154 CELL HEREDITY 



(Total units 0.18) 

 (wf 8.10.11,12.16.19.20 



su^-ad^o riboi anj ad^^ P 3\^| \^paoai (jotal units 0.14) 



i I I ! ' T ^ < r I II 



39 19 7 20 20 0.5 8 0.3 16 0.16 0.7 



Chromosome I 



, acr, aero,., , adi ad-t 



aa23 / W f/7i4 pu ni3 / acr.^ 



I ''^' i n ' ' ' ?'" ! " 



30 0.3 2.5 20 40 34 19 31 01 29 



Chromosome II 



FIGURE 6.1. Linkage maps of two of the chromosomes of Aspergillus nidulans Chro- 

 mosome I is drown on twice the scale of Chromosome II (after Pontecorvo, 1958, Trends 

 in Genetic Analysis, New York, Columbia University Pressj. 



The sequence mav not pass from one chromosome strand to another 

 because of special properties of substances B and C. 



In microorganisms, on the other hand, the intimate biochemical func- 

 tions of genes are sometimes better understood than in higher forms, and 

 it might be expected, therefore, that pseudoallelism could be studied 

 more conveniently by using them. Some cases of close linkage between 

 genes with similar functions or concerned with related steps in the same 

 synthesis have been found in microorganisms and analyzed with informa- 

 tive results. 



In the fungus, Aspergillus nidulans, intensively studied by Pontecorvo 

 and co-workers, chromosome II can mutate at two closely linked sites 

 called ad^ and ad^, either of which causes a requirement for adenine 



(Figure 6.1). In crosses through the heterozvgote — - — —, recombina- 



-I- flflg 



tion occurs in 0.1 per cent of the cases, one of the products being the 

 adenine-independent form, ad^^ad^'^, which can be found in low fre- 

 quency by selection on minimal medium. Although ad^ and ad^ are 

 indistinguishable by nutritional tests, both growing on adenosine, 

 adenine, hypoxanthine, or 4-amino-5-imidazole carboxamide, they com- 

 plement one another and allow the growth of a heterokaryon, which 



carries nuclei or both tvpes in a common cvtoplasm . These 



+ ad^ 



adenineless genes are functionally different, mutate independently, and 



recombine. They are as distinct as loosely linked nonallelic genes such 



