http://www.theage.com.au/articles/2003/11/27/1069825920727.html
How a woman consumed her twin and became two individuals - genetically speaking. By Garry Barker.
The woman was in shock. At the age of 52, she had just been told that she was not the mother of two of her three grown sons, yet she knew she had conceived them with her husband and delivered them naturally.
The case, encountered in Boston and the subject of a report in New Scientist magazine this month, has led researchers to conclude that the woman is a chimera, genetically two individuals in a single, otherwise normal, body.
The US researchers suggest the woman was formed when two non-identical twin embryos - two eggs released into her mother's womb at the same time and simultaneously fertilised - that would have developed into sisters, fused to develop into a single foetus.
But, says Alan Trounson, of Monash University, one of the world's leading experts on human reproduction, the facts could be even more bizarre. The chimera could be the product of what is known as the "disappearing twin" phenomenon, in which one twin foetus consumes the other and, in the process, absorbs some of its cells.
"Disappearing twins are relatively common in human pregnancies - certainly a few in every hundred pregnancies," Trounson said. "They are diagnosed as twins and then one disappears."
Nobody was yet sure why it happened, he said, but the phenomenon had been found "very frequently" in IVF cases where several embryos were implanted at the same time.
"Then they went and looked at general pregnancies and found the disappearing twin was relatively common there, too.
"It happens fairly early in a pregnancy," Trounson said. "Not early, early, but usually in the first third of a pregnancy."
As a result of, say, fusion of blood vessels, "one twin could consume the other so that cells are transferred. One twin becomes parasitic on the other and eventually that second twin disappears."
Trounson discounted the fusion of embryos theory. "But nobody really knows," he said. "Certainly, it can be done artificially in animals by mixing embryonic cells to form a chimera, so it is at least possible. But it might be unlikely.
"Mixing as is suggested in the Boston case would have to happen in the first week and I cannot see that happening because of the impervious shell around an embryo."
But, later in the first third of a pregnancy, "one of the twins could form a fusion with the other. Say the blood vessels fuse, which has been reported, and one twin then consumes the other - basically draws out the material of the other twin and it disappears," Trounson said.
Nothing comparable with the Boston case had so far emerged in Australia, he said, but that did not mean chimeras did not exist here. "We have not tested for them," he said. "I doubt there has been any large-scale testing of IVF children for chimerism. It would be interesting to check it out."
The Boston case emerged after the chimeric woman found she needed a kidney transplant. Her three sons volunteered as donors. Blood tests showed one son to be equipped with the two blocks of genes, known as haplotypes, that humans inherit, one each from the father and from the mother.
But while the other boys had their father's haplotypes, they had different haplotypes from those the tests determined to be their mother's. In fact, their mother had two sets of haplotypes, one dominant.
Chimeras are rare, but far from unknown. Some have eyes of completely different colour, others, according to the New Scientist report, are discovered when they seek medical help for reproductive problems and are found to have both male and female structures in their bodies.
In almost all cases, chimerism causes no problems and remains undetected. Yet there may also be an advantage: when an organ transplant is required, a chimera has twice the chance of finding a match as an ordinary human because they have two sets of haplotypes on which they can be matched.
_Weekly World News_ cover story:
"WOMAN CONFESSES: I ATE MY OWN TWIN SISTER!!
-- Ronn! :)
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