.4. D. HERSHEY AND MARTHA CHASE 



Heterozygosis 



Mixed yields of T2H from bacteria infected with r and r+ phage always contain 

 about two per cent of particles that give rise to mottled plaques. Samples of 

 phage from these mottled plaques consist of approximately equal numbers of 

 typical r and r"*" particles, with only traces of mottling phage particles. This dis- 

 tinguishes motthng phage particles from phage particles containing an unstable 

 genetic factor affecting the r character (Hershey, 1946). These produce sectored 

 rather than mottled plaques, and the sectored plaques contain sectoring particles 

 and r particles, but no r+. 



The mottling phage particles do not consist of clumps. This is shown both by 

 the genetic data to be described presently, and more generally by the following 

 consideration of mactivation data. 



Suppose, for example, that a mottling phage particle really consists of a small 

 clump of j normal r+ and k normal r particles. If the population containing this 

 clump is heated sufficiently to reduce the titer by a factor 100, the chance that the 

 mottling property of the specified clump will survive is at most (1 — 0.99') (1 — 

 0.99*=), which is very much less than the chance (0.01) that a given phage particle 

 will survive. The proportion of mottled plaques originating from clumps in a small 

 surviving fraction of the population will, therefore, be very much less than the 

 proportion of mottling clumps in the original population. Mottling particles do 

 not behave in this way. In experiments in which populations containing mot- 

 tling phage were inactivated by heating, by antiserum, by ultraviolet light, or by 

 beta rays from P^^ no appreciable decrease in the proportion of mottling phage 

 among survivors was seen. (The populations were examined at ten per cent and 

 one per cent levels of survival.) The mottling particles are inactivated as a unit 

 by the agencies mentioned, and possess the same resistance to inactivation as the 

 non-motthng phage particles in the population. 



We conclude that mottling phage coming from bacteria infected with r and r+ 

 virus contains both parental markers in a particle of otherwise normal properties. 

 Since there is adequate reason to call these markers allelic genes, the mottling 

 particles are appropriately termed heterozygotes. We use this word without in- 

 tending to imply any specific structural basis for the observed properties of the 

 particles. 



There is no evidence that the heterozygotes can multiply in the heterozygous 

 condition. The small proportion (roughly two per cent) of mottling phage par- 

 ticles that is found in the mottled plaques originating from heterozygotes is also 

 found in mottled plaques coming from bacteria infected with mixtures of r and r+ 

 phage. The similarity of the two proportions suggests that the mottling particles 

 have been formed in both instances during the development of the plaque. 

 The proportion of r — r+ heterozygotes produced in bacteria infected with 

 mixtures of r and r+ virus does not vary significantly from two per cent for five 

 different r markers (Table 4, left haK), and is also about the same in crosses be- 

 tween h and r mutants, to which we now turn. 



Information about the structure of the heterozygotes can be sought by analyz- 

 ing them in terms of the segregants they yield. For this purpose, the viruses giv- 



184 



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