The genetics of recombination / D.G. Catcheside.

  • Catcheside, D. G.
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.

    48/192 (page 34)
    34 Recombination in eucaryotes locus 46 normally show no reciprocals (cross overs), but in crosses involving 277 about 11% of the 2+ ;6с asci result from crossing over. The polarity of conversion criterion places 277 to the left of the 137 site. The occurrence of the odd segregations (1+:7с) reflect in mutant X mutant crosses the 5+ :3с observed sometimes in + X с crosses. The odd segregations appear to be the property of particular mutant genes or of the stocks in which they are carried. The genes 277 and 1216 at locus 46, gene 60 at locus 19 and several at locus 75 have this property. It is notable that 277 does not produce \-\r'.lc asci in crosses to 137, although it does so with all other locus 46 alleles. The frequencies of conversion differ from one site to another in a locus. Often these are clearly polarized, decreasing from one end to the other. In some cases the differences in frequency of conversion are not in a systematic order (e.g. Rossignol 1969). Other differences found between different alleles concern the direction of conversion. In a cross + X c, four classes of exceptional asci are commonly observed in Ascobolus immer sus, namely, 6+ : 2c, 2+ :6c, 5-1- :3c and 3+ :5c. The preferred direction of conversion is defined by the ratio of the frequencies of 6:2 to 2:6 or of 5:3 to 3:5 segregations, the wild-types being given first in each ratio. Differences of these kinds, comprising: (1) frequency of conversion or recombination; (2) the time of conversion whether meiotic (6:2 and 2:6) or post meiotic (5:3 and 3:5) ; and (3) direction of conversion, whether mutant to wild or the reverse, have been ascribed to the nature of the genetic alterations at the mutant sites and are called 'marker effects'. The time of segregation and the direction of conversion define a 'conversion spectrum'. At the 164 locus, conversion is almost always meiotic (Baranowska, 1970). In crosses to wild-type, 6+ :2w and 2+ :&w frequencies are equal in the crosses 10 X -|-, 94 X + and 115x +. They are unequal in other cases, conversion to + being commoner in W173X-1-, 140x + and 70 X +, while conversion to w is commoner in 24 x +. These differences bear no relation to the position of the mutant site in the locus. They are usually assumed to be due to the mutant itself rather than to any extraneous differences, but there is no proof in general of such 'marker' effects. The contrast with the parity rule shown in yeast is striking. At locus 75, the common exceptional asci in crosses of mutants to wild are also 2-|- :6c and 6+ :2c. The ratio of these two classes (conversion to mutant divided by conversion to wild) constitutes a dissymetry coeflficient (DC). The data depend upon counts of groups of eight ascospores, each group discharged from an ascus. A sample of the 2 + : 6c and of the 6+ : 2c was tested in each case for effects not due to conversion. Most spurious 2 + : 6c asci were due to a tetratype segregation for a new (spontaneous) non-allelic mutant and the parent one. The few spurious 6 + : 2c asci were due to two of the pigmented ascospores being genetically mutant. Rossignol (1969) found five distinct classes at locus 75: a(DC = 11), ß{\), 7(0.7), (5(0.04) and e(2), so that the preferred directions of