Diagnosing Chromosome Disorders

Although scientists have been able to microscopically observe chromosomes since the mid-1800’s, a century passed before staining techniques were developed to examine them on a specific and individual basis. The chromosomes could then be arranged according to size and banding pattern for detailed examination – a display called a karyotype. Once it became possible to accurately identify individual chromosomes, abnormalities in chromosome number (such as trisomy 21, also known as Down syndrome) were discovered. Karyotypes can also identify deletions, duplications, and inversions of chromosomal segments. 

Although abnormalities on the order of millions of base pairs can be detected using the basic chromosomal banding techniques, smaller alterations cannot be discerned. More recent technologies, such as fluorescence in situ hybridization (FISH) and array comparative genome hybridization (aCGH), allow a finer level of resolution, with the ability to identify submicroscopic chromosome changes. 

Array CHG has replaced karyotyping as the standard chromosome diagnostic tool to detect abnormalities in chromosome number, microdeletions and other chromosome imbalances. It is used in both prenatal and postnatal settings. Depending on the specific array, it can detect chromosomal imbalances as small as 1,000 bp in size. The use of array CGH has significantly increased the diagnosis of chromosomal imbalances among individual with clinical anomalies.