Reduced phenotypic effect on partial trisomy 1q in a X/1 translocation.

A girl with psychomotor retardation and minor physical abnormalities, had an unbalanced X/1 translocation resulting in a partial trisomy 1q and partial monosomy for Xp. The four cases of partial trisomy for the distal segment of 1q reported in the literature showed a much more severely affected phenotype. In the present case the translocated X/1 chromosome is preferentially late replicating and there is spreading of late replication to the translocated 1q segment only in a minor proportion of the cells. The possibility of a non univocous correspondence between spreading of genetic inactivation and cytological spreading of late replication is discussed.