Human X-autosome translocations: differential inactivation of the X chromosome in a kindred with an X-9 translocation.

A kindred with an X-autosome translocation and differential inactivation of the X chromosome is described. The phenotypically normal mother has a reciprocal translocation [46,X,rcp(X;9) (q11;q32)] while the daughter's karyotype is unbalanced [46,X,--X,+der(9),rcp(X;9) (q11;q32)mat], indicating adjacent-two type of segregation in the mother. In the mother's cells the normal X is late replicating, while in the daughter's cells almost the entire der(9) is late replicating, indicating the presence of autosomal inactivation. The daughter's abnormal phenotype can be explained by her sex chromosomal complement and the absence of effective trisomy 9. At this stage there is no simple explanation to account for all types of inactivation patterns encountered in the 14 balanced and 15 unbalanced cases of X-autosome translocations reported to date. Selection of X inactivation is not an inherent characteristic of the X chromosome per se, and it is not dependent on the direction of chromosomal exchange, as was suggested previously. Correlation of the phenotypic and cytogenetic features of these patients suggests a pattern of X and autosomal inactivation consistent with the least amount of genotypic and phenotypic imbalance in most cases. The data are most consistent with random X inactivation followed by selection of the most viable cell line.