The 22 pairs of human autosomes were first classified according to length and position of the centromere into seven groups, identified with the letters A-G. All autosomes could be placed satisfactorily within a group, but the numbering within the groups was more or less tentatves (Fig 47a) until 1970. Chromosomes banding techniques, along with new methods of identification, finally distinguished all 46 human chromosomes. Bands are defined as parts of chromosomes that appear lighter or darker than adjacent regions when treated with particular staining methods. Q-staining methods employ quinacrine compounds and products fluorescent Q-bands along the chromosomes. G (Giemsa)-staining methods (fig 47b) result in G-bands and, with some Giemsa techniques, the R (reverse)-staining methods result in R-bands. The centric region and other heterochromatin areas are stained by C-banding (fig 47c). Refinements in staining techniques now permit all human chromosomes to be identified (fig 47d). The amount of DNA packed into one G-band is on the order of 106-107 based-pairs of DNA, and it is clearly not a gene.
In 1971 a conference on the standardization of human cytogenetics was held in Paris. Conferees agreed that to describe extra autosomes, the number of the extra chromosome should be placed after the total number and sex chromosomes with a plus (+) or minus (-) sign before the number of the autosomes involved, for example, 47,XX+21 is the karyotype of a female with trisomy-21. A male with an extra X chromosomes is symbolized 47,XXY. A plus or minus sign is placed following a chromosomes symbol to signify increase or decrease in arm length. The letter q symbolizes the long arm and ? the short arm. For example, 46,XY, 1q+ indicates an increase in the length of the long arm of chromosomes 1. The person represented by this example is a male with 48 chromosomes. The karyotype 47,XY,+14 ? + symbolizes a male with 47 chromosomes, including an additional chromosome (no.14), with an increase in the length of its short arm. The types of chromosomes aberration are represented by abbreviations for convenience in presenting chromosomes formulas: def (deficiency), dup (duplication), r (ring), inv (inversion), and t (translocation).
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