WHAT IS A GENE? 157 



y orfie ads 



I I I 



0.048 0.14 



Heterokaryon Heterozygote 



wild wild 



wild wild 



wild wild 



wild wild 



mutant mutant 



+ ods 



FIGURE 6.2. Map of two mutants at the ad, locus of Aspergillus nidulans and the 

 effect of cis and trans arrangements in heterokaryons and heterozygotes. 



ing over and these are recognized as bi2, biS and bil, in that order. 

 When any one of these units is mutant, the functional gene of which 

 it is a part cannot operate normally. Hence the frarw-heterokaryon and 

 ^mrw-heterozygote fail. In the cw-configuration there is on one chromo- 

 some an array of normal units, and this will function in the synthesis of 

 the pelargonic acid. In other words, the functional unit is large and 

 contains several mutable and recombinable subunits. 



The alternative hypothesis contends that each of the bi genes has a 

 separate but related function. The production of some labile or non- 

 diffusible substance, or of some substance in millimicromolar amounts, 

 might require that the genes need be located next to one another on the 

 same chromosome in order that the sequence of reactions be carried out. 

 For example: 



Pimelic acid >■ X >■ Y >■ 7,8-Diamino pelargonic acid 



612 bi^ bil 



Hence, all ^rans-combinations would fail, including the heterokaryon 

 test, because the products of one bi gene may not be transported to the 



