GENETICS OF BACTERIOPHAGE 



set of genes. In this case the residual heterozygotes are diploid, but are not 

 representative of the zygotes from which they come. 



(3) Residual heterozygotes may be representative zygotes that are doubly 

 heterozygous in structure, but which undergo segregations accompanied by 

 frequent losses to yield parental and recombijiant pairs. 



The third alternative can be excluded. In crosses involving the markers h 

 and rl3, one finds only two per cent of recombinants among the whole progeny, 

 and about 20 per cent of recombinants among the segregants of heterozygous 

 progeny. To explain this in terms of alternative (3), one would have to assume a 

 low frequency of intracellular zygote formation. This assumption is incompatible 

 with the high frequency of triparental recombination observed. 



The questions about structure of heterozygotes can be generalized in the 

 following way. We find that about two per cent of the progeny of crosses are 

 heterozygous for each marker, and that the particles heterozygous for one are 

 mostly not the particles heterozygous for the other, excepting close linkage. Since 

 the frequencies are not specific for individual mutants, they are presumably 

 independent of local structure, and every phage particle must carry doublings at 

 one or more unmarked loci if total map distances are large. The alternatives 

 (1) and (2) are to this extent applicable to all the progeny, and take the simple 

 form: are phage particles diploid or not? 



It is reasonable to assume that the formation of heterozygotes and the for- 

 mation of recombinants are related processes, but there is no evidence that 

 recombinants have their primary origin in structures resembling the residual 

 heterozygotes. Instead, recombinants and residual heterozygotes may be 

 alternative products of other structures about which we have no direct mfor- 

 mation. The residual heterozygotes have one characteristic that is suggestive in 

 this connection : they segregate to yield one recombinant per heterozygote. The 

 recombinants that are produced in crosses also have to be assumed to come from 

 structures yielding one recombinant, to explain the independent or nearly 

 independent distributions of sister recombinants among single cell yields of virus 

 (Hershey and Rotman, 1949). 



The frequency of double heterozygotes provides a measure of linkage that is 

 independent of the results of recombination tests. Both measures show that h is 

 linked to rl3 and that r2 is linked to r4. The new measure is insensitive for large 

 map distances since the crosses involving h and r7, and h and rl, which yield 

 respectively 20 and 40 per cent of recombinants, produce the same number of 

 double heterozygotes. 



Conclusion 



A preliminary analysis of heterozygous particles of the bacteriophage T2H 

 raises new questions about the mechanism of genetic recombination, and suggests 

 that new ideas are needed to explain this phenomenon. 



The work reported in this paper was aided by a grant from the Division of 

 Research Grants and Fellowships, U. S. Public Health Service. 



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