A karyotype refers to the number and appearance of chromosomes in the nucleus of a 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. Karyotypes describe an organism's chromosome count and what these chromosomes look like under a light microscope. Attention is paid to their length, the position of the centromeres, the banding pattern, any differences between the sex chromosomes, and any other physical characteristics. The preparation and study of karyotypes is part of cytogenetics.
The study of complete sets of chromosomes is sometimes known as karyology. Chromosomes are represented (by rearranging a photomicrograph) in a standard format known as a karyogram or ideogram: in pairs, ordered by size and centromere position for chromosomes of the same size.
So, in normal diploid organisms, autosomal chromosomes are present in two copies. There may or may not be sex chromosomes. Polyploid cells have multiple copies of chromosomes and haploid cells have single copies.
The study of karyotypes is important for cell biology and genetics, and the results can be used in evolutionary biology and medicine. Karyotypes can be used for many purposes; such as studying chromosomal aberrations, cell function, taxonomic relationships, and gathering information on past evolutionary events.
Chromosomes were first observed in plant cells by Carl Wilhelm von Nägeli in 1842. Walther Flemming, the discoverer of mitosis, described its behavior in animal cells (salamanders) in 1882. The name was coined by another German anatomist, Heinrich von Waldeyer in 1888.
The next stage took place after the development of genetics in the early 20th century, when it was appreciated that chromosomes (which can be observed by karyotype) were the carriers of genes. Lev Delaunay seems to have been the first person to define karyotype as the phenotypic appearance of somatic chromosomes, in contrast to their gene contents in 1922. The later history of the concept can be followed in the works of CD Darlington and Michael JD White.
