Fungal genetics / J.R.S. Fincham and P.R. Day.

  • Fincham, J. R. S.
Date:
[1963]
Catalogue details

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

Credit: Fungal genetics / J.R.S. Fincham and P.R. Day. Source: Wellcome Collection.

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    80 FUNGAL GENETICS THE ESTABLISHMENT OF GENE ORDER If linkage values could be relied upon to be constant from one cross to another, one could build up a map of a linkage group from a series of crosses each involving only two markers. In Neurospora, at any rate, there is enough genetically determined variability in cross-over frequency to make this an uncertain procedure, and a much sounder method is to build up a picture of the entire gene sequence by a series of 3- or 4-point crosses involving overlapping sets of markers. Whether one is analysing random spores or tetrads, the basic principle in multipoint crosses is that the most probable gene order is that which requires the postulation of the fewest cross-overs. This principle was im¬ plied in the example given in Chapter 1. In Table 11 the various possi¬ bilities in a 3-point cross are set out systematically. When the intervals between markers are short, the classes resulting from double exchanges should hardly occur at all and only when the recombination fraction shown by adjacent markers approaches 0-5 (i.e. no detectable linkage) will the fre¬ quency of double cross-over products approach that of the least frequent single cross-over products. In the latter situation the establishment of an unambiguous gene order will depend on the use of additional markers to shorten the inter-marker intervals. Among the tetrad types, classes 2, 3 and 4 (Table 11) usually provide most of the evidence for gene order; if the intervals are fairly short one of these classes should be very infre¬ quent relative to the other two. When a higher frequency of multiple crossing-over prevails, classes 9,10 and 11 may give a good indication of the most probable order, since an order requiring the postulation of a quadruple exchange is much less probable than one requiring only a double. It was noted above that random spores were generally more efficient than tetrads for the establishment of linkage. The same is generally true for the ordering of genes, though, where linkage is very loose, tetrads may give information not obtainable from random spores because of the greater number of comparisons which may be made between differ¬ ent classes. When one is dealing with auxotrophic markers, and particu¬ larly when the recombinant classes are rather infrequent, random spores have a very marked advantage. Thus, in a cross between two auxotrophic mutants, the prototrophic recombinant class can be estimated by spread¬ ing spores on minimal medium and counting the number capable of forming colonies. If the two loci concerned are unlinked, 25 per cent of the spores capable of forming colonies on complete medium should do so on minimal medium. Selective media can also be used with great sav-