GENE CONVERSION 26-38 



Tetrads M488 - M491 and M500 - M503 (Table 26-18) exemp- 

 lify a situation one would expect as a corollary to the cases of the 

 extra-fermenter tetrads discussed above. The haplophase parent 

 CIA in the crosses analyzed in Table 26-18 is a member of the 

 "converting pedigree" bearing the dominant G and ME alleles. The 

 occurrence of extra -fermenters in these crosses has been explain- 

 ed above as being due to a transfer of gene material from the G to 

 the g (or ME to the me) allele. However, 609 (a parent of M488 - 

 M503, Table 26-19) which is also a member of the "converting pedi- 

 gree", bore the recessive me allele and was mated to a ME clone 

 (3 190) derived from a regularly segregating (ME/me) pedigree. The 

 fact that tetrads M488 - M491 and M500 - M503 obtained from this 

 cross include more than the 2 expected me clones indicates that the 

 recessive allele in the "converting pedigree", i.e., in 609, can de- 

 grade the ME allele of its mate which descended from a pedigree 

 in which the ME/me alleles segregate regularly. At the same time 

 an excess of G phenotypes are produced in the same asci. 



The occurrence of 3 nonfermenter segregants in each tetrad is 

 explained on the following basis: During meiosis the me gene from 

 the "converting pedigree" acquires some essential gene component 

 from the homologous dominant allele of its mate (3190). This ac- 

 quisition is not sufficient to elevate the me allele to functional ac- 

 tivity, but the degradation of the ME gene is sxifflciently severe to 

 negate its normal function. The single fermenter segregant in each 

 tetrad was presumably not affected or, at least, was apparently not 

 degraded below the critical threshold. 



The data presented above eliminate the possibility that non- 

 Mendelian tetrads arise from mutations during the growth of origi- 

 nally Mendelian tetrads on glucose. They support the view that ir- 

 regular segregations of the abilities to ferment galactose and meli- 

 biose may originate in the heterozygous condition owii^ to gene 

 conversions. 



The view that non-Mendelian ratios arise from gene conversions 

 in the heterozygote rather than from chance mutations has been con- 

 firmed above only with respect to galactose and melibiose fermen- 

 tations. No attempt was made to verify irregular ratios of other 

 sugar markers (sucrose, alpha methyl glucoside and maltose) using 

 pour-plates of tetrads as checks. The widespread irregular segre- 

 gations of these markers suggests, however, that conversion of 

 alleles was operative in these cases also. 



In addition to irregularities in segregation of the sugar markers, 

 the irregularities in inheritance of adenine -synthesizing ability are 

 pronounced. Several tetrads comprised 4 white progeny out of a 

 pink X white cross but, in some, 2 clones are ad(W) and 2 are AD(W). 

 This fact further confirms the hybridity of the asci analyzed. 



Disturbances in mating reactions are also noteworthy. In test- 

 ing for mating type specificity, a heavy inoculum of each of the four 



