254 CELL HEREDITY 



ctMl, it is possihlo that the regular segregation ()l)served is that of an 

 oriented nonchromosonial factor. This possihihty is raised for two 

 reasons. First, the induced character of the mutation suggests that it is 

 nonchromosonial, since the weight of evidence supports the view that 

 chromosomal gene mutability is a random process. Second, the exclusive 

 occurrence of first-division segregation locates the y j factor as either 

 verv close to a chromosomal centromere or else as nonchromosomal but 

 still replicating and segregating in a regular way. Only the occurrence 

 of linkage with some chromosomal gene can resolve this question, and 

 in tests thus far of seven chromosomes (out of eight to ten) no linkage has 

 been found. 



In summary, the nonchromosomal determination of some chloroplast 

 properties has been well documented in many species of plants, but in no 

 system has the identification of the determinant been established. Be- 

 cause of the organized complexity of chloroplast structure, it seems likely 

 that some aspects of chloroplast heredity may be determined by copying 

 on a supertemplate of some sort. If such a mechanism exists, it should 

 be localized within the chloroplast. Nonchromosomal genes or steady- 

 state systems influencing chloroplast development, on the contrary, 

 might be found elsewhere in the cell. 



MITOCHONDRIAL HEREDITY 



The basic issues of mitochondrial heredity are very similar to those 

 we have just been considering with respect to chloroplasts: (1) Where 

 are the determinants located which control mitochondrial formation, and 

 (2) do mitochondria replicate? Here too, as with chloroplasts, there is 

 good genetic evidence that both chromosomal and nonchromosomal de- 

 terminants contribute to mitochondrial development, but neither the 

 location nor chemical nature of the nonchromosomal determinants has 

 been established. 



The best genetic evidence about mitochondrial heredity comes from 

 studies of the poky mutants in Neurospora and the petite mutants in 

 yeast, which were described above. The systems have certain features 

 in common, among them the occurrence of altered cytochrome enzymes 

 in the mutant strains and the existence of chromosomal as well as non- 

 chromosomal genes with very similar phenotypic effects. A comparison 

 of the cytochrome components of a number of these strains is given in 

 Figure 9.6. 



In addition to the nonsegregation in crosses, a very important evidence 



