A dark band on a chromosome is caused by staining with a dye that produces alternating light and dark regions along the length of the chromosome. The dye used in karyotyping stains regions of chromosomes that are rich in the base pairs Adenine (A) and Thymine (T), which produces a dark band. These regions of the chromosome that contain fewer genes are usually very tightly packed and produce very dark bands. It is important to note that the thinnest bands contain over a million base pairs and potentially hundreds of genes, so bands do not represent single genes. Karyotyping is one of many techniques that allow us to look for several thousand possible genetic diseases in humans.