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Since the chromosomes
can be delineated only during mitosis, it is necessary to examine
material in which there are many dividing cells. This can usually
be accomplished by culturing cells from the blood or skin. 
After growth, the cells are fixed on slides and then stained with
a variety of DNA-specific stains that permit the identification
of the chromosomes.
The Denver
system of chromosome classification, established in 1959, identified
the chromosomes by their length and the position of the centromeres.
Since then the method has been improved by the use of special
staining techniques that impart unique light and dark bands to
each chromosome. These bands permit the identification of chromosomal
regions that are duplicated, missing, or transposed to other chromosomes.
Gina
Kolata
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