46,XX/46,XY is a chimeric genetic condition characterized by the presence of some cells that express a 46,XX karyotype and some cells that express a 46,XY karyotype in a single human being. The cause of the condition lies in conception or utero with the aggregation of two distinct zygotes or blastocysts (one of which expresses 46,XX and the other of which expresses 46,XY) into a single embryo,[1] which subsequently leads to the development of a single individual with two distinct cell lines, instead of a pair of fraternal twins. This is not to be confused with mosaicism or hybridism, neither of which are chimeric conditions, but is considered as an intersex condition.
In humans, sexual dimorphism is a consequence of the XY sex-determination system. In typical prenatal sex differentiation, the male and female embryo is anatomically identical until week 7 of the pregnancy, when the presence or the absence of the SRY gene on the Y chromosome causes the undetermined gonadal tissue to undergo differentiation and eventually will become either a pair of testes or ovaries respectively.[1] The cells of the developing testes produce Anti-Müllerian hormone, causing the regression of the Müllerian ducts.[2] As individuals with 46,XX/46,XY partially express the SRY gene, the normal process by which an embryo normally develops a phenotypic male or phenotypic female may be significantly affected causing variation will affect in the gonads, the reproductive tract, and the genitals.[3] Despite this, there have been cases of completely normal sex differentiation occurring in 46,XX/46,XY individuals reported in the medical literature.[4][5][6] 46,XX/46,XY chimerism can be identified during pregnancy by prenatal screening or in early childhood through genetic testing and direct observation.[7]