222 ROBERT L. SINSHEIMER 



molecule is made, patterned in part after one parent and in part after 

 another, remains a moot question. In attempts to answer this question 

 several studies have been made of the association of a physical label with 

 a genotype in a mixed infection. 



If bacteria are mixedly infected with P 32 -labeled T4 and unlabeled T2 

 and the progeny put through a second cycle at low multiplicity to elimi- 

 nate phenotypic mixing, 149 * the distribution of P 32 among the T2 and T4 

 type phages can be measured by specifically adsorbing the phage to speci- 

 fically susceptible bacteria. About two-fifths of the transferred P 32 appears 

 in T2 type progeny while three-fifths is found in the T4 type. 104 



Similar experiments have been carried out using phage of the same type 

 (T2), differing only as to the host range locus, h. If bacteria are mixedly 

 infected with equal numbers of P 32 -labeled h and unlabeled h + phage, 

 again two-thirds of the transferred label is found in particles of the origi- 

 nally labeled (h) parental genotype; (h + and h may be separated by the 

 use of bacteria with specific adsorptive ability). However, this transferred 

 label does not appear to be indissolubly linked to the h locus, for if this 

 progeny is used in a second generation experiment, only 59% of the 

 phosphorus transferred from the first generation h progeny is contained 

 in the second generation progeny of the h genotype. 152 



Levinthal has reported 55 that if heavily P 32 -labeled T2 h + phages are crossed with 

 T2 h phage, at least 90% of the "stars" among the progeny phage are associated with 

 phage (after a second generation to eliminate phenotypic mixing) of the h + genotype. 

 This evidence supports his conclusion that the large DNA piece, or star-forming 

 particle, carries at least the h locus and may indeed carry all of the known genetic 

 loci of T2. In this case the P 32 which appears in the progeny of the originally unlabeled 

 genotype may come from the smaller, nongenetic DNA components. 



An alternative experiment to test the hypothesis that a single large piece of the 

 DNA of T2 carries all of the genetic loci was performed by Sato and Stent. 153 Heavily 

 P 32 -labeled phage bearing 4 markers were crossed with an excess of a nonradioactive 

 phage bearing, at each of the 4 loci, a distinctive allele. Under these conditions very 

 few progeny phage are formed which have all 4 of the markers associated with the 



149a T^e progeny of a mixed infection of T2 and T4 will contain both genotypes and 

 both phenotypes. A small fraction of the progeny will be genotypically T2 but 

 phenotypically T4 and vice versa. 150 ' 1S1 Presumably this is a consequence of in 

 elusion of a T2 genome in a T4 protein coat. This phenotypic mixing can be elimi- 

 nated by a second generation of growth under conditions of single infection, so 

 that all of the phage from a given cell will have both the phenotype and genotype 

 corresponding to the genotype of the infecting particle. 



150 M. Delbriick and W. T. Bailey, Jr., Cold Spring Harbor Symposia Quant. Biol. 11, 

 33 (1946). 



161 A. Novick and L. Szilard, Science 113, 34 (1951). 



152 A. D. Hershey, E. Burgi, J. D. Mandell, and N. E. Melechen, Carnegie Inst. Wash- 

 ington Yearbook 55, 297 (1956). 



153 G. Sato and G. S. Stent, in "The Chemical Basis of Heredity" (W. D. McElroy 

 and B. Glass, eds.), p. 724. Johns Hopkins, Baltimore, 1957. 



