28-4 THE YEAST CELL 



the precise timing of steps in ontogeny, for the cell would respond 

 automatically through substrates which produced changes in gene" 

 product to gene equilibria, as they occurred. Orderly develop- 

 ment would be based on a series of automatically induced environ- 

 mental reactions. 



GENE-TO-GENE TRANSFER OF GENE COMPONENT 



A second fact which must be reconciled to classical genetics is 

 that tetrad analyses of yeasts have revealed non-mendelian ratios 

 which result from the transfer from one chromosome to another of 

 components of the hereditary apparatus. It is suggested that the 

 components are attached to loci along the chromosome by attractive 

 forces. A locus with a maximal attraction for its specific complex 

 is dominant and one with no attraction for it is ^recessive. Inter- 

 mediate reactions produce- irregular or non-mendelian segregations. 



THE TRIPARTITE NATURE OF THE GENE 



The new theory proposes that the gene consists of (1) the locus 

 and (2) the chroaogene which produces (3) the gene-product. The 

 locus is the place of attachment for the chromogene. Chromogenes 

 may be attached to the keyed skeleton of the chromosome and may 

 be transferred from one locus to its allele on the homologous chrom- 

 osome, depending on the strength of the attraction of the two allelic 

 loci. Both the locus and the chromogene may be quite complex with 

 many sub -components. The desoxyribosenucleoprotein in the chrom- 

 osomes studied by Mirsky, the Stedmans, and Stern may comprise 

 the fibrous keyed -skeleton. The concentrated attention on the desoxy- 

 ribosenucleoprotein has proved very fruitful, but these fibrous struc- 

 tures may merely serve as places of attachment for the chromogenes. 



DEGRADATION OF THE GENE 



The theory proposes that the gene may be affected by at least 

 three different degrees of degradation: (1) alteration of the gene- 

 product to gene equilibrium which can often be repaired in the 

 haplophase, (2) injury or loss of the chromogene which may be re- 

 paired in the heterozygote, in the presence of the dominant allele, 

 (3) destruction of the locus which is irreparable. The third type of 

 injury produces "good" genes which segregate well and give regular 

 ratios because activity of the gene ceases and repair is impossible. 



MENDEUSM AS A SPECIAL CASE OF 

 THE GENERAL THEORY 



Mendel's law is based on the analysis of regularly -segregating 

 hypomorphic, recessive genes which are presumably not affected 



