196 CELL HEREDITY 



Genetic Map of the Pan -2 Locus 

 96 Prototrophs 0008 09 08 10 06 



in total viable < )< h ** ** >— 



spores 



75 7 19 



CompI 23/9 10 25 3 20 71 



39 



/, 



NoncompI ^ "\ 13 12 21 18 49 32 22 27 34 30 



28 \ 



CompI 23 66 5 69 57 70 61"- 9 



l_j_l III ^11 I 



NoncompI 2 6 1 



FIGURE 7.7. Mutants at the pan-2 locus in Neurosporo (after Case and Giles, 1960, 

 Proc. Natl. Acad. So. Wash., 46:659). 



forms in every respect except one: the linearity of the genetic material. 

 With the application of fine structure mapping to Neurospora and other 

 microorganisms, instances were regularly found which were rather similar 

 to the results obtained with phage. Let us summarize these observa- 

 tions, aberrant from the view point of the chiasmatype theory of crossing 

 over (Chapter 4). 



Recombination within a Gene 



The appearance of wild-type progeny in crosses between two allelic 

 mutants was described in Chapter 6. Alleles behaving in this way were 

 initially called pseudoalleles, because it was not considered possible 

 for recombination to occur between true alleles, which were considered 

 indivisible. Pseudoalleles were first detected by E. B. Lewis in Droso- 

 phila, and pursued in this organism by him and by M. M. Green, in 

 intensive studies, which were very laborious because of the low fre- 

 quency of recombinants recovered. Most of the current studies are being 

 carried out with microbial systems in which selective techniques permit 

 the screening of many millions of offspring with ease. 



Wild-type progeny have been recovered from crosses between allelic 

 mutants at a number of different loci in Neurospora, yeast, and Asper- 

 gillus, as well as in phage. Do these exceptional progeny actually result 

 from recombination? The strongest evidence for recombination is pro- 

 vided by the rll region of phage (Chapter 6) in which mutants have been 

 mapped by recombination and also by deletion analysis virtually to the 

 nucleotide level. 



Similar results, but less detailed, have been obtained in Neurospora, 

 for example, at the pan-2 locus shown in Figure 7.7. All mutants at 



