Monday 17 October 2005
Definition: Karyotype is a photomicrograph of an individual’s chromosomes arranged in a standard format showing the number, size, and shape of each chromosome type.
It is used in low-resolution physical mapping to correlate gross chromosomal abnormalities with the characteristics of specific diseases.
Human somatic cells contain 46 chromosomes; these comprise 22 homologous pairs of autosomes and two sex chromosomes, XX in the female and XY in the male. The study of chromosomes-karyotyping-is the basic tool of the cytogeneticist.
The usual procedure of producing a chromosome spread is to arrest mitosis in dividing cells in metaphase by the use of mitotic spindle inhibitors (e.g., colcemid) and then to stain the chromosomes. In a metaphase spread, the individual chromosomes take the form of two chromatids connected at the centromere.
A karyotype is a standard arrangement of a photographed or imaged stained metaphase spread in which chromosome pairs are arranged in order of decreasing length.
A variety of staining methods that allow identification of each individual chromosome on the basis of a distinctive and reliable pattern of alternating light and dark bands along the length of the chromosome have been developed.
The one most commonly employed uses a Giemsa stain and is hence called G banding. A normal male karyotype with G banding is illustrated in Figure 5-20. With G banding, approximately 400 to 800 bands per haploid set can be detected.
The resolution obtained by banding techniques can be dramatically improved by obtaining the cells in prophase. The individual chromosomes appear markedly elongated, and up to 1500 bands per karyotype may be recognized.
The use of these banding techniques permits certain identification of each chromosome as well as delineation of precise breakpoints and other subtle alterations.