NONCHROMOSOMAL GENES 241 



presence of oxygen as if they were growing anaerobically. Detailed 

 studies of their respiratory enzymes have shown that those which are of 

 mitochondrial location in normal strains are absent in petite strains, 

 although morphological structures corresponding to mitochrondria can 

 be found by electron microscopy. 



When petite strains are crossed with normal yeast (Figure 9.3) the 

 petite phenotype does not segregate regularly among the progeny. With 

 strains called neutral petites, all the progeny are normal: the petite 

 factor is not transmitted through the cross. In other strains, called 

 suppressive petites, the petite factor may be transmitted to all progeny 

 or to none, depending upon the precise environmental conditions during 

 germination. 



The fact that the transmission of petite can be altered by the environ- 

 ment may eventually shed some light on the mechanism by which non- 

 chromosomal determinants are distributed at meiosis. At present, one 

 may only speculate that a common mechanism of transmission for non- 

 chromosomal determinants at meiosis may exist in all organisms, despite 

 the apparently special feature that uniparental transmission is linked 

 with mating type in some species and not in others. 



In summary, it is the absence of segregation in meiosis, the disap- 

 pearance of one of the two parental phenotypes, and the consequent non- 

 mapability of the hereditary factor which distinguish nonchromosomal 

 inheritance so sharply from that of determinants carried on chromosomes, 

 in the cited examples. The same criteria apply in most other examples of 

 nonchromosomal inheritance, of which there are some hundred well- 

 studied instances in the literature, most of them from plant material. 

 There are also some examples of erratic segregation patterns, as in the 

 iojap system in maize, to be discussed below. 



Additional criteria which have been employed to distinguish between 

 chromosomal and nonchromosomal genes include the occurrence of segre- 

 gation during vegetative growth, giving rise to sectors (not associated 

 with somatic recombination), chemical induction of mutation, and in- 

 fectivity in the sense of transfer of the determinant from cell to cell 

 in transient fusions not involving chromosome transfer. 



Operationally, this mode of inheritance can only be referred to as non- 

 chromosomal, although it is often spoken of as cytoplasmic. It should 

 be understood, of course, that many mechanisms can produce aberrant 

 segregation ratios, such as physiological effects of parental cytoplasm, 

 multifactorial determination of phenotype, and various sorts of chromo- 

 some aberration. These must all be excluded by suitable experiments 

 before the existence of a nonchromosomal determinant may be properly 

 invoked to account for the data. 



