X-Chromosome Inactivation

Abstract

Female mammals have two X-chromosomes, but male mammals have only one X-chromosome. This could result in females having twice as many X-chromosome transcripts as males, but this does not occur because one of the X-chromosomes is largely inactive. (The tip of the short arm of both X-chromosomes is active.) The inactive X-chromosome is highly condensed, replicates late in S phage, and can be seen as the dense Barr body in interphase nuclei. The active region of the inactive X-chromosome is not late replicating and can be detected in chromosome spreads by its increased sensitivity to nucleases (Kerem et al., 1983). In XXY males with an extra X-chromosome (Klinefelter’s syndrome), one X-chromosome is inactivated.