Pericentric inversions involving the secondary constriction (qh) region of chromosome 9 are considered to be normal variants. The evolutionary mechanisms and conservation of these inversions via Mendelian fashion have been investigated since the advent of banding techniques. Routine cytogenetic techniques cannot provide the fine characterization necessary to determine the type of genetic material involved in these rearrangements. Therefore, the fluorescence in situ hybridization technique with the human centromere-specific alpha satellite and the beta satellite (D9Z5) and classical satellite (D9Z1) human DNA probes were used to identify the breakpoints of chromosome 9 pericentric inversions. Four unique types of pericentric inversions involving the 9qh region were observed, and the mechanism may be due to breakage and reunion at the proposed breakpoints. They are: type A inversions consist of breakpoints within the alpha and beta satellite DNA regions; type B consist of breakpoints within the beta satellite DNA region and band 9q13; type C involve breakage within the beta and classical satellite DNA regions, and type D have breakpoints within the alpha and classical satellite DNA regions. Obviously, reshuffling of satellite DNA sequences has occurred, which has given rise to a variety of heteromorphisms whose clinical significance remains obscure.
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