6 SEX IN MICROORGANISMS 



The procedure of making a cross between two different strains 

 consists in infecting a certain number of growing bacteria with two 

 kinds of virus. These bacteria will lyse, yielding new phage. We 

 refer to the infecting phage as the parent and to the phage which is 

 liberated upon lysis as the progeny. Different ratios of parental phage 

 types can be used to infect the bacteria, but the average procedure 

 is to use equal multiplicity, let us say 5 particles of one type and 5 

 particles of the other per single bacterium. 



A suspension of bacteria in buffered saline, containing 10^ cells 

 per milliliter, is mixed with an equal volume of a suspension of phage 

 containing 10^ of each parental type. Under these conditions, adsorp- 

 tion of the phage to the bacteria occurs without starting the growth 

 cycle of the phage in the bacterial cell. After adsorption is completed, 

 growth of the phage is started by adding nutrient broth. At the end 

 of the latent period the bacteria will burst, yielding phage progeny. 

 This progeny is plated and the different types of plaques scored. The 

 ratios between the different types will give the result of the cross. 

 As the mating occurs not between two particles but between groups 

 of particles, a multiparental cross is possible if the bacterial cell is 

 infected with more than two parental types. Hershey for the first 

 time obtained results of triparental crosses, showing that some prog- 

 eny particles contain markers derived from all three parental types. 



HETEROZYGOTES 



The phage particles recovered in the yield of a cross are con- 

 sidered haploids. This statement is made for the sake of simplicity, 

 because Hershey and Chase (1951) have shown that 2 per cent of the 

 phage particles originating from a cross are heterozygous for any 

 character for which the two parents are different. Except for this 2 

 per cent, all the other particles give, on a subsequent infection, a pure 

 yield of either one parental type or the other. 



At first sight one is tempted to interpret this finding by assuming 

 that a certain random fraction of the particles are diploids. In a two- 

 factor cross this leads to the prediction that heterozygosis with respect 

 to one of the two factors should be strongly correlated with heter- 

 ozygosis with respect to the other factor. In contradiction to this, 

 Hershey and Chase found zero correlation when the two factors are 

 unlinked. Only when the factors are closely linked does some correla- 



