TOPIC 2

2. Karyotypes:

Karyotypes are images of chromosomes to display their banding patterns. When a nucleus is in during metaphase of mitosis, its chromosomes are condensed and the banding of the chromosomes can be visualized when certain dyes (e.g. Giemsa dye) are added to the chromosomes. There are several classical methods available to visualize the banding pattern as well as a more genomic one called chromosomal painting.

For those of us who are unaccustomed to seeing real chromosomes, often they are drawn in a cartoon fashion called an ideogram. Below is an ideogram of the X chromosome. The short arm of any chromosome is called the "p" arm which stands for the French word for small - petite. The long arm is called the "q" arm. Many years ago, histologist numbered the bands for each arm so we can refer to particular bands as genomic locations and everyone will be looking at the same band.


A karyotype is the number and appearance of chromosomes in the nucleous of a eukaryote cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.

Karyotypes describe the number of chromosomes, and what they look like under a light microscope. Attention is paid to their length, the position of the centromeres, any differences between the sex chromosomes, and any other physical characteristics. The preparation and study of karyotypes is part of cytogenetics.

The study of whole sets of chromosomes is sometimes known as karyology. The chromosomes are depicted (by rearranging a microphotograph) in a standard format known as a karyogram or idiogram: in pairs, ordered by size and position of centromere for chromosomes of the same size.

Karyotypes can be used for many purposes; such as, to study chromosomal aberrations,cellular function, taxonomic relationships, and to gather information about past evolutionary events.

The karyotype is characteristic of each species, as the number of chromosomes, humans have 46 chromosomes (23 pairs because we are diploid or 2n) nucleus of every cell, [1] organized into 22 autosomes and 1 pair peer sexual (male XY and female XX). 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 with only a Y chromosome is incapable of survival.

Staining: The study of karyotypes is possible due to staining. Usually a colorant is applied after they have been arrested during cell division by colchicine solution. For the human white blood cells are the most frequently used because they are easily induced to grow and divide in tissue culture. Sometimes the comments can be made when cells are not dividing. The sex of a newborn can be determined by observation of cells at the interface. Most species have a standard karyotype.


Classic Karyotipe: In the "classic" (depicted) karyotype, a dye, often Giemsa (G-banding), less frequently Quinacrine, is used to stain bands on the chromosomes. Giemsa is specific for the phosphate groups of DNA. Quinacrine binds to the adenine-thymine-rich regions. Each chromosome has a characteristic banding pattern that helps to identify them; both chromosomes in a pair will have the same banding pattern. 

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 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, of 5.

Spectral Karyotype: Spectral karyotyping is a molecular cytogenetic technique used to simultaneously visualize all the pairs of chromosomes in an organism in different colors. Fluorescently labeled probes for each chromosome are made by labeling chromosome-specific DNA with different fluorophores. Because there are 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 used to identify structural chromosome aberrations in cancer cells and other disease conditions when Giemsa banding or other techniques are not accurate enough.

Virtual Karyotype: detects genomic copy number variations at a higher resolution level than conventional karyotyping or chromosome-based comparative genomic hybridization.