Karyotype

A karyotype is the complete chromosome makeup of an organism's cells: how many chromosomes there are and what each one looks like. The word also names the image prepared in a cytogenetics lab, in which the chromosomes of a single dividing cell are photographed and arranged as matching pairs from largest to smallest. Each chromosome is identified by its length, where its centromere sits and the stripes of light and dark bands left by staining. A typical human karyotype shows 46 chromosomes, 22 pairs of autosomes plus two sex chromosomes, written 46,XX or 46,XY.

To prepare one, technicians grow cells such as white blood cells, add colchicine to halt division at metaphase, when chromosomes are most condensed, then spread them on a slide and stain them, usually with Giemsa dye. The result can reveal an extra or missing chromosome, as in trisomy 21 (Down syndrome) or Turner syndrome (45,X), and large structural changes such as deletions and translocations.

Students meet karyotypes in genetics courses and in clinical work, including prenatal testing and the diagnosis of leukemias, where the Philadelphia chromosome, an exchange between chromosomes 9 and 22, is a classic finding. Smaller changes need finer methods such as FISH or chromosomal microarrays.