238 PHYSICAL BASIS OF HEEEDITY 



It has been pointed out that the evidence shows not 

 only that the genes are carried by the chromosomes, but 

 that there may be interchanges between paternally- 

 derived and maternally-derived chromosome pairs. The 

 evidence shows that this interchange is a normal feature 

 of the germ-cell, and not peculiar to hybrids, or to a 

 heterozygous condition of the pairs. 



This analysis leads then to the view that the gene is 

 a certain amount of material in the chromosome that may 

 separate from the chromosome in which it lies, and be 

 replaced by a corresponding part (and by none other) 

 of the homologous chromosome. It is of fundamental sig- 

 nificance in this connection to recognize that the genes 

 of the pair that interchange do not jump out of one chro- 

 mosome into the other, so to speak, but are changed 

 by the thread breaking as a piece in front of or else 

 behind them, but not in both places at once, as would 

 be the case if only a single pair of allelomorphs were 

 involved each time. 



That the gene does not stand for the whole length of 

 the chromosome between two other known genes is shown 

 by the fact that new genes arising by mutation in the inter- 

 mediate region do not affect the character of the gene 

 already known. This fact recurring continually in Droso- 

 phila, where new mutations frequently appear, reassures 

 us that the idea of the gene as a very small part of the 

 thread is a legitimate conclusion, even if we can not tell 

 how large or how small that region is. 



1. The Manifold Effects of Each Gene 

 If we examine almost any mutant race, such as the 

 race of white -eyed Drosophila, we find that the white eye 

 is only one of the characteristics that such a mutant race 

 shows. The productivity of the individual is also much 

 affected, and the viability is lower than in the wild fly. AU 

 of these peculiarities are found whenever the white eye 

 emerges from a cross, and are not separable from the 



