Our DNA containing about 20 000 genes, consisting of 3 billion base pairs, constitutes our genetic code, is packed into 23 pairs of chromosomes within the cell. Cytogenetics is a genetic method that investigates the numerical and structural anomalies of these chromosomes. Cytogenetics is the evaluation of chromosomes by culturing any living tissue in the laboratory (blood, bone marrow, etc.), inducing the division of cells which are visible in the light microscope, in terms of fracture, loss or gain. Some chromosomal changes may be a sign of a genetic disease or condition or some types of cancer. With cytogenetic test results, it is possible to diagnose the disease or condition and to help plan of the treatment or to monitor how well the treatment works. Cytogenetic studies begin by removing all chromosomes from the nuclei of cells. These chromosomes are then transferred to glass slides, painted with special paints and examined by making them visible under the microscope. Different staining and banding techniques can be used to diagnose the changes detected during the examination (NOR banding, C banding etc.).

Using computer software, the images of chromosomes in the slides are taken and the picture is divided into pieces, so that the pairs of chromosomes can be matched according to the size and the the way they painted as indicated on the ideogram. Each chromosome pair is assigned a special number (1 to 22, plus X and Y) to prepare the karyotype of the individual.

There are many diseases that can be diagnosed by examining the karyotype of the individual. For example, Down's syndrome in which an individual has extra chromosome 21 can be identified by cytogenetic studies. In a karyotype, instead of a pair of chromosomes if there are three chromosomes this is called "trisomy". It can also be determined by this method that a woman has only one X chromosome as in the case of Turner syndrome. If there is only a chromosome instead of a pair, it is called "monozomy".

Sometimes a chromosome fragment and sticks to another chromosome. This is called "translocation". As an example of a disease caused by translocation, a part of chromosome 9 breaks down and connects to chromosome 22 called chronic myeloid leukemia (CML). Chromosome 8 to chromosome 14 translocation called Burkitt lymphoma. The cause of the disease caused by translocations is usually loss of gene at breakpoints or at the point of attachment of a specific gene that converts normal cells into tumor cells. Occasionally, transloxion occurs as a balanced translocation that does not cause loss or activation of the gene, which is harmless to the individual but may cause recurrent pregnancy losses.