Changes in Chromosome Number

A numerical chromosomal aberration, called ploidy, is any change in the number of chromosomes present in a cell, and it comes in two fundamentally different forms. Aneuploidy is a change confined to individual chromosomes within an otherwise normal diploid set, produced by adding or losing one or a few single chromosomes; organisms showing it are called aneuploids or heteroploids, and it is split into hyperploidy, an addition of chromosomes, further divided into simple trisomy (2n+1, a single extra chromosome, first reported by Blackeslee in 1910 in the jimson weed Datura stramonium), double trisomy (2n+1+1, two different chromosomes each present in an extra copy), tetrasomy (2n+2, an extra full pair of one chromosome type, seen in every possible combination in wheat), and pentasomy (2n+3); and hypoploidy, a loss of chromosomes, divided into monosomy (2n-1, one chromosome missing), double monosomy (2n-1-1, documented in maize), and nullisomy (2n-2, an entire homologous pair missing, usually produced by self-fertilising a monosomic plant, and generally lethal). Euploidy, by contrast, is a change involving one or more complete basic chromosome sets rather than individual chromosomes, and includes monoploidy (a single basic set, denoted x), the familiar diploidy (2n, two sets), and polyploidy (more than two sets, denoted 2n+n+n... and named by the number of sets present -- triploid 3x, tetraploid 4x, pentaploid 5x, hexaploid 6x, and so on), which is common in plants (many ornamentals and crop species are polyploid) but comparatively rare in animals. Polyploidy is further divided by the origin of the extra sets: autopolyploidy, where every set traces back to the same species (autotriploids, such as the seedless cultivated banana, arise from crossing an autotetraploid with a diploid and are highly sterile because their odd chromosome number cannot pair evenly at meiosis; autotetraploids, such as rye, grapes, alfalfa, groundnut, potato and coffee, are typically produced by artificially doubling a diploid's chromosome number), and allopolyploidy, where the sets come from two or more different, though usually closely related, species and fertility is restored after the initial sterile hybrid by doubling its chromosome number with the alkaloid colchicine (extracted from Colchicum autumnale, and remarkably harmless to that source plant itself, which carries a natural anticolchicine). The classic example is Raphanobrassica, produced by the Russian geneticist G.D. Karpechenko in 1927 by crossing radish (Raphanus sativus, 2n=18) with cabbage (Brassica oleracea, 2n=18): the sterile F1 hybrid became fertile once its chromosome number was doubled to 2n=4x=36, though the resulting plant disappointed Karpechenko by combining the leaves of radish with the root of cabbage rather than the other way around. Triticale, the first successful man-made cereal, is produced by crossing wheat with rye and then doubling the sterile hybrid's chromosomes with colchicine; depending on which wheat is used, the result can be tetraploid (from diploid wheat and rye), hexaploid (from tetraploid durum wheat and rye, combining wheat's high protein content with rye's high lysine content, which wheat lacks), or …