Chimerism - When fraternal twin embryos fuse

A 2007 article, linked here:

Chimerism and tetragametic chimerism in humans: implications in autoimmunity, allorecognition and tolerance

Quote:  Demonstration of persistent chimerism is usually discovered in cases of inter-sexuality due to fusion of fraternal twins or in cases of fusion of embryos with demonstrable allogeneic monoclonality of blood which, excluded maternity or paternity when blood alone is used as the source of DNA. In single pregnancies it is possible to produce two kinds of microchimerism: feto-maternal and materno-fetal, but in cases of fraternal twin pregnancies it is possible to identify three different kinds which are related to cases of vanishing twins that can be identified during pregnancy by imaging procedures; (1) hematopoietic, (2) gonadal, and (3) freemartins when the twins have different sex and the individual born is a female with either gonadal or both gonadal and hematopoietic tissues. Fraternal twin pregnancies can also produce fusion of embryos. Such cases could be of different sex presenting with inter-sexuality or in same sex twins

Simply, this means that fraternal twins can fuse in early pregnancy, leaving one sole survivor with mixed DNA. Even one blood stem cell from a vanishing twin can mean that your blood is chimeric - ie. of a mixed type. Or you may have a teratoma that is made out of cells from your twin, or a dermoid cyst, or specific tissues, such as your ovaries, that are not your own.

The implications are that autoimmune disease could be related to this, also how tolerant you are if you have some kind of transplant.

I have noticed how many womb twin survivors are sensitive to their environment, to chemicals, pollution, certain foods, caffeine and so forth.

The vanished twin leaves a legacy at a psychological level, thats  for sure,  but there are also many physical signs.   Some of them are  at a microscopic level, but they are very profound, nonetheless. For example, being intersex creates an existential crisis - "Who am I?  Man or woman?"  That's hard.