BACTERIOPHAGE GENETICS 345 



The possibility that the mottled plaques might have originated 

 from mixed clumps of r and r+ phage particles, rather than from 

 single heterozygous particles, was excluded by the fact that the 

 kinetics of inactivation by heat, beta rays, ultraviolet light, and 

 antiserum were the same as those of normal T2 phage particles. 

 There was no evidence that the mottling phage particles gave 

 rise to mottling progeny, but rather the evidence suggested that 

 their progeny were segregants. However, it now seems possible 

 that a heterozygous particle may be used one or more times as a 

 pattern for the synthesis of segregant progeny before its career 

 is interrupted by maturation. 



Hershey and Chase (1951) examined five different r loci and 

 one h locus for evidence of heterozygosis and in each case about 

 2 per cent of the total phage progeny was heterozygous for the 

 marker examined. The proportion of heterozygotes remained 

 constant at about 2 per cent whether the average burst size was 

 10 (premature lysis), 250 (normal lysis), or 1,700 (lysis inhibi- 

 tion). This was confirmed by Levinthal and Visconti (1953) 

 with a different gene locus. This constant proportion of hetero- 

 zygotes indicates that heterozygous vegetative phage do not 

 produce heterozygous replic'^s. It is consistent with the assump- 

 tion that there is a constant probability per heterozygous mating 

 of producing a phage particle heterozygous for a particular 

 genetic region, and that this phage particle has a constant prob- 

 ability of remaining intact until it matures. 



When the parental phage particles differ at two genetic loci, 

 such as h and r, the proportions of different kinds of heterozygotes 

 depend on the linkage between the markers. If the markers are 

 unlinked, the proportion of double heterozygotes is about 3 per 

 cent of all particles heterozygous for either the h or r character. 

 The proportion of double heterozygotes expected on the basis 

 of chance is 1 per cent. The excess is due to the same cause as 

 low negative interference ; the frequencies of single and double 

 heterozygotes are calculated on the basis of the total phage 

 population whereas they should be calculated on the basis of 

 heterozygous matings only. For the distantly linked markers h 



