CYTOPLASMIC INHERITANCE 27-2 



no part of which (not even the genes) can be formed except from 

 preexisting precursors originating from the cell nutrients. The 

 failure of any one of a number of cellular components to duplicate 

 itself would result in the failure of the cell and the genes in it to 

 multiply. Not too long ago geneticists found it difficult to under- 

 stand how the non-genic parts of the chromosome which connected 

 the chromomeres (supposedly genie macromolecules) could dupli- 

 cate themselves because only the genes, and not the intergenic con- 

 nections were supposed to have the capacity for self-duplication. 

 Obviously many parts of the cell must have this ability. The differ- 

 ence between the gene and other self-duplicating parts of the cell 

 is that the gene controls the path which self -duplication shall follow. 

 It confers specificity to the enzymes which control the metabolism 

 of the other self -duplicating entities which, like the gene, multiply 

 by accretion of the substances which accumulate in the cell. The 

 gene is also the basis for change, for by mutation it can alter the 

 specificity of the enzyme it produces and thus the direction of its 

 own self -duplication and, as a result, of the self -duplication of other 

 cellular components. All self -duplicating structures in Ihe cell are 

 dependent on the gene, for all control of cellular metabolism resides 

 in the gene. Although many structures besides genes are self -dupli- 

 cating, none except genes have significance in the alteration of cellu- 

 lar activities or in heredity. All self -duplicating structures pro- 

 vide essential skeletons for their own continued production, but they 

 depend on gene control for the forms which they assume; each per- 

 manent structure acts as the ''starter" on which its "crystallization* 

 occurs. On this basis, we may define self -duplication as a general 

 biological phenomenon and reserve the adjective ''autonomous"for 

 those cellular components which, like the genes and chloroplasts, 

 are capable of directing metabolism and originating changes in cell 

 metabolism by mutation. A rickettsial parasite like the kappa sub- 

 stance would also be considered autonomous in this terminology. 



HETEROSIS AND "RUNNING OUT" OF HYBRID VIGOR 



The degeneration or "running out" of hybrids showing heterosis 

 has been one of the principal problems of hybrid vigor (East, 1910). 

 The fact that some degenerated potato hybrids were found to be virus - 

 infected led to a general acceptance of the view that virus infection 

 provided a satisfactory explanation because genetic theory had as- 

 sumed that drastic alteration of a genotype does not occur during a- 

 sexual propagation. 



Haplophase yeast cultures (isolated in our laboratory in June, 

 1944) were mated to produce many hybrids. The matings are shown 

 in fig.-10-2. Duringthe ensuing summer individual tubes were se- 

 lected from the collection stored in the cold room and tested for their 



