GENETIC CONTROL 13 



within a linkage group, from the frequency of recombinations. A most 

 remarkable outcome of studies based on such reasoning is that all the 

 experimental data could be accounted for by a linear arrangement of the 

 genes. Cytological observations revealed changes in the bands of giant 

 chromosomes of salivary glands of diptera at corresponding positions 

 (Bridges and Bridges, 1939; Demerec, 1941; Lewis, 1945). 



In this outlook, crossing over between the members of a pair of homo- 

 logous chromosomes at meiosis involves breakage and reassociation of the 

 chromosome at the level of intergenic material; the genes are implicitly 

 considered as discrete units which are small compared to the distance 

 between them. 



Developments of the genetics of micro-organisms made it possible to go 

 one step further in the analysis of the structure of the genome. 



Moulds and yeasts form zygotes by fusion of haploid forms. In bacteria, 

 three processes have been discovered in which part of the genome of a bac- 

 terium can be introduced into another and give rise to recombinants. Thus 

 in 'conjugation', part of the genome of the 'male' Hfr bacterium enters the 

 F~ recipient cell (Lederberg and Tatum, 1946; Wollman et al., 1956). In 

 'transduction' (Zinder and Lederberg, 1952), a fragment of the bacterial 

 genome is carried over from one bacterium to another by a bacteriophage 

 produced in the donor bacterium. In 'transformation', a piece of DNA 

 extracted from a bacterium by the experimenter, is absorbed by a receptive 

 bacterium and thus a new piece of genetic material is introduced which may 

 eventually be integrated in the genome. In all these cases, the transfer is 

 unidirectional and only part of the genome is usually carried over from the 

 donor to the recipient bacterium. In bacteriophage, recombination of 

 genetic markers is observed when two related phages develop within the 

 same bacterium. (For a brief review, see Braun, 1953.) 



The genome of bacteria, to say nothing of bacteriophage, is much smaller 

 than that of higher organisms. Bacteria and bacteriophages do not possess 

 chromosomal structures comparable to those found in animals or plants, 

 and recombination in these micro-organisms probably occurs by a process 

 which is quite different from crossing over as observed in higher organisms 

 (Hayes, 1960). Nevertheless, application of the same mapping principle as 

 used for higher organisms again led to results compatible with a congruent 

 linear arrangement of the genes. The linear order of the genes was more- 

 over confirmed by the observation of a progressive linear transfer of the 

 genetic markers during bacterial conjugation (Jacob et al, 1956; Jacob and 

 Wollman, 1958; Wollman and Jacob, 1958). 



With moulds, yeast, bacteria and bacteriophages it was possible to in- 

 crease the resolving power of mapping procedures enormously. As many as 

 10^ individuals can be handled in one experiment. With well chosen selec- 

 tive media, extremely rare recombinants ca'n be recovered and isolated. It is 



