Rare Y Chromosome Discovery in Bloodstains at Crime Scene Shutterstock/PeopleImages
Forensic investigations into the murder victim have revealed a fascinating case of Chimerism. This means her body harbored genetically distinct cells, resembling those from two different individuals.
The unidentified woman’s cellular composition displayed varying male and female cell ratios across tissues. The most plausible explanation is that she developed from one egg fertilized by two sperm—one carrying an X chromosome and the other a Y chromosome, according to biologists from New Scientist.
“This is an intriguing case, but not entirely unprecedented,” noted David Haig from Harvard University.
Visible signs of chimerism can be rare, though singer Taylor Mule has raised awareness about the condition. Often, genetic testing is the only way to identify it.
This was also true for the murder victim shot and killed in China, where blood analysis at the scene revealed the presence of a Y chromosome, prompting further investigation.
Subsequent tests showed the female (XX) to male (XY) cell ratios varied throughout her body. In one hair sample, the majority were XY cells, while the kidney revealed a balanced mix. The other tissues examined predominantly contained XX cells, albeit in varying amounts.
Typically, XX/XY chimerism is linked to ambiguous sexual characteristics. However, in this instance, the woman’s anatomy offered no indication of her condition, and she had a son—hinting that she may have been unaware of her chimerism.
One known mechanism for XX/XY chimerism formation is through the fusion of non-identical twins. Here, two separately fertilized eggs combine to form a single embryo.
Nevertheless, the X chromosome in the victim’s XY cells matched one of the X chromosomes found in the XX cells, indicating both could have originated from the same egg—thus excluding the fusion theory.
Initially, it was believed that one egg split into two eggs, each of which was fertilized to create two separate embryos that later fused. This theory has been challenged by Chinese forensic experts.
Michael Gabbett at Queensland University of Technology in Brisbane argues that this possibility is negated.
“When this type of chimera was first documented in humans, this was the prevailing theory. However, no one has been able to provide substantial evidence for it occurring in humans or other mammals,” Gabbett stated.
Instead, he proposes that one egg was fertilized by two sperm, leading to a triploid fertilized egg that ultimately divided into three. Two cells retained one set from the egg and one from sperm while the third likely contained two sperm sets, leading to its eventual demise.
This rare occurrence, termed “trigametic chimerism,” involves an egg and two sperm, a theory supported by Haig.
This phenomenon is exceedingly rare, and on occasion, the embryo can split, resulting in semi-identical twins and even chimerism. There are only two recorded cases of semi-identical twins, one of which Gabbett was involved in identifying.
For the murder victim, the chimeric cells were present throughout her body, and the Chinese research team asserts this is the first extensive examination of various organs in such cases of triplet chimerism.
Another form, microchimerism, is more prevalent than trigametic chimerism. It occurs when maternal cells invade the fetus or vice versa during pregnancy and can also happen when siblings exchange cells.
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Source: www.newscientist.com
