General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
Catalogue details

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

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

    324/384 (page 306)
    306 General cytogenetics §6.4 approach is still possible. Assumptions are: ( 1) Independent for¬ mation of the first chiasma in the segment considered (compare section 2.8.1), i.e. there is no effect of interference on this chias¬ ma, and (2) Poisson distribution of chiasmata disturbed only by interference. Since Poisson distributions are additive, chiasma lo¬ calization is not in conflict with this assumption as long as it can be assumed that for each point of the chromosome the formation of a chiasma is independent of that in other points, except for interference. Then the segments without chiasmata (classifiable as such) amount to in which м is the expected crossing-over frequency in absence of interference. The class with one or more chiasmata equals m =(1—e^'^). This is twice the recombination fre¬ quency in Haldane's mapping function: y =|(1—e^-*), that, similar¬ ly, does not take interference into account. In order to include interference, this function may be replaced by one of the more modern mapping functions, for instance that of Kosambi {y=^th 2x), which is rather simple to apply. Then the frequency of segments with at least one chiasma, m, equals th 2x. The corre¬ sponding genetic map length л' can be read from tables of th. Application, of course, is possible only wlien the segments involved can be recognized individually or when the frequency of being associated with its homologue can be determined by other means. This is possible in translocation heterozygotes (section 4.5.3 and Sybenga, 1970), in double telocentrics where one chromosome is replaced by the corresponding to telocentrics (section 6.4.1), and even in telocentric trisomies (section 5.2.2). Using telocentric tri¬ somies, the length of the short (nucleolar) arm of the satellite chromosome of rye was found to be 76 map units, the long arm 123 units. In another genotype, using translocations, the length of the short arm was 91 units, that of the long arm 124 units.