138 CELL HEREDITY 



The method of interrupted mating allows easy mapping of that part 

 of the genetic material which usually gets into the F~ cells (Table 

 5.5). The order of genes in such maps corresponds to that found by the 

 more classical techniques of recombination analysis, which depend upon 

 the assumption that recombination occurs in a zygote with two complete 

 sets of genes. The procedure is as follows. Two mutant strains differing 

 in a number of genes are mixed together. The recombinants are rare and 

 must be selected for by a suitable choice of parental strains. For ex- 

 ample, one might be threonineless-leucineless {thr~ leu~) and the other, 

 methionineless-biotinless (met~ bio~). Such strains by themselves will 

 not form colonies on agar devoid of the required substances because 

 double back mutations of the thr~ leu~ or of the met~ bio~ in the same 

 cell are too rare to be ob.served. But when the two strains are mixed and 

 then plated on minimal agar, some prototrophs (met'^ hio'^ thr^ leu^) 

 with genes from each parent are recovered (Figure 5.16). These re- 

 sults do not provide linkage data because only one recombinant type is 

 selected, it is not known how many effective matings occurred, and no 

 parental frequencies are observed. 



However, it is possible to establish linkage relations by crossing strains 

 which are double auxotrophs and also carry other markers which are not 

 deliberately selected for or against. The unselected markers should be 

 found in the prototrophs with a frequency that is a function of their 

 linkage to the selected markers. If a gene is very closely linked to 

 a selected marker, it will be found among the offspring with a high fre- 

 quency. In a reciprocal cross it will, of course, be represented less 

 frequently. For example, in a cross of met~ bio~ thr^ leu^ lac^ T\-r x 

 met^ hio^ thr~ leu~ lac~ Tl-s, where Tl is a locus determining resistance 

 to phage Tl, the prototrophs can be selected for and the frequency with 

 which the lac and Tl markers appear among them can be determined. 

 Typical results are shown in Table 5.6. It will be seen that most of the off- 

 spring were lac~, showing a linkage to the met bio loci. Tl-r tvpes are also 

 in preponderance, indicating a linkage of that locus with thr leu, which in 

 this method must be considered a unit. The frequency of recombination 

 between the lac and Tl loci indicates free exchange between them. But 

 there is a marked deficiency of one class of recombinents, lac^ Tl-s, com- 

 pared to the reciprocal, lac~ Tl-r. This can be explained by assuming 

 that the lac and Tl loci are between those for met bio and thr leu as the 

 diagram in Table 5.6 shows. It can also be seen that the parental 

 coupling determines the frequency of a given gene among the progeny 

 because of the selection for one and against the other marker to which 

 the gene is linked. Nonetheless, the exchanges in a given region of the 

 linkage group are about the same in each cross, and a map can be con- 



