Chromosomes and Genes 97 



quently been used to avoid the consequences of modem discoveries 

 for the classic theory of the gene. 



The difference between the classic definition and recent views 

 has been put very well in a discussion between Whiting and Lewis 

 (see Lewis, 1951) in which Lewis defends the classic point of view 

 and Whiting breaks a lance in my behalf. I quote: 



"Whiting: The speaker has defined the gene as the unit within 

 which there is no crossing over. Three years ago I questioned the 

 existence of genes [P. W. Whiting, Biol. Bull, 95:257] as units of 

 hereditary transmission, pointing out that ideologically the gene is 

 the lineal descendant of the subcellular units of past philosophies — 

 micellae, pangenes, etc. — and that we have no evidence that crossing 

 over does not occur in the homozygote within that portion of the 

 chromosome acting as a unit in the heterozygote. The 13 sex alleles 

 found by Goldschmidt in the Z-chromosome of Lymantria and the 

 8 in the W-chromosome act as units in the hybrids, but there may 

 well be intragenic crossing over in the pure races. Similarly in 

 Drosophila, the sex gene is diffuse, scattered throughout the active 

 portion of the X. This acts as a unit segregating from the Y or the lack 

 of X in the sex-heterozygote, the male, but it is not a unit in the 

 sex-homozygote, the female, as shown by the locaHzed sex-linked 

 genes for which the female may be heterozygous. The germ plasm is 

 genie material but it does not consist of genes. It produces genes by 

 such structural reorganizations as may subsequently Mendelize with 

 the original condition." 



'%ewis: The speaker's remark about the definition of the term, 

 gene, was given as a very brief aside to the main paper. Since there 

 are conflicting viewpoints on the matter of this definition, we will 

 amplify here our reasons for employing the crossing over process as 

 an essential part of a working definition of the gene. The definition 

 of any particulate unit must be in terms of its indestructibility by 

 some breakage or splitting process. The crossing over process and any 

 which leads to chromosomal arrangements are the only yet known 

 processes of this kind which can be used in defining the unit of 

 heredity. The point we wish to emphasize is that the discovery of 

 what we have called position pseudoallelism relegates the rearrange- 

 ment process to second place. Thus, if a chromosomal rearrangement 

 separates units which must lie close together in order to function 

 normally, then the effect of that rearrangement will be to cause an 

 apparent destruction of one or more of the units which are acting 



