18 SEX IN MICROORGANISMS 



ance to phages. Such characters will be called unselected, since their 

 segregation is regulated by the internal mechanism of recombination 

 rather than the exigencies of the technique. 



The first unselected marker to be used in our experiments was 

 resistance to phage Tl (Tatum and Lederberg, 1947). According to 

 conventions, resistance and sensitivity are symbolized as V/ and Vi% 

 respectively. Vi' is a specially convenient marker, as it can be pro- 

 duced in any stock by the selection of spontaneous mutants with Tl. 

 Before they can be used in these experiments, such stocks must be 

 carefully purified and, as for any marker, the stability and repro- 

 ducible scoring of the mutation must be verified. A variety of crosses 

 was carried out in which one parent was V/, the other Vi^. In each 

 case we found a segregation for this marker, i.e., some of the proto- 

 trophs displayed the V/ trait, from one parent, and others F/ from 

 the other. In control crosses, V/ x F/' gave only Vx% and V/ x 

 V/ gave only Vi'. Such a segregation in the first filial generation, 

 the f-1, indicates a haplobiontic life cycle, similar to that of many uni- 

 cellular organisms. 



If an unselected marker were associated with one chromosome 

 independent of others carrying the selected, nutritional mutations, the 

 f-1 should show a mendehan ratio of 1:1. Different ratios were ob- 

 served for each of the markers tested, the first evidence of linkage. 

 The observed frequencies varied from one marker to another, and 

 with a given marker, from one parental combination to another. In a 

 given cross, however, the f-1 segregation ratios have been as repro- 

 ducible as in any genetic material, which is to say they are subject 

 mainly to sampling error. 



A simple test of the significance of f-1 ratios can be made by 

 reverse crosses, whereby a marker is introduced first in one, then in 

 the other parent. For example, BM— V Z x TL— F/' is compared 

 with BM— F/ X TL— V/. About 70 per cent of the prototrophs 

 from the first cross are F/'". In the second cross, about 30 per cent 

 are V/, i.e., this ratio is inverted. The same result has been obtained 

 in many reverse crosses involving different parental lines, and different 

 markers and combinations of markers. It sho^^'s that the f-1 ratios have 

 nothing to do with the physiological effects of the markers, but that 

 they are due entirely to the mechanics of segregation. It also shows 

 that dominance plays no role, and more generally that a genetic parti- 



