A karyotype lines up every chromosome in a cell, and ours reveals an ancient fusion
Humans carry one fewer pair of chromosomes than chimpanzees and other great apes. The reason is visible in a karyotype, the lined-up portrait of a cell's chromosomes: our chromosome 2 appears to be two ancestral ape chromosomes joined end to end. A dye, a microscope and a camera are enough to reveal it.
A karyotype is the full chromosome set of a cell or species, described by number, size and shape. To see it, technicians halt dividing cells with colchicine at the stage when chromosomes are most tightly packed, strip away proteins, and apply Giemsa stain. White blood cells are the usual choice for humans because they are easy to coax into dividing in culture. The resulting picture, called a karyogram, arranges chromosomes in pairs by size and by where the centromere, the pinched waist, sits.
The stain produces a striped pattern called G-banding. Lighter bands are richer in guanine-cytosine pairs, hold more genes and are more active; darker bands are more tightly packed. Those bands give every region an address, which is how cytogeneticists describe exactly where an abnormality lies. A normal human body cell has 46 chromosomes, 22 matched pairs plus two sex chromosomes, usually written 46,XX or 46,XY, while sperm and eggs carry 23 unpaired ones.
Karyotypes are remarkably variable across life, even though the underlying DNA machinery is nearly universal. Two legumes, Lotus tenuis and the broad bean Vicia faba, each have six chromosome pairs, yet the broad bean's are many times larger, and chromosome sizes can differ twentyfold between genera in one family. Mitochondria add another twist: a red blood cell holds none of their small genome, while an egg cell may carry up to 1,500,000 copies.
Some organisms rewrite their karyotype on purpose during development. Many sciarid flies discard whole chromosomes, and in the pig roundworm Ascaris suum every cell destined to become body tissue throws away chunks of its chromosomes and builds new ends. Biologists admit they still understand poorly why karyotypes evolve as they do.
Source: Karyotype
