Transcriptionally dormant X chromosome as found within individuals possessing more than one X chromosome.
Such individuals, of course, usually are female but also can be Klinefelter syndrome males. The reason for the formation of the Barr body is one of dosage compensation, which is to say that most of the genes associated with the X chromosome are expressed to an appropriate extent when found in a single copy, i.e., as one sees in males. To assure that females do not receive a double dosage of the associated gene products, only one X chromosome is transcribed.
Which X chromosome is converted to a Barr body occurs randomly during embryo development and can be a different X chromosome in different tissues. Once converted, the Barr body retains that status through mitotic divisions. Females, as a consequence, are considered to be mosaics with regard to X chromosome inactivation, with some tissues expressing one X chromosome and different tissues the other. The classic example of such mosaicism is seen with calico cats, where fur coloration is a sex-linked trait.
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