CROSSING. 



"daughters has typically "notch" wings, etc. It was possi- 

 ble to ascertain that the purple stock had not been con- 

 taminated. The character was accordingly considered 

 "as due to a reappearance of the old notch gene". 

 As can be seen from this sample, the new character is un- 

 questionably identified with a gene. The gene itself is called 

 notched. This is quite typical of the publications on Drosophila. 

 The same characters appear quite frequently and apparently 

 spontaneously. Loss-mutations are followed by return-muta- 

 tions, the "lost character" reappearing. Such cases are often 

 casually alluded to without much reference to the fact that, 

 should they actually correspond with loss and recovery of 

 genes, or with change in dominance of genes, the bearing upon 

 our theoretical notions about genes should be of the utmost 



interest. 



So long as a stock is not genotypically homogeneous, that is, 

 so long as the potential variability of it is not zero, both reces- 

 sive novelties and dominant novelties can be expected to turn 

 up in it, and it depends largely upon the care with which the 

 material is analyzed and studied, how great the number of 

 discoverable "unit-characters" is. 



How is it possible to account for the enormous genetic var- 

 iability in this Drosophila material, if we do not have recourse 

 to the mutation-hypothesis for an explanation? Is it possible 

 to explain the origin of the new characters wholly on the basis 

 of recombination of Mendelian factors, genes? It is plain that 

 the difficulties in the way of such an explantion are made to ap- 

 pear unnecessarily great by a strict adherence to the determin- 

 ant-conception of heredity. We have had many an argument 

 with American geneticians about the use of mnemotechnic 

 symbols to denote particular genes. To a great many people it 

 appears that, when it is demonstrated that a given gene produ- 

 ces the difference between black and white animals, calling it 

 "Black" or B helps to memorize this action of the gene. And 

 they are of the opinion, that this system of naming genes after 

 characters does not produce the confusion which we fear it 



