Breakpoints in α, β, and satellite III DNA sequences of chromosome 9 result in a variety of pericentric inversions

Human chromosome 9 with a pericentric inversion involving the qh region is considered normal. It has probably evolved through breakage and reunion and is retained through mendelian inheritance without any apparent phenotypic consequences. Fluorescent in situ hybridisation (FISH) technique using α, β, and satellite III DNA probes showed that the breakpoints are variable and can be localised in the α or in the satellite III and β DNA regions or both. Three types of inversions are proposed which appear similar by CBG banding: pericentric inversions with two alphoid, one β, and one satellite III hybridisation signals were classified as type A. Type B were those with two β, one α, and one satellite III hybridisation signals, while type C was complex, and most likely involved two inversions, since two separate hybridisation signals were detected in each of the alphoid, β satellite, and satellite III DNA regions. Based on eight cases, type A is likely to be the most frequent, but the frequencies, which at present appear non-random for these different types of inversions in the population, can only be estimated by studying a larger sample size. Inversion heteromorphisms may promote reshuffling of tandem arrays of DNA repeat sequences, thereby giving rise to new heteromorphic domains. Alternatively, the repetitive nature of the sequences lends to the structural variations observed within the inv(9) chromosomes (or any other abnormal chromosome that is the result of recombination between, or breakage within, repetitive DNA).