We report our observations about familial segregations of chromosomal aberrations: the simple forms and complex rearrangements. Congenital malformations and mental retardation, can be present both in unbalanced and in balanced translocations. Various hypotheses have been proposed to explain this phenomenon: in particular a possible "position effect" or genic mutation or genomic imprinting. In our study we have used both standard techniques and techniques with high resolution banding to investigate if the rearrangements were balanced or not. Molecular study and gene dosage have been used when possible, to define the correlation with the clinic phenotype.