Karyotype analysis is a techniques where chromosomes are visualized under a microscope. Cells are collected from an individual, induced to divide, and then arrested at metaphase (a stage of cell division when the chromosome are condensed and therefore visible). The chromsomes are stained with certain dyes that show a pattern of light and dark bands (called the banding pattern). These bands reflect regional differences in the amounts of A and T versus G and C. The banding pattern for each chromosome is specific and consistent allowing identification of each of the 24 chromosomes.

Picture of human
chromosome 4
           Karyotype analysis can detect large chromosomal abnormalities such as loss or gain of an entire chromosome or portions of a chromsome and translocations (when a portion of a chromosome breaks off and rejoins with another chromosome, a common occurrence in leukemias). However, only big changes in chromsomes can be visualized by karyotype analysis. Many of the changes in our genetic material that cause disease are very small and require other methods to be detected.
The figure on the left shows a typical karyotype from a male. Note that he has two copies of all autosomes (non-sex determining chromosomes) and just one copy of the X and Y chromosome.
           Some conditions, such as Down Syndrome and Turner Syndrome, are caused by large chromosomal changes. These syndromes are often diagnosed or confirmed by karyotype analysis.
This karyotype is from a female (2 copies of the X chromosome) with Down Syndrome. Notice she has three copies of chromosome 21 (trisomy 21).