46,XX



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.

Most 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 karyotype 45,Y does not occur, as an embryo without an X chromosome cannot survive.

Classic karyotype
In the "classic" (depicted) karyotype, a dye, often Giemsa, is used to stain bands on the chromosomes. This is also referred to as G-banding. Each chromosome has a characteristic banding pattern that helps to identify them (notice that the two chromosomes of one pair have the same banding pattern).

Also visible in the karyotype are the individual chromosomes that match up with identical ones. There are some cases where deletions, extra, or missing chromosomal material, cause disease, such as in Down Syndrome, Angelman Syndrome, Edward's Syndrome and Patau Syndrome.

Karyotypes are arranged with the short arm of the chromosome on top, and the long arm on the bottom. Some karyotypes call the short and long arms as p and q, respectively. In addition, the differently stained regions (and sub-regions) are given numerical designations, from proximal to distal on the chromosome arms. For example, Cri du chat syndrome involves a deletion on the short arm of chromosome 5. It is written as 46,XX,5p-. The critical region for this syndrome is deletion of 15.2, which is written as 46,XX,5p15.2- or 46,XX,del(5p15.2).

Chromosomal Diseases
Down syndrome, a common chromosomal disease, is caused by trisomy (three copies) of chromosome 21.

Spectral karyotype (SKY technique)
Spectral karyotyping is a molecular cytogenetic technique used to simultaneously visualize all the 23 pairs of human (or the 20 pairs of mouse) chromosomes in different colors. Fluorescently-labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Due to there being a limited number of spectrally-distinct fluorophores, a combinatorial labeling method is used to generate many different colors. Spectral differences generated by combinatorial labeling are captured and analyzed by using an interferometer attached to a fluorescence microscope. Image processing software then assigns a pseudo color to each spectrally different combination, allowing the visualization of the individually colored chromosomes.

This technique is widely used to identify structural chromosome aberrations in cancer cells and other disease conditions.

Reference

 * E. Schröck, S. du Manoir, T. Veldman, B. Schoell, J. Wienberg, M. A. Ferguson-Smith, Y. Ning, D. H. Ledbetter, I. Bar-Am, D. Soenksen, Y. Garini, T. Ried, SCIENCE, 26 July 1996; 273 (5274):494