Chapter 20 

 IRREGULAR RATIOS 



NON-MENDELIAN INHERITANCE OF ABILITY TO 

 FERMENT GALACTOSE 



In contrast to the regularity with which fermenter and nonfer- 

 menter alleles were segregated in the hybrids described in Chap- 

 ter 19, it is interesting to note the irregular segregations when the 

 nonfermenter allele is introduced from Saccharomyces bayanus. 

 S. bayanus is incapable of fermenting galactose, hybrids between 

 it and S. cerevisiae make it possible to study the inheritance of 

 galactose fermentation. S. bayanus produces large cylindrical cells 

 both in the haplophase and diplophase, providing an additional gen- 

 etical marker. The hybrid between the large cylindrical gametes 

 of S. bayanus and the round gametes of S. cerevisiae produced a 

 large cylindrical diplophase, proving that the bayanus -type cell is 

 dominant (fig. 20-1). One difficulty is that our culture of S. bay- 

 anus sporulated only rarely and only one ascospore of a very large 

 number that was isolated grew. The fact that many of the single 

 ascospore cultures of the hybrid produced viable 4-spored asci 

 considerably complicated the genetical analysis. It is notable as 

 an evidence of hybrid vigor that the original hybrid and the progeny 

 all sporulated very abundantly in spite of the poor sporulation of 

 the original S. bayanus. 



Table 20-1 is a pedigree showing the progenies of a hybrid be- 

 tween S. bayanus and S. cerevisiae. All of the haplophase cultures 

 from the hybrid fermented galactose. Half of the single ascospore 

 cultures had large cylindrical cells like S. bayanus and half resem- 

 bled haplophases of S. cerevisiae. It is apparent that the gene con- 

 trolling cell shape segregated in a regular Mendelian way and af- 

 fords the necessary proof that a hybrid between the two species had 

 been produced. However, none of the expected segregations of G 

 from g occurred. All the asci were GGGG when all were expected 

 to be GGgg. I first interpreted this phenomenon as a result of cy- 

 toplasmic transfer. Since that time accumulating evidence has re- 

 vealed a number of similar circumstances and a more satisfactory 

 interpretation will be brought forth in later chapters. It will be 

 sufficient at this time to establish the fact that irregular ratios oc- 

 cur in yeasts. As an example of a loss, the haplophase culture No. 

 9 (which fermented galactose) produced an illegitimate diploid which 

 on dissection yielded four spores all nonfermenters of galactose. 

 Three produced the typical long cylindrical cell shape confirming 

 their descent from their haploid parent. In the fourth the cell shape 



20-1 



