AN INTERSTITIAL NUCLEOLUS ORGANIZER REGION IN THE LONG ARM OF HUMAN-CHROMOSOME-7 - CYTOGENETIC CHARACTERIZATION AND FAMILIAL SEGREGATION

An unusual NOR-bearing chromosome 7 was detected in a phenotypically n ormal, healthy 29-year-old male proband. Differential banding techniqu es as well as in situ hybridization employing various DNA-probes were performed in order to characterize the chromosome in detail. The nucle olus organizer region was found to be located between bands 7q21.3 and 7q22.1. No further rearrangements were detected in this chromosome. A nalysis of spontaneously occurring micronuclei revealed 9% of them to contain a 7q fragment distal to (or including) the inserted NOR, sugge sting that the inserted secondary constriction represents a potential hot spot for chromosomal breakage and rearrangement. Segregation analy sis of the variant chromosome 7 in 51 members of the probands' family showed transmission in a Mendelian fashion. 27 individuals were found to be heterozygous for the inserted chromosome. A three-year-old child in the consanguineous marriage of two heterozygous carriers exhibited the NOR-insertion in both of his chromosome 7 homologues. To our know ledge, this is the first report on a homozygous carrier of a non-acroc entric NOR-bearing chromosome.