Karyotype

Karyotype, expressed as the aggregate of chromosomes, the chromosomal complex, a cytological (see Cytology) characteristic, the idiogram of a species (according to Delone, who introduced the term). A survey of known K. reveals great diversity. K. can differ in the number, size, and shape of their components - chromosomes. Differences in number are quite significant: from the smallest possible for diploid (see Cytology) number - two (Ascaris megalocephala, var. monovalens) - to several hundred (some lower crustaceans, ferns, some protists).- Chromosome forms also show considerable diversity; spherical (Fig. 3) and rod-shaped chromosomes are known, and in the latter, the relationship between length and thickness varies within very wide limits (Figs. 4, 7, 8 and 9). In longer chromosomes, constrictions are often found, dividing the chromosome body into a varying number of segments (arms) of different relative lengths (Figs. 1, 2, 10 and 11). Some chromosomes have a small appendage attached to one end by a thin thread ["satellites", "trabants" (Figs. 1 and 10)]. Finally, some chromosomes have typical constant bends, which are mostly located at the constrictions.- As for size, the differences are also very significant: if the smallest are at the limit of microscopic observation, not exceeding 0.3-0.4 μ in diameter, then the largest reach 20 μ or more in length, with a diameter of 2-3 μ.- The chromosomal complex can consist of both uniform and diverse elements; there are K. composed entirely of spherical or entirely of rod-shaped chromosomes, but often both are mixed in the most varied proportions and combinations, and usually all components have only the same diameter. In diploid complexes, all chromosomes except the sex chromosomes (autosomes) are necessarily represented by at least a pair (and sometimes a larger number; see below) of morphologically identical elements. Sex chromosomes, depending on sex and K., can be paired or unpaired. The unpaired nature can be expressed in different K. in various ways: inequality in size of partners or absence of one of them (Figs. 3,5,6); in some K., sex elements are represented even by a whole group (for a more detailed description of the sex chromosome apparatus and its functions - see Sex determination). The study of K. of closely related forms reveals extremely interesting relationships in individual branches of the systematic tree and sometimes allows a glimpse into the processes accompanying their evolution. Sometimes K. of neighboring species are morphologically completely identical, but in other cases it can be established that the formation of species or races was accompanied by either the fragmentation of a varying number of chromosomes (fragmentation) or their connection, gluing together (association); sometimes certain irregularities in the functions of the chromosomal apparatus lead to one or more chromosomes of the original complex being repeated in the derivative more than twice times (tri-, tetra-, penta- and generally polysomy). Finally, in other cases, undoubtedly also conditioned by irregularities in the mechanism of mitosis during maturation of sex cells, fertilization or cleavage, a haploid set is added to the basic diploid complex (the number of chromosomes increases by 1½ times - a triploid form) or a second diploid set [tetraploidy, each chromosome is repeated 4 times in the K., the total number in the complex is doubled (Figs. 8 and 9)]. Forms occur in which the entire basic complex is repeated an even greater number of times (penta-, sexa-, octo- and generally polyploidy). Combinations of all these processes can complicate and obscure the relationships between the numbers of components of chromosomal complexes of closely related forms, which in other cases are extremely simple, as for example in chrysanthemums (9, 18, 27, 36,45), sorrels (8,12,16, 36, 40) or wheats (14, 28,42), forming so-called polyploid series. The human karyotype began to attract the attention of researchers soon after the first descriptions of karyokinesis (see); however, the great difficulties associated with counting complexes consisting, like this one, of relatively high numbers of extremely small and closely located components, allowed it to be considered finally established only very recently (1929). As has invariably been the case in counting a number of other similar complexes, the first researchers distinguished only the largest chromosomes; until the 1920s, most authors counted 16 to 24 elements in the human diploid complex (Flemming observed up to 28 in humans as early as 1881). Only in 1912 was the diploid number 48 first indicated by Vinivarter, subsequently confirmed by Evans, Painter, Oguma and Kihara, again by Painter, Evans and Swezy. Painter assigned humans by sex chromosomes to the XX-XY type (see Sex determination), with all egg cells containing 23 autosomes + X, while half the spermatozoa carry the same set, and the other half instead of the X-element (medium size) contains a very small Y. Upon fertilization, spermatozoa of the first type give girls (diploid set 46 autosomes + 2 X), and spermatozoa of the second type - boys (46 autosomes + X + Y). Vinivarter, as well as Oguma and Kihara, denied the presence of Y-elements in men; however, this was confirmed by the recent work of Evans and Swezy (1929). The works mentioned here from recent years have examined complexes not only of whites, but also of colored races (Negroes, Japanese), and no differences in number and form of chromosomes were found between them.