RECOMBINATION IN SEXUAL ORGANISMS 



75 



of alb, and the order alh^ alb" alb^ alb~ was found. Whenever a 

 second-division segregation is observed, we know that crossing over was 

 responsible for it. By counting the number of asci with second-division 

 segregations, we can approximate the frequency with which crossover 

 events take place between a gene and its centromere. 



It is observed that some meioses do not involve a crossover event in 

 a particular region under investigation whereas others do. For example, 

 in an actual experiment with Neurospora, 54 asci were analyzed from a 

 cross of alb~ mt'^ X alb^ mt~ . Of these 24 (or 44 per cent) showed 

 first-division segregation of both allelic pairs and only parental combina- 

 tions of them. We may suppose that no crossover event between either 

 locus and the centromere happened in these meioses. Therefore, 56 per 

 cent had such crossovers. Among these, if crossing over occurred be- 

 tween one or the other locus and the centromere, we would expect half 

 of the spores produced to contain parental combinations of genes, and 

 half to contain recombinations. The over-all frequency of new combina- 

 tions expected can be calculated as follows: 



Actually, 0.31 new combinations were found among the 432 spores 

 produced. The reason for the discrepancy between 0.28 and 0.31 is that 

 three asci in which recombinants were found did not have the usual 

 content of half parental and half new combinations; they had all new com- 

 binations in this arrangement: alb^ mt^, alb^ mt^, alb~ mt~, alb~ mt~ 

 These asci may be explained if we consider the possibility of more than 

 one crossover event between the centromere and a given locus. Figure 

 3.6 shows that four-strand double crossing over between either locus and 

 its centromere will give the above result, i.e., all new-combination 

 spores and only first-division segregation. We can see that, if the dif- 

 ferent kinds of double-crossover events occur at random in the proportion 

 1:2:1 as illustrated, the over-all frequency of new combinations is still 

 0.50 as it was with a single-crossover event. This is true for all numbers 

 of crossover events, as is shown in Table 3.2. No matter what the 



