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A Barr body is the inactivated X chromosome that is usually found in the nuclei of female somatic cells. It can also occur in males when there is an abnormality and the male is XXY instead of the normal XY, a condition known as Klinefelter's syndrome.
The one with the suppressed Xist remains active, the other becomes the Barr-body (the process of selecting which chromosome becomes the Barr-body is random, hence female mosaicism). So, the point is, that for correct X-inactivation, the interplay of two X chromosomes are needed, and people with Turner syndrome have only one.