chinese baby boy with two brains is doing well, but hardly sleeps because his brains work in rotation. The boy, born in Chaoyang city in northeastern Liaoning province in July 1995, was growing well and did not need surgery, the official Xinhua news agency said.
Source: The Dominion 14 October 1996
He (they?) would make a fascinating study.
Would each of his brains need to be taught separately? Can each brain read the other's thoughts, or must Brain 1 speak aloud or write messages to Brain 2?
Is this in fact the ultimate conjoined twin, actually two people sharing one body? If so, can each of them marry? The same girl? Or different girls? (Would one wife be jealous of the other?) Should they get two votes? Two superannuations?
Does each take turns "using" the body while the other is asleep? I suppose they must share it during at least eight hours. Does one or the other of them have to lose consciousness in that case? Could Brain 1 become so dominant that it never lets Brain 2 take control except when Brain 1 is asleep? (And would that be some criminal or civil offense that Brain 2 could pursue in court?) Is it possible that they are each unaware of the other?
Will the body wear out so that both die before full maturity because their shared body never really gets to rest?