348 BACTERIOPHAGES 



typically r. Since only r heterozygotes containing both r2 + 

 and r7+ need be considered, the possible segregation patterns 



are (1) r2+rl+/r2rl+ (r2 heterozygotes), (2) r2+r7+/r2+r7 {rl 

 heterozygotes), (3) rl'^rl / rlrl '^ (double heterozygotes), (4) ?2+- 

 rl'^/rlrl (double heterozygotes). Patterns (1) and (2) will lead 



The Cross 



_h TZ* ^7 _ 



produces heterozygotes by 



Model I Modem 



./J2_+_ .h /_2+_ 



r2 r?-^ (I) r2 



h Tz*_ r_7_ h r 2 '*^ r^7_ 



r?-^" (2) ' "r7 + 



Heterozygotes for the r markers segregate to give 



(2) 



Figure 1 1 . Predictions of two models for the structure of heterozygotes. 

 Unpublished drawing by M. Delbriick. 



to mottled plaques. Pattern (3) will produce primarily r 

 segregants and will produce mottling only infrequently. Pattern 

 (4) is a possible type of segregation but was never observed by 

 Hershey and Chase (1951) who reported that double hetero- 

 zygotes for linked characters always segregated to give the 

 parental genotypes. As may be seen by inspection of Figure 1 1 

 the rl heterozygotes will carry only the h allele in either model 

 I or model II, whereas the rl heterozygotes will carry the h^ 

 allele in model I and the h allele in model II. Because the fre- 



