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Karyotype refers to the arrangement of chromosomes in their matched (homologous) pairs. For the purposes of this definition, we will be referring to human chromosomes, although there is a karyotype characteristic for each species. The human chromosomes are arranged and numbered according to the International System for Human Cytogenetic Nomenclature (ISCN). The most recent recommendations of the ISCN are from 1995. Karyotype either refers to the actual composition of the chromosomes in a body cell of an individual or species, or to the actual diagram or photograph of those chromosomes, arranged in their pairs.
The normal human karyotype consists of 23 pairs of chromosomes. There are 22 pair of autosomes, which are the chromosomes that are not the sex chromosomes. The genes on these chromosomes instruct our bodies as to how they look and function. The 23rd pair of chromosomes are the sex chromosomes. Typically, females have two X sex chromosomes and males have one X sex chromosome and one Y sex chromosome.
In the construction of the karyotype, the chromosomes are numbered 1 to 22 from longest to shortest. The last pair are the sex chromosomes and are placed on the karyotype after the 22nd pair. The chromosomes can be separated into groups, based on their length and the position of the centromere. Group A consists of chromosome pairs 1, 2 and 3. They are the longest chromosomes and their centromeres are in the center of the chromosomes (metacentric). Group B consists of chromosome pairs 4 and 5. They are long; however, their centromeres lie toward the top of the chromosomes (submetacentric). Group C consists of chromosome pairs 6, 7, 8, 9, 10, 11 and 12 and also includes the X chromosome. They are medium-sized and their centromeres either lie in the middle or toward the top of the chromosomes. Group D consists of chromosome pairs 13,14 and 15. They are medium-sized and their centromeres lie at the top of the chromosomes (acrocentric). Additionally, the D group chromosomes have satellites. Group E consists of chromosome pairs 16, 17 and 18. They are relatively short chromosomes and their centromeres lie in the center or towards the top of the chromosomes. Group F consists of chromosomes 19 and 20. They are short chromosomes with centromeres that lie in the center of the chromosome. Lastly, group G consists of chromosome pairs 21, 22 and the Y chromosome. These are short chromosomes with their centromeres at the top. Chromosome pairs 21 and 22 have satellites. The Y chromosome does not have satellites.
The actual chromosomes are only individually distinguishable during a certain stage of cell division. This stage is called the metaphase stage. Chromosome preparations are made from pictures of the chromosomes during the metaphase stage of division. The metaphase spread is what the technician sees in one cell under the microscope and what the photograph of that one cell is referred to. Usually, the chromosomes in a metaphase preparation are banded by special staining techniques used in the laboratory. Each numbered chromosome is unique in its banding pattern so that all number 1s look the same and all number 2s look the same, etc. Although, there can be small normal familial variations in chromosomes. Because of banding, the chromosomes are more easily distinguishable from each other and the banding makes it is easier to see differences or abnormalities. For example, if a chromosome is missing a piece, or two chromosomes are attached to each other (translocation), it is much easier to see with banded chromosomes than with unbanded chromosomes.
Chromosome preparations can be made from any potentially dividing cells, including; blood cells, skin cells, amniotic fluid cells (the fluid surrounding an
ISCN formulas exist to describe any chromosome complement. The basic formula for writing a karyotype is as follows. The first item written is the total number of chromosomes, followed by a comma. The the second item written is the sex chromosome complement. The typical female karyotype is written as 46,XX and the typical male karyotype is written as 46,XY.
Author Info: Renee A. Laux MS, The Gale Group Inc., Gale, Detroit, Gale Encyclopedia of Genetic Disorders Part I, 2002This feature is for informational purposes only and should not be used to replace the care and information received from your healthcare provider. Please consult a healthcare professional with any health concerns you may have.
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