What can be considered rare is the motivation to detect a chimera. Indeed, in his book "How New Humans are Made", Dr. Boklage asserts most chimeras have no physical abnormalities and indeed look like everyone else. He also adds that Chimerism need not be present in the blood. So without physical abnormalities or strange blood typing to act as a motivation, there is little or no reason for a clinician to pursue the diagnosis.
While there may be no physical signs of chimerism, the condition might be expressed in the behavior of a chimera. The creation or artifical chimeras lends creedence to this idea. The creation of artificial “brain chimeras” is becoming a routine practice in the field of neurobiology. One such study conducted by German scientists created a brood of Japanese quails with female brains and male physiology.
“Male Japanese quails with female brains do not show male sexual behaviors”
The male quail chimeras with female brains did not display mounting or crowing behavior. It is a scientific fact the hypothalamus–pituitary–gonad (HPG) axis coordinates displays of sexual behavior with reproductive physiology. Sexual brain development therefore defines the ability of brain areas, including the neural part of the HPG axis, to respond to particular physiological signals in adulthood.
Could it be a that spontaneous human chimera arising from a plural pregnancy (boy & girl) might display similar behaviors? Indeed, in "Stem Cells for Dummies" Goldstein and Schneider describe the formation of a chimera:
Two zygotes travel down the fallopian tubes towards the uterus, growing and dividing into multiple celled embryos as they go. The growing embryos come into contact with each other, and one absorbs the other or they fuse together. The absorbed embryo supplies cells with it's own DNA to the body of the developing embryo. The resulting baby (and eventually adult) is composed of cells with two different genetic libraries -- one set of cells from each of the two embryos that fused.When merging occurs, the embryos are little more than brain primordia. It is possible the merging of brain primordia from a boy and girl could create a hybrid brain structure or more appropriately a hybrid HPG axis – – both male and female at once.
I believe sometimes the merging creates a human with same-sex attraction, other times merging creates a human with gender confusion, and sometimes perhaps no effect at all. It would largely depend on the physics of the collision of the two embryos as to how the resulting chimera would form.