RECOMBINATION ANALYSIS IN MICROBIAL SYSTEMS 



55 



the effectively paired region, a new recombination phenomenon, re- 

 vealed by microbial systems, should be mentioned. This is tlic phe- 

 nomenon of non-reciprocal recombination, or gene conversion ( Mitch- 

 ell, 1955; Roman, 1956; Case and Giles, 1958 ) . Where all four products 

 of meiosis can be routinely recovered and identified, as in the as- 

 comycetes, abnormal segregations can be readily recognized. Figure 6 

 shows the type of rare, irregular, non-reciprocal segregation that has 

 been observed in quite a few instances in crosses involving markers in a 

 single gene locus. A reciprocal recombination event in the locus would 

 have produced one non-mutant recombinant and one doubly mutant 

 one. The segregation in Figure 6, A, could be explained by supposing 

 that the normal ai site of the second parental chromosome has been 

 copied twice, and that the extra copy of this region has been inserted in 

 the place of the mutant ai site in the new chromatid initiated from 

 tlie Parent 1 chromosome. This is imagined to occur as shown in figure 

 6, B. During replication, the two new chromatids are postulated to 

 grow at slightly unequal rates. The more advanced chromatid switches 

 to the Parent 2 chromosome, copies from it for a short distance, and 

 then returns to the Parent 1 chromosome. Following this, the more 



Parents 



Meiotic products 



<: 



X 



-h 



ai + 



o- 



O 



+ + 



31 + 



y 



-I- 



y 



-t- 



+ 



I I 



+ 32 



I I 



4- 





o 



o- 



+ 



+ 



-f 32 

 I I 



+ ^2 



-i — I— 



+ 



-4- 



-I- 

 -t- 



B 



+ 32 



a 



^ 



o= 



ai + 



+ + 



+ ^2 



Figure 6. Diagram of the results of a non-reciprocal recombination between two closely 

 linked markers, Oj and Cz- 



