GENETIC CONTROL 15 



the size of a muton, how close together two different mutons can be located, 

 and finally to translate this genetic concept into chemical terms. This is 

 already partly accomplished, as we shall see later. 



The recoil is the smallest element which can be interchanged, as a unit, by 

 genetic recombination. It is of interest for the analysis of recombination 

 processes and it is related to the limit of divisibility of the genetic material. 

 It has, so far, little bearing on studies of protein synthesis. 



The cistron, which will be considered more closely in the following pages, 

 is the unit of function. It can be defined as a piece of genetic material which 

 must be present as a unit to accomplish its function. This function is the 

 control of a certain observable character or phenotype. The character con- 

 sidered may be e.g. a morphological feature, or the capacity of growing on 

 a defined medium, or the production of a pigment, etc. The observable 

 character may depend on the genetic material in a simple way or in a very 

 indirect way; it is clear therefore that the meaning of the 'genetic unit of 

 function' will depend on the character which is being considered. The 

 character we are interested in is the synthesis of a specific protein. The 

 cistron, in this particular case, is a piece of genetic material which specific- 

 ally controls the synthesis of this particular protein, and which ceases to 

 fulfil this function if it is fragmented. The size of this unit has deep implica- 

 tions for the mechanism of protein synthesis. Let us examine how the 

 cistron can be studied experimentally. 



In the process of recombination, whatever the underlying mechanism, a 

 unique genetic structure is reconstituted or copied from two pieces of 

 genome which come each from one of the parents. 



But it is also possible to confront within the same cell two genomes or 

 parts of genomes coming from two different cells, under such conditions 

 that they do not recombine or that recombination is negligible. This occurs 

 for instance in the formation of a zygote by fusion of two gametes. In 

 moulds, two haploid hyphae of the same mating type can fuse and form a 

 heterocaryon, in which each cell contains two types of nuclei, one type 

 coming from each organism (Beadle and Conradt, 1944). When a piece of 

 an Hfr genome enters a F" bacterium, a system resembling a zygote is 

 formed. Again when two different bacteriophages enter the same bacterium, 

 they multiply mostly independently, this is somewhat comparable to a 

 heterocaryon. Also in abortive transduction, part of the genome is intro- 

 duced into a bacterium and coexists with the bacterial genome without 

 recombining (Demerec and Ozeki, 1959). 



The study of such heterozygotes or heterocaryons makes it possible to 

 find out whether two different mutations concern the same functionally 

 indivisible unit of genetic material or whether they affect two units which 

 can operate separately. 



Let us consider two mutation sites A and B on the same linkage group 



