The parental origin and mechanism of formation of three dicentric X chromosomes. 
Cytogenetic and molecular analyses of three dicentric X chromosomes were performed in an attempt to identify the parental origin and mechanism of formation of the aberrant chromosomes.  Results indicate that, in these three cases, the dicentric chromosomes were formed by chromatid breakage and reunion of sister chromatids at the breakpoint.  In two cases the abnormal chromosomes were paternal in origin; in the third case the dicentric originated from the maternal X chromosome.