Barr body: the color-staining spot (the sex chromatin) located at the edge of the nucleus of cells taken from individuals with more than one X chromosome. It is normally found in female cells, and so is used as a sign of female genetic sex. It is also found in men with the 47,XXY (Klinefelter's) syndrome. It is missing in girls with the 45,X (Turner's) syndrome. The Barr-body test is rapid and inexpensive as compared with actual chromosome counting, and so is used as a method of preliminary X-chromosome screening.

Dictionary of Sexology Project: Main Index

A Barr body is a condensed, inactive X chromosome found in the somatic (non-gamete) cells of normal females and genetically abnormal males who have more than one X chromosome. Barr bodies are visible when the nucleus of the cell is not dividing.

None of the genes in a Barr body are used, since only one X chromosome can function within a cell. In every cell with more than one X chromosome, one is randomly turned off to create a Barr body. Because of this phenomenon, identical twin girls display more phenotypic differences than identical twin boys.

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