346 BACTERIOPHAGES 



and rl the proportion of double heterozygotes is also 3 per cent of 

 the total heterozygotes but for the closely linked markers h and 

 rl3 the proportion of double heterozygotes is much greater, 59 

 per cent of the total number of heterozygotes for the two loci. 

 This information about the frequencies of single and double 

 heterozygotes was derived from observations of the segregation 

 patterns of heterozygous phage particles by sampling the phage 

 progeny in mottled plaques. For instance in the linked cross 

 hrl by h'^r'^, 44 per cent of the mottled plaques gave only hr'^ 

 and hr particles, 50 per cent gave only A+r and /z+r+ particles 

 and 6 per cent gave hr and h^r^ particles plus some recombi- 

 nants. None of 150 mottled plaques that were sampled gave 

 predominantly the two recombinant types Ar+ and h'^r. This 

 means that 6 per cent of the particles that were heterozygous 

 for the r locus were also heterozygous for the h locus, and all of 

 these segregated to give the parental combinations rather than 

 recombinants. The proportion of the different genotypes 

 among the heterozygous particles was controlled by the ratio of 

 the parental particles as expected. For instance, in the unlinked 

 cross with five hr\ particles to one A+r+ particle, about 80 per 

 cent of the heterozygotes segregated to give the majority parent 

 and recombinant, hr and Ar"^, and 20 per cent gave the minority 

 parent and recombinant, A+r and A+r+. 



Hershey and Chase (1951) suggested two reasonable genetic 

 structures for the heterozygous particles. One proposed struc- 

 ture is essentially haploid with a small piece of added genetic 

 material derived from the second parent to form a diploid region. 

 The second structure is diploid throughout and contains one 

 parental set and one recombinant set of genes. In each struc- 

 ture the heterozygous region must be limited in extent, being 

 larger than 2 map units, but shorter than 20 units, as judged by 

 the incidence of doubly heterozygous particles in various crosses. 

 The possibility that the formation of heterozygotes is related to the 

 formation of recombinants was pointed out by Hershey and Chase. 

 The singly heterozygous particles as a group give rise in their prog- 

 eny to half recombinants and half parental types regardless of the 



