Kinetics of Phenotypic Expression 201 



some to recipient cells with high frequency, to form partial 

 zygotes. The transferred fraction of donor chromosome 

 carries on it, in the order of their arrangement, genes deter- 

 mining the synthesis of the amino acids threonine and 

 leucine (T, L), resistance to valine (Val), resistance to sodium 

 azide (Az), resistance to the virulent bacteriophage Tl and 

 the ability to ferment lactose (Lac); the recipient cell differs 

 from the donor in all these characters. The zygotes formed 

 when cultures of the donor and recipient strains are mixed 

 together in broth can be represented thus : 



Donor T L Val Az Tl Lac 



Recipient + + r r r + 



When such zygotes are plated on synthetic minimal agar 

 devoid of threonine and leucine, only those recombinants can 

 grow which have inherited from the donor chromosomal con- 

 tribution the two linked genes which control synthesis of 

 these amino acids; i.e. selection is made for the gene T+L-[- 

 from the donor parent. The other genes on the transferred 

 segment of donor chromosome are not selected and w^ill be 

 inherited among T4-L+ recombinants with a frequency 

 proportional to their distance from the selective markers 

 T-I-L4-. In other words, the closer an unselected donor gene 

 is situated to the selected genes T+L-j-, whose inheritance 

 is obligatory, the less the probability that it will be separated 

 from these genes by the occurrence of a cross-over between 

 them and the greater the probability that it will be included 

 in recombinants. 



The donor genes controlling resistance to valine, sodium 

 azide and phage Tl are all closely linked to the selective genes 

 T-f-L+ and are inherited by about 100 per cent, 90 per cent 

 and 75 per cent of T+L-|- recombinants, respectively. For 

 this reason, and because they are concerned with very dif- 

 ferent aspects of cellular function, these markers are well 



