HOME › WIKI› CONCEPTS› KARYOTYPE

Karyotype

One-sentence definition. A karyotype is the full chromosome complement of an organism, characterized by chromosome number, size, shape, and, where applicable, banding pattern.

One-sentence analogy. A karyotype is like a floor plan of a genome: it tells you how many rooms there are, how large each one is, and where the load-bearing walls (centromeres) sit, without yet revealing what is inside any room.

Why it matters. Karyotypes are the entry point to chromosome evolution. Before we can ask why chromosome number changes, why fusions are favored in some lineages, or why sex chromosomes degenerate, we need a shared vocabulary for describing what a chromosome complement looks like and how to measure it. Karyotype variation within and among species is one of the most accessible signals of genome evolution, visible under a light microscope, and the karyotype database our lab maintains for Coleoptera depends entirely on this shared description system.

Where you meet it in the wiki.

Prerequisites: none (this is foundational) Next, learn about: autosome, chromosome fusion, heterogamety

Background

Karyotyping as a practice predates a correct human count by decades. Cytologists in the early twentieth century estimated the human chromosome number at 48, an error that persisted because metaphase spreads were difficult to prepare and chromosomes clumped. In 1956, Joe Hin Tjio and Albert Levan published a correction: the correct number is 46. Their technical advance (a hypotonic solution to swell cells before spreading) turned chromosome number into a stable, measurable character that could be tabulated across species and compared.

The karyotype became far more informative with the banding revolution. Torbjörn Caspersson and colleagues showed in 1969 that Giemsa stain produces reproducible light-and-dark bands along each chromosome, giving every chromosome a visual identity independent of size. The International System for Human Cytogenomic Nomenclature (ISCN) codified band nomenclature so that a karyotype description written in one lab could be read in another. Fluorescence in situ hybridization (FISH) and spectral karyotyping extended this further, painting chromosomes with sequence-anchored DNA probes. Our lab’s focus is Coleoptera, where karyotype diversity is exceptional: the historical record reaches back to Nettie Stevens, who in 1905 first identified sex-determining chromosomes in the yellow mealworm beetle Tenebrio molitor, and we continue building a comparative karyotype database for the order using these same foundational measurements.

How it works

Karyotypes are prepared from cells caught in mitotic metaphase, the stage when chromosomes are maximally condensed and individually visible. In many vertebrates, bone marrow provides a convenient source of rapidly dividing cells. In beetles and other insects, gonads (testes, especially) are the standard tissue because spermatogonial divisions yield clean, well-spread metaphase plates. Cells are treated with a spindle poison (colchicine) to arrest division at metaphase, then swollen in a hypotonic salt solution, fixed, dropped onto a slide, and stained with Giemsa or a related dye. Under a microscope, chromosomes appear as paired rods or dots whose size and shape can be measured.

The standard notation is 2n = N, where 2n is the diploid chromosome number; sex chromosome systems are appended (e.g., 2n = 46, XY in humans; 2n = 10, X0 in a beetle lacking a Y). Each chromosome is classified by centromere position: metacentric (centromere near the middle, roughly equal arms), submetacentric (offset centromere, one long arm q and one short arm p), acrocentric (centromere near one end), and telocentric (centromere terminal, single functional arm). One important exception is the holocentric chromosome, found in many insects including some beetles and all Hemiptera. Holocentric chromosomes lack a localized centromere; kinetochore activity is distributed along the entire chromosome length, and this changes how fissions and fusions behave evolutionarily.

A worked example

The human karyotype (2n = 46) and the chimpanzee karyotype (2n = 48) differ by exactly one fusion. Human chromosome 2 is syntenic with two ancestral ape chromosomes that remain separate in chimpanzees, gorillas, and orangutans. The fusion is detectable as a vestigial internal telomere near the midpoint of chromosome 2 and as a degenerate centromere in the corresponding region. Within Coleoptera the range is far wider: beetle karyotypes span from 2n = 4 in Mecynorhina (scarab beetles) up through 2n = 50 and beyond in other lineages. That range, across more than 400,000 described species, is why Coleoptera is a productive system for studying karyotype evolution statistically.

Common misconceptions

Chromosome number reflects genome size. It does not. A species with 2n = 40 can have a smaller genome than one with 2n = 10 if the chromosomes differ in length. Chromosome number and genome size are related only loosely, and they evolve under partly different forces.
A karyotype describes the full DNA sequence of an organism. It describes chromosome number, morphology, and (with banding) broad structural organization. Sequence content is a finer level of description that a karyotype does not capture.
Karyotype changes are always deleterious. Most are, at least while segregating heterozygously. A fusion heterozygote, for example, can suffer meiotic problems when the three-chromosome chain must segregate. Many karyotype changes are likely fixed by drift in small populations despite an initial fitness cost, not because they are advantageous.
Holocentric chromosomes are rare or exotic. Holocentricity is the ancestral state in several large insect orders, including Hemiptera and Lepidoptera, and has evolved independently in nematodes, plants, and other lineages. It is not a curiosity but a distinct chromosome architecture that affects how we interpret karyotype evolution in those groups.

How to spot it in papers

A table of 2n values in a systematic or comparative study is a karyotype survey. Look for associated sex chromosome system notation (XY, X0, ZW, etc.) alongside the diploid number.
Methods mentioning testis squashes, bone marrow spreads, hypotonic treatment, colchicine arrest, or Giemsa staining describe primary karyotyping. These papers are reporting new karyotype data rather than compiling it from the literature.
The term “chromosome complement” is a near-synonym for karyotype and appears in older literature where “karyotype” was reserved for the photographic plate of sorted chromosomes (a karyogram).
Chromosome morphology described by centromere position (metacentric, acrocentric, etc.) or reported as a fundamental number (NF, the number of chromosome arms) tells you the paper is working at the level of karyotype structure, not just count.