Molecular studies of the parental origin and nature of human X isochromosomes.

X-chromosome restriction fragment length polymorphisms were used to determine the parental origin of the isochromosome in nine individuals with an i(Xq) or idic(Xq). We were able to specify the parental source of eight of the nine isochromosomes, with six being maternal and two paternal in origin. In two cases, one i(Xq) and one idic(Xq), we used Xq markers to determine the level of heterozygosity in the isochromosome. Each was homozygous at all tested loci, suggesting that each originated from a single X chromosome and not from an exchange of material between two X's.