A 21-year-old female dizygotic twin was referred for cytogenetic evaluation because of mild mental retardation. Significant history, clinical, and physical findings included irregular menses, mildly coarse facies, and microcornea. Chromosome analysis revealed a pericentric inversion of the X chromosome, 46,X,inv(X)(p11;q22). Her twin who is phenotypically normal was also found to carry the same inversion. The twins differ significantly in X chromosome inactivation and menstrual cycle function.
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