336 BACTERIOPHAGES 



obtains as recombinants both T2 + + and T2rlr7. The double 

 mutant Tlrlrl particles are phenotypically like the T2r parents 

 but can be identified because on back crossing they give no wild 

 type recombinants with either parent. The proportions of the 

 two recombinant types from a two factor cross are equal when 

 averaged over the yields of many bacteria. However, in single 

 cell bursts the proportions of the complementary recombinants 

 may be far from equal. If the recombination frequency is low, 

 many of the bursts may contain only one of the two possible 

 recombinant types (Hershey and Rotman, 1949). This may 

 mean that the complementary recombinants are produced in- 

 dependently of each other in different recombination events; 

 or it may mean that each recombination event produces both 

 kinds of recombinants but that random multiplication and 

 random maturation may result in unequal numbers of the 

 complementary recombinants in the yield from a single cell. 

 The available data do not furnish a crucial test of this important 

 point but favor the concept of the independent production of 

 complementary recombinants. 



c. Multiple Recombinations 



In general, multiple factor crosses give results that might have 

 been predicted from the two factor crosses. Crosses of the type 

 T2rlr5 X T2r\r\3 yield no wild recombinants, whereas crosses 

 of the type T2rl X T2r4r7 do so. Multiple factor crosses in- 

 volving mixed infection with three distinct kinds of phage such 

 as r7rl3 X r4rl3 X r4r7 give a small proportion of wild type 

 recombinants. This result indicates that at least three different 

 phage particles can participate in reproduction and recombina- 

 tion within the same host cell. It also demonstrates that at 

 least two successive recombination events or matings can con- 

 tribute genes to a single progeny phage particle ; that is, vegeta- 

 tive phage may undergo several matings with different partners 

 before maturation. That triparental matings inxoKe successixc 

 events rather than a single event involving three partners is a 

 more reasonable assumption because of kinetic and steric con- 



