These are inactivated in females in a process called lyonization, resulting in the formation of Barr bodies.  
A mutation of the FMR1 gene can result in Martin-Bell syndrome, also named for the "fragile" form of this structure. These are inactivated in females in a process called lyonization, resulting in the formation of Barr bodies.  Klinefelter syndrome sees the presence of one or more extra copies of this chromosome in males, resulting in reduced levels of testosterone. Turner syndrome is a condition in which females may only have one of these chromosomes. For 10 points, identify this sex chromosome that is present in both males and females.
A mutation of the FMR1 gene can result in Martin-Bell syndrome, also named for the "fragile" form of this structure.
X chromosome