RECOMBINATION IN VIRUSES AND BACTERIA 137 



and lac^. This is found to be the case. But it is otherwise with the mal 

 locus. The heterozvj^otes from a mal^ X ma/~ cross are almost in- 

 variably either mal'^ or mar, and they produce only the same type of off- 

 spring. If a mutant to the mal~^ condition is selected from the hetero- 

 zygote that is mar, it produces only mal^ offspring, unlike the mutated 

 lac~ homozygote which after mutation segregated both types. The 

 heterozygote that was mar must, therefore, have had that gene repre- 

 sented only once; heterozygous for other genes, it was hemizygous at the 

 mar locus. Such strains could have become hemizygous after a com- 

 plete holozygote was formed, or, because the mal gene in the hemizygote 

 is usually that of the F~ parent, such a condition may never have existed 

 and the genetic contribution from the Hfr parent may have been only 

 partial, giving rise to a merozygote. 



Although genetic material may sometimes be lost from the zygote, 

 Wollman and Jacob, at the Pasteur Institute in France, showed that the 

 absence of particular genes among the recombinant progeny of a cross 

 results from incomplete transfer of genetic material. Their method con- 

 sisted of separating the members of pairs at different times during the 

 conjugation process by agitating them vigorously in a Waring Blendoi. 

 It was found that the number of genes transferred from the Hfr was di- 

 rectly proportional to the time elapsing before the cells were separated. 

 Remarkably, one gene was always transferred first, and the others 

 followed in order (Figure 5.15) as though the genetic material were ar- 

 ranged in linear sequence and passed lengthwise across the cytoplasmic 

 bridge. When the pairs are agitated, the bridge is broken, leaving in the 

 Hfr cell whatever genes have not yet been transferred. No matter how 

 long the cells are allowed to pair, some genes rarely enter the F~ cell. 

 Apparently spontaneous breakage usually prevents distant genes from 

 entering. 



TABLE 5.5 

 Genetic Map of the Chromosomal Segment Injected with High Frequency in 



One Strain of Hfr 



(From Jacob and Wollman, 1958, The Biological Replication of Macromolecules. 

 New York: Academic Press, p. 75) 



thr leu azi Tj lac^ gal 



A is the p)er cent of thr'^ leu^ str-r recombinants which have inherited the 



different H/r alleles. 



B is the time in minutes at which indixidua! Hfr characters start penetrating 

 the F~ recipient. 



