The term "karyotype" refers to the chromosomal pattern inside the nucleus of an animal cell (eukaryote), as well as to describes the set of chromosomes in a species or in an individual organism. The study of karyotypes in eukaryotic cells is a branch of science termed cytogenetics, from "cyto" meaning cell and "genetics" meaning the cell's chromosomal make up.
A karyotype will be shared by organisms from the same species, but the following intra-species variations are seen:
- The karyotype of males and females may differ. For example, in humans the male karyotype contains an X and a Y chromosome while in human females there are two X chromosomes
- There are karyotypic differences between body (somatic) cells and egg and sperm cells (gametes). The sperm and egg cells each contain half the amount of chromosomes a somatic cell contains, and only make a complete cell with the full number of 46 chromosomes when they combine during fertilization
- Karyotypes may also differ within a population due to genetic polymorphism
- The karyotype of a species may vary by geographical location and racial differences are also seen
- Genetic abnormalities may also give rise to abnormal and different karyotypes
Karyotyping refers to the process used to examine a cells' karyotype while it is locked in metaphase during cell division. This is done using the drug colchicine or demecolcine (colcemid). At this stage of cell division, the chromosomes are condensed into dense thread-like structures that are easily distinguishable on staining and imaging. The autosomes are then arranged in order of their length followed by the sex chromosomes to create a karyogram.
The human karyotype and number of chromosomes was first discovered early in the 20th century. By the mid 1950's, the human karyotype composed of its 46 chromosomes was finalized. Chimpanzees are the closest relatives to humans with a karyotype comprising 48 chromosomes.