Cytological Maps and The Cytology of Crossing Over

cytological maps of these chromosomes, which were consistent with the … rapidly gave more and more detailed cytological maps of all the chromo …

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CYTOLOGICAL MAPS AND THE CYTOLOGY OF CROSSING OVER
Rearrangements of the chromosomal material were first detected in Drosophila by genetic methods. Deficiencies were reported by Bridges (1917) and by Mohr (1919), duplication by Bridges (1919), translocation by Bridges (1923), and inversion by Sturtevant (1926). These were all of spontaneous occurrence, and none of them were cytologically identifiable by the methods then available. The first successful cytological attempt to analyze the chromosomes from a genetic point of view, in terms of units less than a whole chromosome, was made by Belling. By 1924, Belling and Blakeslee reported a series of extra-chromosome types in Datura. In this paper they developed the idea that the meiotic pairing of chromosomes, even at a late stage (diakinesis), could be used to determine the homologies of separate arms. They described \”secondary trisomics,\” in which the extra chromosome was made up of two like arms of a normal chromosome, presumably having arisen by a somatic division that was normal except that the centromere had divided transversely to the long axis of the chromosome instead of parallel to that axis. These types gave evidence as to the phenotypic effects of the separate arms. Unfortunately, however, there were few mutant genes available in the plant, and the more critical earlier stages (pachytene) of meiosis were not favorable for study in Datura. A different attempt to get cytological information on the genetic composition of individual chromosomes was developed for maize by McClintock, Randolph, Longley, and others. Here it was possible to study the pairing of homologous chromosomes at pachytene, when they were longer and showed more recognizable detail than at the later stages

A large series of mutant genes was known, with linkage maps well understood, and a series of chromosome rearrangements was collected. This was the most hopeful material for a detailed correlation between cytologically visible structures and linkage maps– until the development of the salivary gland chromosome technique for Drosophila in 1933 (see p. 75). In Drosophila, the first evidence relating a cytologically visible structure to a linkage map was Anderson\’s proof (1925) that the centromere end of the X is the right end of the linkage map– as pointed out in Chapter 8. Muller\’s original report on the mutagenic effects of X rays (1927) stated that he had also recovered \”a high proportion of changes in the linear order of the genes.\” The presence of rearrangements was confirmed by cytological studies (Painter and Muller, 1929). They showed, especially by a study of long deletions, that the genetical map of the X, based on crossing-over frequencies, does not correspond with the intervals measured on the metaphase chromosomes– although the sequential order is mutually consistent. By 1931 they showed that a large section of the right (centromere) end of the X is \”inert\” (contains very few genes) and in 1930 produced a \”cytological map\” of the X. Dobzhansky (1929, 1930) studied X-ray-induced translocations involving the second and the third chromosomes with the small fourth. The positions of the break-points were determined cytologically, and also genetically, by determining the apparent locus of a fourth-chromosome gene (eyeless) on the maps of the longer chromosomes. There resulted cytological maps of these chromosomes, which were consistent with the sequences of loci on the genetical maps but showed that, as Muller and Painter found for the X, the intervals were not proportional. That is, there were relatively long sections with relatively little crossing over, and relatively short ones with much crossing over. In 1931 two papers that appeared independently demonstrated that recombinants arising from genetic crossing over are accompanied by exchange of cytologically visible markers. The first of these, by Creighton and McClintock, utilized a translocation and a \”knob\” (heterochromatic end) in maize; the second, by Stem, utilized an X of Drosophila with an arm of Y attached to its right end, and an X-IV translocation. In both cases, two marker genes between the cytologically identifiable regions were available, and it was demonstrated that recombination between the marker genes was regularly accompanied by recombination between the cytological markers. These papers gave the final cytological proof that

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