.1. D. HERSHEY AND MARTHA CHASE 



FIG. 1. Progeny of the cross h X rl plated on mixed indicator. The large clear plaque 

 is hr; small clear, h; large turbid, r; small turbid, wild-type. The eccentric clearings in the 

 r plaque result from secondary h mutations. 



virus (Luria, 1945). The h mutants are normal with respect to type of plaque. 

 By successive mutations, double (hr) mutants are readily obtained. The four 

 kinds of virus, wild, h, r, and hr, can be recognized by plating on agar plates seed- 

 ed with a mixture of sensitive (B) and indicator (B/2) strains of bacteria. Inde- 

 pendently of the r character, h virus lyses both B and B/2 to produce clear 

 plaques, and h^ virus lyses only B to produce turbid plaques. Independently of 

 the h character, r virus produces large plaques, and r"*" virus produces small ones. 



Genetic recombination is observed in a "cross" in which sensitive bacteria are 

 infected with a few particles of h and a few particles of r per cell. The viral proge- 

 ny coming from the mixedly infected bacteria contains both parental types, to- 

 gether with a certain proportion of the two recombinants (Fig. 1). Analogous re- 

 combinants are found when two different r mutants are crossed. For example, 

 the cross rl X r2 gives rise to the double mutant rlr2 and wild-type (Hershey and 

 Rotman, 1948). 



In both types of cross, the yield of recombinants is characteristic for the 



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