Comparative Genomic Hybridization (CGH) is an important technology as part of genetic diagnosis because it allows the examination of all 24 chromosomes in the embryo. Previous technologies like FISH, while still a useful tool, limited the number of chromosomes that could be evaluated.
CGH is a technique to examine the DNA content of a cell by breaking the DNA into small pieces and comparing the results to a known sample. The test sample is deferentially labeled with a fluorescent marker and combined with the known sample. The color of the fluorescence is quantitated to produce the test results. Because the technique measures “copy numbers” and the results are normalized to the most frequent ratio level of test and control samples, CGH cannot detect polyploidy (the presense of an even number of copies).
CGH has the advantage that it can quantitate the the number of all the chromosomes. We can use this to our advantage for aneuploidy screening, because we want to know about all the chromosomes, if possible for embryo screening.