RECOMBINATION IN SEXUAL ORGANISMS 



77 



number of crossover events, the frequency of recombinants produced is 

 0.5, provided that the exchanges occur between the different chromatids 

 at random. 



The deviation from the expectation of 0.28 new combinations men- 

 tioned above is due in part to the fact that the three- and four-strand 

 double-crossover events were recognized because they gave rise to re- 

 combinant spores, while the two-strand doubles which give rise only to 

 parental combinations and to first-division segregations were not identi- 

 fied. The meioses in which they occurred would therefore be recorded 

 as noncrossover. If the two-strand doubles equal the four-strand 

 doubles, as the hypothesis of randomness predicts, and there were 3 of 

 the latter among the 54 asci analyzed, then we may assume that there 

 were 3 of the former. Thus 3 of the 24 asci showing first-division 

 segregation and only parental combinations of genes were in reality 

 caused by two-strand double-crossover events. To get the real number 

 of asci in which no crossover events occurred, we subtract 3 from 24 

 and obtain 21. Subtracting 21 from 54 yields the number of asci in 

 which at least one crossover event occurred. The frequency of meioses 

 with crossing over is thus raised to 0.61 and the frequency of new 

 combinations expected from all meioses to 0.31, which happens to be 

 exactly what was found. In one of the 54 asci there were 0.5 new com- 

 binations but second-division segregation of both allelic pairs. This is 

 expected from three-strand doubles if one crossover event occurs on each 

 side of the centromere. 



We can see in this sample of data from Neurospora an extraordinary 

 correspondence between the behavior of genes and chromosomes. In 



TABLE 3.2 



The Relation between the Number of Crossover Events and the Frequency with 



which DifFerent Types of Chromatids are Produced 



(After Belling, 1931, U. Cal. Publ. Bot., 16:311) 



Types of Chromatids 



