As part of cytogenetic diagnostics, the karyotype of a person is shown by microscopic analysis of the chromosomes. The karyotype of a chromosomally normal person comprises 46 chromosomes: 22 pairs of autosomes and two sex chromosomes (female karyotype: 46,XX, male karyotype: 46,XY). For a chromosome analysis, cells capable of division must be cultivated and prepared in such a way that the chromosomes in the metaphase stage of mitosis can be assessed under the light microscope after staining (e.g. GTG banding). This allows changes in the number (e.g. trisomies) and structure of the chromosomes (e.g. deletion, translocation) to be recognized. Balanced translocations can be an important cause of infertility or repeated miscarriages and lead to an increased risk of disability in the child. A karyotype analysis can detect deletions or duplications of 5-10Mb in size.

The chromosome analyses are carried out postnatally from blood lymphocytes, fibroblasts (skin biopsy) and prenatally from amniotic fluid cells, chorionic villi or cells from umbilical cord blood or fetal abortion tissue.