52 MOLECULES, VIRUSES, AND BACTERIA 



developed by Benzer (1955), Chase and Doermann (1958) have pre- 

 sented an extraordinary body of data demonstrating this phenomenon. 

 Crosses were performed with markers in various patterns along the 

 pairing chromosomes. Three of the simpler types of crosses are shown 

 in Figure 5. Because of the large number of alleles in the series, it is 

 possible to vary not only the patterns of the markers a, b, and c but 

 also the distances separating them. The frequency of formation of a 

 non-mutant chromosome is scored by selective plating in which only 

 non-mutant phage particles give progeny. One readily sees that the 

 formation of a wild type chromosome requires the same cross-over 

 event in crosses 1 and 2, However, in Cross 2, a second cross-over be- 

 tween locus b and locus c would place mutant c in the lower chromo- 

 some, and thus both chromosomes would be mutant. Consequently, 

 the frequency of formation of non-mutant particles will be the same in 

 the two crosses only if the occurrence of two cross-over events in this 

 small region is rare. Experiments show that Cross 2, performed with a 

 variety of different markers, almost invariably gives fewer wild-type 

 progeny. Thus the probability of a second cross-over in the region 

 between b and c is very high indeed. In Cross 3, two cross-overs are 

 required in order to produce a non-mutant chromosome. The predicted 

 frequency of both events occurring in a single pair of mating chromo- 

 somes can be calculated from the observed frequency of the single 

 events, measured in crosses of type 1. The ratio between the observed 

 and predicted values, known as the coincidence, should be one if the 

 two events are really independent, greater than one if the occurrence of 



•"a 



^b 



•"a 



•"a 



f Figure 5. Types of crosses in 



T2 performed by Chase and 

 Doermann (1958). The vari- 



f.^ ous mutational sites are in a 



single gene locus. The dotted 

 — lines indicate the positions of 



cross-overs which will pro- 

 duce a wild-type recombinant 

 chromosome. 



