240 



Parents 



FIGURE 9.3. Nonchromosomal inheritance in yeast. In crosses of wild type with 

 neutral petites, all the progeny are wild type regardless of time of sporulation, but 

 the suppressive petites behave differently. If the zygotes are sporulated immediately, 

 all progeny are petite, but if zygotes are allowed to grow vegetatively for a number of 

 generations and then sporulated, all the progeny are wild type, as in the case of neutral 

 petites. Chromosomal genes segregate 2:2 in all crosses (after Ephrussi, Hottinguer, 

 and Roman, 1955, Proc. Nafl. Acad. Sci. Wash., 41:1065). 



either the pattern of inheritance or in the phenotype upon repeated 

 transfer, indicating that the hereditary determinant is stable, as in the 

 Chlamydomonas system. In the presence of the poky factor, the activi- 

 ties of the cytochrome enzymes are altered, and terminal oxidation is 

 decreased, accounting probably for the slow growth. The primary action 

 of poky is not known, but there is evidence that the structure as well as 

 the functional abilities of the mitochondria have been affected. 



Petite strains of yeast have been described which exhibit non- 

 chromosomal inheritance. These strains, which produce tiny colonies on 

 agar, have no cytochrome oxidase activity and ferment glucose in the 



