.4. D. HERSHEY AND MARTHA CHASE 



(e) In the cross hrlS X wild-type (closely linked markers) the doubly hetero- 

 zygous class makes up about 59 per cent of the total number of heterozygotes. 

 This cross yields about two per cent of recombinants, but the pooled hetero- 

 zygotes segregate to yield about 20 per cent of recombinants. 



(f) If the crosses involving unlinked or distant factors are varied by introduc- 

 ing a 5-fold excess of one parent, the effect is to increase the frequency of the 

 single heterozygotes segregating to yield that parent. This effect is not visible if 

 the markers are closely linked. 



(6) The frequency of heterozygosis is independent of the yield of virus per bac- 

 terium when this is decreased by premature lysis with cyanide, or increased under 

 conditions of lysis-inhibition. 



Discussion 



Information about inheritance in bacteriophage T2H comes from the analysis 

 of mutations and from recombination tests. These two techniques agree in show- 

 ing that mutations occur in localized genes. Recombination tests reveal that the 

 genes are organized into linkage groups. For one of these groups, it appears that 

 the arrangement of genes is linear. Inheritance in bacteriophage is therefore 

 amenable to the same kind of genetic analysis that has served to elucidate nuclear 

 organization in other organisms. The limitations peculiar to viral genetics should 

 not be overlooked. It is not possible to recover the immediate products of re- 

 combination, unless the heterozygotes prove to be such; the mechanism of re- 

 combination is unknown ; and cytogenetic techniques are inapplicable. 



The analysis of heterozygotes raises new questions about the mechanism of 

 genetic recombination. The surprising result is that the great majority of the 

 heterozygotes recovered from a two-factor cross segregate as if they were 

 homozygous or hemizygous for one of the marked genes, unless these are very 

 closely linked. This means that the heterozygotes found, which should perhaps 

 be called residual heterozygotes, may not be representative of the heterozygotes 

 formed in the cell. 



One feature of the residual heterozygotes is reassuring in this respect. The 

 total frequency of heterozygosis is the same for five different r markers and one 

 h marker. This makes it unlikely that the formation of residual heterozygotes is 

 contingent on structural differences between different mutants. 



The questions raised by the peculiar segregation pattern of heterozygotes are 

 clarified somewhat in terms of the following alternatives. 



(1) Residual heterozygotes may not be diploid particles, but particles con- 

 taining one or more small extra pieces of genetic material. Double heterozygotes 

 for distant markers contain two or more pieces. These pieces are substituted for 

 the homologous pieces in one of the very early progeny of the segregating 

 heterozygote. The residual heterozygotes need not differ from intracellular 

 heterozygotes, and their production need not involve zygote formation. 



2) Residual heterozygotes may be formed preferentially from zygotes in 

 which recombination has occurred, and receive one parental and one recombinant 



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