karyotype and ploidy
/ KAIR-ee-oh-type / PLOY-dee /
If you stained a dividing cell, photographed its chromosomes, then cut them out and arranged them by size into neat pairs, you would have a sort of family portrait of that organism's whole chromosome set. That organized picture is the karyotype, and counting and pairing the chromosomes leads to a second idea — ploidy, how many complete sets a cell carries.
A karyotype is the full set of chromosomes of a cell or species, displayed by number, size, and shape (banding patterns from staining help tell them apart). Humans have a karyotype of 46 chromosomes: 22 matched pairs of autosomes plus one pair of sex chromosomes (XX or XY). Ploidy describes how many copies of the basic chromosome set are present. Most human body cells are diploid (2n) — two of each chromosome, one inherited from each parent — while eggs and sperm are haploid (n), carrying a single set so that fertilization restores the diploid number. Many plants and some animals are polyploid, carrying three, four, or more full sets, which is one route to bigger genomes and new species.
Karyotype and ploidy matter clinically and evolutionarily. Examining a karyotype reveals whole-chromosome abnormalities: an extra copy of chromosome 21 (trisomy 21) causes Down syndrome, and missing or extra sex chromosomes cause other conditions. Such errors usually arise when chromosomes fail to separate properly during cell division. In evolution and agriculture, polyploidy has been a major force — many crops like wheat are polyploid, and whole-genome duplications have shaped the history of many lineages. The karyotype is, in effect, the genome's table of contents at the level of whole chromosomes.
A human karyotype is written 46,XX (a typical female) or 46,XY (a typical male). Down syndrome shows up as 47,XX,+21 or 47,XY,+21 — an extra copy of chromosome 21 visible at a glance in the arranged chromosome portrait.
A karyotype lays out every chromosome; ploidy counts the complete sets.
Chromosome count is not a measure of complexity: a fern can have over a thousand chromosomes and some deer just a handful. And humans are diploid, not haploid — each body cell normally holds two copies of the genome, not one.