198 CELL HEREDITY 



Parents a+ x +6 

 (a and b are alleles at the same locus) 

 (A) Reciprocal recombination 



a + 

 a + 

 + b 

 + + 



FIGURE 7.8. Aberrant tetrad 

 types found in Neurospora. 



7.8, which also includes data from a study of the pdx region of Neuro- 

 spora. Class 1 includes asci with only one recombinant and both 

 parental types; class 2 consists of asci with only one parental and one 

 recombinant type; class 3 contains only parental types, segregating 3:1 

 instead of the expected 2 : 2. 



Two mechanisms have been proposed to account for these data. One 

 mechanism, called gene conversion, postulates some sort of directed 

 mutation in which one allele influences the configuration of its homolog. 

 This mechanism does not account for classes 1 and 2, in which new 

 phenotypes are found unlike either parent. Rather than directed muta- 

 tion, the other mechanism involves recombination by a copy-choice 

 method, similar to that proposed for phage recombination. At the level 

 of individual nucleotides, the distinction between directed mutation and 

 copy-choice recombination is probably meaningless. 



Negative Interference 



Another new phenomenon associated with intragenic recombination is 

 negative interference: the increased frequency of outside marker ex- 

 change in the vicinity of an intragenic recombination event over that 

 occurring in unselected material. In Neurospora there is almost random 



