A molecular study of X isochromosomes: parental origin, centromeric structure, and mechanisms of formation.

Fourteen individuals with an i(Xq) or idic(Xq) were studied using RFLP analysis in order to determine both parental origin and extent of heterozygosity of the isochromosome and to search for the presence of short-arm material. In five cases the isochromosome was paternally derived, while nine patients had a maternal i(Xq). The analysis of heterozygosity of the nine maternally derived isochromosomes by using Xq markers showed heterozygosity in two cases, suggesting an origin from two homologous X chromosomes. Homozygosity was found at all informative loci in seven cases, which therefore are probably the product of either centromere misdivision or sister-chromatid exchange. Presence of Xp markers was seen both in the three i(Xq) chromosomes which appeared dicentric by cytogenetic analysis and in three additional cytogenetically monocentric cases. Mean parental ages were greater for the maternally derived cases as compared with the paternally derived cases.