136 



CELL HEREDITY 



100 



I I 80 



c c 



< 5 



42 E 



a> o 



is <" 



^1 



o + 



60 



40 - 



0) 



+ 



£. ^ 



i:20- 



10 20 30 40 50 



Time of Separation in Minutes 



60 



FIGURE 5.15. The time course of transfer of genes from an Hh to an f strain as 

 determined by the method of interrupted mating. A mixture of H\t thr^ leu azi-s TT-s 

 /oc 1^ 90/^' s/r-s and F fhr leu oz/'-r 77-r /oc , 90/^ sfr-r bacteria is made, agitated 

 in a Waring Blendor and plated on minimal medium plus streptomycin so that only the 

 thr' leu sfr-r recombinants are recovered (after Wollman and Jacob, 1958, Ann. 

 Inst. Pasfeur, 95:641 I. 



in the clone formed from mated bacteria, manv of the cells remain hetero- 

 zygous for a number of genes. Even after single cell isolation the re- 

 sulting clone consists of a mixture of cells heterozygous for some genes, 

 and two kinds of haploid cells containing one or the other of the genes 

 of a heterozygous pair. For example, if a cell heterozygous for lac and 

 Iac~ is isolated, it produces some progeny still heterozygous and others 

 in which the lac~ has segregated from the lac^ . Among the segregants 

 all combinations of parental genes are found, indicating that the hetero- 

 zygote is not just a heterokaryon that carries the two different pa- 

 rental nuclei in their original form with a chance that the\ will separate 

 at division. 



The heterozygote lac' /lac^ has a /or"*" phenotype because that gene is 

 dominant. But sometimes, when the parents in the cross are lac~ and 

 /flc'*", the unreduced progeny are lac' and produce only lac~ offspring, 

 although they continue to segregate the two alleles of other markers for 

 which they are heterozygous. It can be shown in the following way that 

 such strains are actually homozygous for lac~ . From the strain in ques- 

 tion a lac^ mutant is selected. Since mutations occur independently, if 

 the strain were of the genotype lac~ /lac~, it should ha\e become lac~/ 

 lac^ by mutation, and as a consequence should now segregate both lac 



