Standard Chromosome Testing

A routine chromosome analysis involves obtaining a blood sample. The white blood cells are isolated from the sample and undergo a special preparation in order to view the chromosomes under a microscope.  As part of this preparation the chromosomes are stretched out and a chemical called Geimsa stains the chromosomes in a pattern made up of light and dark bands, which is specific to each chromosome.  This technique is referred to as G-banding.

Once the chromosomes are G-banded, metaphase cells, such as the one shown below, are analyzed on a microscope by cytogenetic technologists. 

 After microscope analysis, pictures of the metaphase chromosomes are captured using a digital camera connected to a computer with special software that allows the technologist to systematically arrange the chromosomes in what is known as a karyotype, which is shown below: 

Routine chromosome analysis allows for the identification of each the chromosomes and the detection of any abnormalities such as translocations, deletions, and duplications.  However, the resolution of this technique is limited.  A clinically significant amount of genetic material may be gained or lost, but not detected by the routine chromosome analysis. This is especially true for the telomeric regions of human chromosomes since most of these regions stain pale by G-banding, making rearrangements between telomeres very difficult, if not impossible, to detect using this method.

A newer technique has been developed called fluorescence in situ hybridization (FISH) which can be used to detect smaller changes.  FISH is the technique that is utilized in subtelomere analysis.