karyotype

A karyotype is the number and visual appearance of the chromosomes in the nucleus of a cell. It is used to characterize an organism’s chromosomal complement and can reveal species- or individual-specific features.

In humans, the typical karyotype contains 46 chromosomes arranged in 22 pairs of autosomes and one pair of sex chromosomes, designated as 46,XX in females or 46,XY in males.

Chromosome analysis is usually performed on dividing cells. Cells are cultured and arrested in metaphase, swollen with a hypotonic solution, fixed, and stained to reveal distinctive banding patterns, most commonly G-banding. A karyogram is produced by sorting and pairing chromosomes by size, banding pattern, and centromere position.

Karyotypes can reveal numerical abnormalities (aneuploidy) such as trisomy 21 or monosomy X, and structural abnormalities such as deletions, duplications, inversions, and translocations.

Applications include prenatal testing (for example, amniocentesis or chorionic villus sampling), diagnosis of congenital disorders, and cancer cytogenetics, where characteristic rearrangements define certain cancers.

Advances in molecular cytogenetics have added techniques such as fluorescence in situ hybridization (FISH) and spectral karyotyping, and array-based methods provide higher resolution for copy-number changes, though they do not produce a conventional karyotype image. Limitations include dependence on dividing cells, limited resolution for small changes, and the potential for cryptic rearrangements.