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When a syndrome is suspected, for which the causative gene or locus (genomic position) is known, fluorescence in situ hybridization (FISH) can allow assessment of duplications, deletions or translocations on a smaller scale. For instance, a locus for congenital hypoparathyroidism, as part of DiGeorge syndrome, exists on the long arm of chromosome 22 (22q). FISH utilizes the principle that complementary DNA sequences will hybridize together by hydrogen bonding. Stretches of DNA from the region of interest are fluorescently labelled and hybridized to the patient’s DNA. The fluorescence is visible as a dot on each sister chromatid of each relevant chromosome (Figure 4.4). Therefore, normal autosomal copy number is viewed as two pairs of two dots; one pair indicates a deletion; and three pairs indicate either duplication or potentially a translocation breakpoint (where the probe detects sequence either side of the breakpoint on different chromosomes). As technologies based on genome‐wide microarray and, especially, next‐generation sequencing have become more prevalent (see below), FISH is now less often used as the main methodology but its description is still useful to explain diagnosis based on DNA hybridization and fluorescence detection.

Figure 4.4 Fluorescent in situ hybridization in a patient with congenital hypoparathyroidism due to DiGeorge syndrome causing hypocalcaemia and congenital heart disease. Metaphase chromosomes were hybridized with a fluorescent probe from chromosome 22q11. The two bright dots indicate hybridization on the sister chromatids of the normal chromosome 22. The arrow points to the other chromosome 22 that lacks signal, indicating a deletion.

Images kindly provided by Professor David Wilson, University of Southampton.