CHROMSOME DUPLICATION AND GENETIC RECOMBINATION 197 



this locus require pantothenic acid for growth, and are found in a small 

 region of linkage Group VI, between the genes ylo and tryp-2. Some 

 40 mutant alleles were mapped in mutant x mutant crosses, on the basis 

 of the frequency of wild-type progeny (assumed to be recombinants) as 

 well as recombination with outside markers. The linearity of the result- 

 ing map provides strong evidence that these offspring did arise by recom- 

 bination. Had they resulted from some other process, their frequency 

 should not have been a function of distance along a linkage map. 



In Neurospora and in yeast, most of the data on recombination be- 

 tween alleles come from enumeration of random ascospores. For ex- 

 ample, the results listed in Table 7.3 are based upon plating hundreds of 

 thousands of ascospores onto minimal medium, and counting the number 

 of colonies which form. In a few systems, fortunately, tetrad analysis 

 was also undertaken, and unexpected results of great importance were 

 obtained. For example, over 1700 asci were dissected in crosses of 

 alleles at the pan-2 locus. Nineteen of these asci showed evidence of 

 recombination within pan-2. In only two of these asci was the recombina- 

 tion event within the locus reciprocal, giving rise to the expected four 

 types of progeny. These two asci provide evidence of reciprocal exchange 

 within a locus comparable to that observed between loci in classical 

 genetic analysis. 



Aberrant Tetrads 



The remaining 17 exceptional asci show nonreciprocal recombination 

 within the pan-2 region, and fall into three classes, as shown in Figure 



TABLE 7.3 



Wild-Type Recombinants Recovered in 



Mutant A. Mutant Crosses in Neurospora 



^ Total Number of 



Cross % Wild 1 ype „ r^i j 



^ Progeny Observed 



