Californian model and singer Taylor Muhl has two skin tones, having fused with her twin sister in the womb
The forensic examination of a homicide victim revealed that the woman had chimerism — that is, her body contained genetically distinct cells, as if they came from two different people.
A female homicide victim, whose identity was not disclosed, presented variable proportions of male and female cells in different tissues.
The most likely explanation is that it developed from a single egg fertilized by two sperm — one carrying an X chromosome and the other a Y chromosome, explained the biologist David Haigfrom Harvard University, at .
The case, which was presented in a publication in the magazine FSI Genetics“It’s fascinating, but not entirely unprecedented,” Haig added.
Sometimes there are visible signs of chimerismas in the case of singer Taylor Muhl, who has spoken publicly about her condition to raise awareness of the issue. In most situations, however, it is only detected through genetic testing.
That’s what happened to this victim, who lived in China and died after being shot. Analysis of blood collected at the crime scene revealed the presence of a Y chromosome, which led to additional tests. Tests showed different proportions of female cells (XX) and male (XY) throughout the body.
In one example, in a hair sample most cells were XYwhile in kidney was found to be a mixture in equal parts. In the remaining 16 tissues analyzed, XX cells predominated, although in variable ratios.
Most of known cases of XX/XY chimerism was identified because people present ambiguous sexual characteristics. In this case, however, the woman’s anatomy gave no indication of the condition and she had a son. It is likely that never knew it was a chimera.
One of the ways in which XX/XY chimeras arise is the fusion of non-identical twins — when two eggs are fertilized separately, creating two embryos that, under normal conditions, would produce fraternal twins, but which, instead, end up fusing.
However, in this victim’s XY cells, the X chromosome was identical to one of the X chromosomes of your XX cells. The only way these X chromosomes are the same is that they came from the same eggwhich rules out the hypothesis of fusion of non-identical twins.
For some time, it was considered possible for a single egg to divide into two and for each one was then fertilizedforming separate embryos that would eventually fuse. This is what the forensic team suggests it happened here.
But This possibility can be excludeda, according to Michael Gabbettresearcher at the Queensland University of Technology, in Brisbane.
“When these types of chimeras were first observed in humans, this was the dominant theory, but since then no one has been able to demonstrate that this could occur in humans or other mammals”, he states.
Gabbett argues, alternatively, that an egg will have been fertilized by two sperm, producing one fertilized egg with three sets of chromosomes. These sets would have replicated, originating six sets, and the embryo would then have divided into three.
Two of these cells would have received a set of chromosomes from the egg and the other from a sperm, which can develop normally. The third would have received both sets of spermcausing anomalies that would probably have eliminated this cell lineage.
The phenomenon is sometimes called trigametic chimerismas it involves three gametes — an egg and two sperm. Haig agrees that this is probably the most plausible explanation.
It is a extremely rare phenomenonand even rarer is the embryo dividing, dI’m giving birth to semi-identical twinsor sesquizygotes, which can also present chimerism.
This is so unusual that only two pairs of semi-identical twins are known, one of which was identified with Gabbett’s help.
In the case of this victim, the different cell lineages remained together and contributed, in varying degrees, to all parts of the body. There are some other known cases of trigametic chimerismbut, according to the team in China, this is the first time that such an extensive analysis of different organs has been carried out.
There is yet another form of chimerism, known as , which is much more common than trigametic chimerism or the fusion of non-identical twins.
O microchimerism arises during pregnancywhen cells from the mother pass to the fetus, or cells from the fetus pass to the mother, and integrate into the other’s organism. Younger siblings can even receive cells from older siblings — or from aunts and uncles.
