Except your cited link is not about sex being messy or not messy, but about microchimerism being linked to cancer and autoimmune diseases.
For those following this thread, below is from the Intersex Society of North America clarifying that transgenderism is not an intersex condition.
From the ISNA,
As for the subject of chimerism: based on scientific papers and studies, chimerism in an individual refers to the coexistence of DNA cells from two distinct organisms, brought about by transplantation or blood transfusion, or physiologically, between mother and fetus during pregnancy, which would be maternal fetal microchimerism, or between twin to twin that shared a circulatory system in gestation.
This is known to happen in non-human species, e.g., cats, mice, marmoset monkeys, cattle, reportedly resulting in intersex condition. Cattle raisers come across freemartins, as an example.
Scientists and researchers believe chimerism happens in humans when fraternal twins fuse together, with the end result of the fused twin being either male, female, or a hermaphroditic individual.
I can see how investigators are drawn to chimerism as also explaning the phenomenon that is transgenderism which leads to gender dysphoria. It has been hypothesized but there is no scientific study or paper that said unsettling condition is caused by chimerism or micro chimerism.
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