12-21 THE YEAST CELL 



ferentiate from cases in which the recessive gametes or the homo- 

 zygous recessive progeny were slightly less viable. Deficiencies 

 in the number of expected recessive progeny are assumed to result 

 from low viability of the recessive allele rather than non-Mendelian 

 segregations producing an excess of dominant gametes. 



The assumption that each diploid sperm mother cell of a hetero- 

 zygous Aa individual always produce 2 A and 2 a sperms is an infer- 

 ence based on statistical evidence. Although the direct evidence de- 

 rived from the tetrad analysis of hybrid yeasts proves that regular 

 Mendelian tetrads occur often, non-Mendelian segregations are not 

 infrequent occurrences; heterozygous A/a hybrids may produce 

 other than AAaa tetrads, in opposition to the induction derived 

 from statistical analysis of indirect evidence. 



In a discussion with Dr. Auerbach, she pointed out that the ab- 

 sence of detectable crossing -over in male Drosophila shows that 

 conversion does not occur in male Drosophila. If one recessive 

 gene, for example **b", in an ife^ft heterozygote were converted 

 into the dominant B, it would appear as a double cross-over. She 

 has surveyed the literature and found no evidence of crossing-over 

 in male Drosophila and, therefore, no conversion of alleles. How- 

 ever, this does not negate the positive evidence derived from tetrad 

 analysis of yeasts. 



