Cells other than ovaries and sperm that make up the human body all contain the same series of 23 pairs of chromosomes: 1 pair of different sex chromosomes for women (XX) and men (XY) and 22 other pairs (autosomes) identical in both sexes. The karyotype is used to visually analyze a large number of chromosomal changes such as their number (e.g., three copies of chromosome 21 in Down syndrome or a single copy of the X chromosome in women [Turner syndrome]). A large number of additional abnormalities called deletions, duplicates, translocations and genetic rearrangements are detected by chromosome band analysis, with each band representing one fragment of the DNA forming our chromosomes. The technique used indicates the resolution of the analysis, meaning the number of bands observed (typically between 300 and 600). A karyotype analysis is useful in a number of situations, including screening for abnormalities in fetuses and newborns. It is also indicated in early or delayed puberty, cases of ambiguous genitalia, male and female infertility, and in several situations where certain genetic diseases are suspected.
Certain genetic rearrangements such as balanced exchanges (translocations) between two chromosomes may not cause any particular symptoms in the individual, but may cause problems for their offspring. Some abnormalities are too subtle to be detected by karyotyping. These abnormalities require further analysis using other techniques (FISH, mutation screening through DNA analysis, DNA sequencing, etc.).