MECHANISMS OF THE ORIGIN OF A G-POSITIVE BAND WITHIN THE SECONDARY CONSTRICTION REGION OF HUMAN-CHROMOSOME-9

We report on a so-called rare variant where a G-positive band was sand wiched within the secondary constriction (qh) region of chromosome 9 a nd is apparently different from previous cases when characterized by t he fluorescence in situ hybridization technique. The major differences included duplication of beta-satellite and satellite III DNA sequence s and bands 9q13-->q21.1, without duplication or inversion of the alph oid sequences. Based on the reported cases, at least four types of var iations can be accounted for. A variety of mechanisms have been propos ed to describe the origin of a G-positive band within the 9qh region, which appears to be similar when studied by routine cytogenetic techni ques but differs by molecular methods. It is hypothesized that the cli nical consequences depend upon the size of the G-positive band(s) dupl icated, and a genetic inactivation mechanism might have some sort of i nfluence during the so-called heterochromatinization process. It appea rs that heterochromatin, once thought to be composed of junk DNA, may have some role after all in suppression of gene(s) and/or spreading of inactivation, if genes are embedded within the heterochromatic region . Apparently, the mixture of different types of DNA creating patches o f genetic debris have become a fundamental hidden treasure, where gene tically active chromatin could be inactivated without dire consequence s. The variable nature of heterochromatin has resulted in cytogenetic heteromorphisms of a number of human chromosomes. Their characterizati on by molecular techniques is becoming imperative, because fetal wasta ge have occurred in many situations where variant chromosomes were wro ngly identified as chromosomal abnormalities.