CHROMOSOME DUPLICATION AND GENETIC RECOMBINATION 201 



assortment between outside markers among progeny showing recombina- 

 tion within a gene, and the total amount of recombination of Hnked 

 markers is much higher in their exceptional progeny than in the general 

 population. Some representative data are given in Table 7.4. 



These results are clearly at variance with the massive evidence of posi- 

 tive interference observed between exchanges which are in the range of 

 1 to 10 map units apart. It may be recalled that when two loci are 

 linked, the frequency of double exchanges between them is generally 

 lower than that expected from the frequencies of single exchanges. The 

 occurrence of one exchange appears to decrease the probability of a 

 second exchange in the vicinity. This phenomenon is found in the yeasts 

 and fungi as well as in higher forms. Any general theory of recombina- 

 tion must account for both positive and negative interference. 



Aberrant Asci with 5:3 and 6:2 Ratios 



Another phenomenon which must be accounted for is the occurrence 

 of 5:3 and 6:2 segregation ratios of alleles. Although such ratios have 

 been observed in Neurospora, the most extensive data come from studies 

 by Olive with the related fungus, Sordaria. In this beautiful material, 

 mutations occur which alter the pigmentation of the ascospore wall and, 

 consequently, the segregation of alleles affecting spore color can be 

 scored by direct observation of the undissected asci. A photograph of 

 asci with 4:4 and 5:3 segregation ratios is shown in Figure 7.9. Some 

 data on frequencies of various ascus types are given in Table 7.5. 



M. Mitchell has performed a careful analysis of two unlinked loci in 

 Neurospora to look for aberrant ratios of this type. She found that one 

 locus showed 25 irregular segregations in 109 asci dissected, whereas the 



TABLE 7.5 

 Aberrant Tetrads in Sordaria 



(From Kitani, Olive, and El-Ani, unpublished") 



Aberrant Ratios Number of Asci 



6 + 



5+ 



7+ 



2g 39 



3g 50 90 with excess -|- 



Ig 1 



2+:6g 8 



3+:5g 12 



Total 110 



20 with excess g 



In this cross, 0.12 per cent of the asci were aberrant for g/+ segregation. 



