RECOMBINATION IN VIRUSES AND BACTERIA 



119 



clonal reproduction. Therefore, unlinked genes and those 50 or more 

 units from one another may give recombinations between them in a fre- 

 quency of less than 50 per cent; some of the old combinations come from 

 unmated phages and are counted along with those from matings, al- 

 though, of course, they should not be. The more phages that are formed 

 in a single cell, the closer the frequency of recombinations between dis- 

 tant genes approaches 50 per cent, because as reproduction continues the 

 chance increases that every phage will have had a mating in its ancestry. 

 The figure of 50 per cent recombination for freely recombining genes can 

 be obtained at will if we take advantage of a three-factor cross. By 

 considering only the progeny that show recombination between two 

 genes, we select viruses whose ancestors have mated. Among such 

 progeny, the third gene shows 50 per cent recombination with any other 

 with which it freely recombines. 



When we make allowance for the fact that a phage cross constitutes 

 a population experiment, many surprising results can be understood. 

 For example, it then becomes clear how a single offspring can have three 



TABLE 5.2 



A Linkage of Genes in a Three-Factor Cross of T4 Bacteriophage in E. co/i 



(From Doermann, 1953, C.S.H. Symp. Quant. Biol., 18:3) 



Region of Genotype Total Phages 



Category Exchange of Progeny of each Type Per Cent 



m = minute plaque mutant 



r = sharp margin plaque mutant 



tu = turbid plaque mutant 



