BACTERIOPHAGE GENETICS 337 



siderations. The fact of multiple matings affects the calculation 

 of recoinbination frecjuencies because a mating of complemen- 

 tary recombinants would produce parental types again (Hershey 

 and Rotman, 1948). Additional data on triparental matings 

 are given by Hershey and Chase (1951). 



d. Linkage and Negative Interference 



The proportion of recombinants to total progeny in matings 

 involving a particular pair of gene loci in T2 is essentially con- 

 stant from experiment to experiment and is not affected by the 

 presence of other mutant loci elsewhere in the parental phage 

 particles. The proportion of recombinants is the same whether 

 the cross involves the parents h X rl or hr X h'^r'^. The propor- 

 tion of recombinants to total progeny varies greatly from one 

 pair of gene loci to another, ranging from less than one per cent 

 to as high as 40 per cent. On the basis of the various recom- 

 bination frequencies observed, Hershey and Rotman (1949) 

 proposed a linkage system involving three linkage groups, and 

 suggested that the gene loci were probably arranged in linear 

 order in each linkage group. 



In studies of recombination in phage T4 Doermann and Hill 

 (1953) found evidence for three linkage groups that were ho- 

 mologous to the three linkage groups of Hershey and Rotman 

 (1949) as demonstrated by crosses between T2 and T4. The 

 recombination values between different pairs of loci were strongly 

 indicative of a linear arrangement of the loci in two of the linkage 

 groups. Not enough loci were available for testing linearity in 

 the third linkage group. 



Hershey and Rotman (1948, 1949) had found that 8 of the 

 15 T2r mutants they isolated formed a single closely linked 

 "cluster," giving a low frequency of recombination when tested 

 pairwise. Doermann and Hill (1953) isolated 26 independent 

 tu mutants in phage T4 and found these to fall into five distinct 

 clusters. The frequency of recombination between loci within a 

 single cluster was of the order of 1 per cent. They also found an 

 r cluster in linkage group H that was homologous with the r 



