A karyotype is the complete set of all chromosomes of a cell of any living organism. The chromosomes are arranged and displayed (often on a photo) in a standard format: in pairs, ordered by size. Karyotypes are examined in searches for chromosomal aberrations, and may be used to determine other macroscopically visible aspects of an individual's genotype, such as sex (XX vs. XY pair). The study of karyotypes is known as karyology.
Normal human karyotypes are denoted 46,XX (for most women) and 46,XY (for most men). However, some individuals have other karyotypes with added or missing sex chromosomes, including 47,XYY, 47,XXY, 47,XXX and 45,X. The other possibility, 45,Y, does not occur, as an embryo without an X chromosome is incapable of survival.
In the "classic" (depicted) karyotype, a dye, often Giemsa, is used to make bands on the chromosomes visible. This is also referred to as G-banding. Each chromosome has a characteristic banding patern which helps to identify them (notice that the two chromosomes of one pair have the same banding pattern).
In this newer technique, several different probes specific of one chromosome pair, carrying different amounts of a set of fluorescent dyes, are hybridized to the chromosomes in a technique known as fluorescent in situ hybridization (FISH). This gives each chromosome pair unique spectral characteristics due to the relative amount of each of the fluorochromes. Chromosomes can be automatically identified in fluorescence microscopy through an interferometer and a computer analysis (spectral imaging )