The genetics of recombination / D.G. Catcheside.

  • David Guthrie Catcheside
Date:
[1977]
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: The genetics of recombination / D.G. Catcheside. Source: Wellcome Collection.

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    Introduction 3 A few species of fungi are the chief contributors. The other main source has been the study, especially in bacteria and their phages, of mutants which affect recombination. Behind both, in interpretation, was the realization that each chromatid is a double stranded structure, a molecule of DNA. Therefore purely mechanical break and rejoin, or even copy choice, theories were defective and had to give way to biochemical systems in which molecular events were catalysed by enzymes. In some species of fungi, such as Neurospora crassa and Sordaria fimicola, the products of each meiosis are retained within a linear ascus in such a way that, subject to accidents, products each from one of the DNA strands of the homologues are arranged in a linear order. Counting the spores as 1 to 8 from apex to base of the ascus, the plane of the first division of meiosis lies between 4 and 5, while the planes of the second division lie between 2 and 3 and between 6 and 7 (Figs 1.1 and 1.2). A В a b О 0 □ Fig. 1.2 Diagram to show relation of first and second division segregation of loci to a chiasma (or cross over). The centromeres and loci proximal to the chiasma show first division segregation, while distal loci show second division segregation. A and a are alleles at a proximal locus; В and b are alleles at a distal locus. Each pair of spores (1 and 2, 3 and 4, 5 and 6, 7 and 8), which are sisters by a post-meiotic mitosis, represents at each homologous point the information carried by the two chains of DNA of a given chromatid as it goes through the later stages of meiosis. Segregation for one factor difference (sayH- X m) normally shows six types of ascus (Fig. 1.3), two with first and four with second division segregation of the factor followed (Table 1.1), all asci showing a 4:4 ratio. The frequency of second division segregation is characteristic of the particular locus, ranging from zero for those adjacent to the centromere to about 67% for those remote from the centromere. Moreover, the two first division classes are equal in freq¬ uency and the four second division classes are equal.. These equalities hold unless there is some genetic cause which biases the orientation of bivalents at meiosis and so makes the direction of segregation non- random, as in Bombardia lunata (Catcheside, 1944) where the segregation of a factor {rubiginosa) affecting ascospore colour can be followed (Table l-l). When regular segregation at more than one locus is followed in ordered asci, precise information is obtainable with respect to linkage and possible
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