Murder victim discovered to have two sets of DNA due to rare condition

A forensic examination of the murder victim determined that she was chimerism – this means that her body contained cells that were genetically different, as if they came from two different people.

In this case, the unidentified woman had different proportions of male and female cells in different tissues. The most likely explanation for this is that it developed from a single egg that was fertilized by two sperm, one of which carried an X chromosome and the other a Y chromosome, biologists said. New scientist.

“It's a fascinating case, but not entirely unprecedented,” says David Haig at Harvard University.

Sometimes there are visible signs of chimerism, as in the case of singer Taylor Muhl, who emphasized her chimerism raise awareness of this condition. However, most often this is only detected through genetic testing.

This was the case with a murder victim who lived in China and was shot and killed. Blood tests from the scene revealed the presence of a Y chromosome, so additional tests were performed.

They found different proportions of female (XX) and male (XY) cells throughout a woman's body. For example, in one hair sample the majority of the cells were XY, while the kidney contained the same number of cells. The remaining 16 fabrics tested were mostly XX, in varying ratios.

Most known cases of XX/XY chimerism were identified due to ambiguous sexual characteristics in humans, but in this case, the woman's anatomy did not indicate her condition, and she gave birth to a son. She probably didn't know she had chimerism.

One way to form XX/XY chimeras is through the fusion of non-identical twins. This is when two eggs are fertilized separately, resulting in two embryos that would normally be non-identical twins but are instead fused together.

However, the murder victim's X chromosome in her XY cells was identical to one of the X chromosomes in her XX cells. The only way these X chromosomes could be identical is if they both came from the same egg, which rules out the fusion of non-identical twins.

It was previously thought that one egg could divide to form two eggs, each of which would then be fertilized to form separate embryos, which would then fuse. This is exactly what a forensic team in China suggests.

But this possibility can be ruled out, says Michael Gabbett at the Queensland University of Technology in Brisbane.

“When these types of chimeras were first discovered in humans, [this] was the prevailing theory, but no one subsequently succeeded in demonstrating that this could happen in humans or other mammals,” says Gabbett.

Instead, he believes that one egg was fertilized by two sperm, resulting in a fertilized egg with three sets of chromosomes. These sets then replicated, resulting in six sets of chromosomes, and then the egg split into three.

Two of these cells would receive one set of chromosomes from the egg and the other from the sperm, so both could develop normally. The third cell received both sets of sperm, resulting in abnormalities that likely killed its line.

This phenomenon is sometimes called trigametic chimerism because it involves three gametes—one egg and two sperm. Haig agrees that this is probably what happened.

This phenomenon is extremely rare – and even rarer, the embryo then splits, resulting in the development of half-identical or one-and-a-half twins, which can also have chimerism. It is so unusual that there are only two known pairs of semi-identical twins. one of which Gabbett helped identify.

In the murder victim's case, the cells stayed together and contributed to varying degrees to all parts of her body. There are a few other known cases of trigametic chimerism, but this is the first time such extensive testing of different organs has been done, according to the team in China.

Another form of chimerism. known as microchimerismis much more common than trigametic chimerism or the fusion of fraternal twins. Occurs during pregnancy when cells from the mother enter the fetus or fetal cells. enter the motherand become part of the body of another. Younger brothers and sisters may even get cells from older siblingsor from an aunt and uncle.

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