There are many critical chromosomal aberrations present at the site of nucleotide sequences. For example - tuner syndrome, down syndrome, Cystic Fibrosis, Tay - Sachs disease [13], so in the testing method of fluorescence in situ hybridization (FISH). The labelled DNA probes reciprocal, to regions of the chromosome in question are allowed to hybridize to the preparation of the test sample's chromosomes. A hybridization signal verifies the presence of that chromosomal material in the test sample, whereas the absence of a signal indicates the absence of the material. Diagnostic FISH for Down syndrome would involve the use of labelled probes with sequences complementary to regions traverse chromosome number 21. The presence of three labelled chromosomes would, therefore, be diagnostic of Down syndrome [6].