A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species or in an individual organism and for a test that detects this complement or measures the number.
What Is a Karyotype?
A karyotype is simply a picture of a person’s chromosomes. In order to get this picture, the chromosomes are isolated, stained, and examined under the microscope. Most often, this is done using the chromosomes in the white blood cells. A picture of the chromosomes is taken through the microscope. Then, the picture of the chromosomes is cut up and rearranged by the chromosome’s size. The chromosomes are lined up from largest to smallest. A trained cytogeneticist can look for missing or extra pieces of chromosome.
karyotype[1]There are 22 numbered pairs of chromosomes called autosomes. The 23rd pair of chromosomes are the sex chromosomes. They determine an individual’s sex. Females have two X chromosomes, and males have an X and a Y chromosome.
How are the Chromosomes Numbered?
Each chromosome has been assigned a number based on its size. The largest chromosome is chromosome 1. Therefore chromosome 18 is one of the smallest chromosomes in humans.
Human karyotypes: 2n = 48 or 46?
Early studies of the human karyotype simply stained chromosomes within cells with Giemsa and "squashed" them between the cover slip and slide. Most cells were not at the proper mitotic phase for chromosomes to be observed, and chromosome separation was poor. The exact count was uncertain: most workers accepted the number 48. The breakthrough came in 1952 (left) when a technician in the lab of T. C. Hsu accidentally substituted distilled water for the normal saline solution used in washing the cells just before "squashing". This "hypotonic" treatment caused the cell nuclei to swell, and allowed the chromosomes to separate before squashing. A further refinement was "dropping" the cells onto the slide at arm's length, which caused the nuclei to burst on impact, further separating them (middle slide). Finally, the use of a plant spindle-poison Colchicine allows chromosomes to be arrested at mitotic metaphase, during their maximum state of compaction [right] These experiments quickly established the human chromosome number as 2n = 46 chromosomes.
Classification of chromosomes into seven groups by size and relative centromere position established the so-called "Denver System" (right) in 1960. Chromosomes within groups B - G were not readily distinguishable from each other. The X chromosome is in the C group, and the Y is in the G group: males are reconizable by five small G-type chromosomes. Modern banding techniques allow each chromosome in the karyotype to be distinguished individually.