GENETICS 255 



the same character are in different chromosomes, and these chromosomes 

 meet in a pair in maturation. It has also been shown that in one of the 

 maturation divisions the tetrad that is formed by a pair of chromosomes 

 is divided in such a way that the original chromosomes are separated 

 from one another. In this separation is the mechanism by which the 

 genes are segregated — if only the genes are in the chromosomes. 



Secondly, the phenomena of linkage can be explained if the genes arc 

 in the chromosomes. Cl^^racters that hang together in their distribution 

 to the germ cells, to a greater degree than would be expected on the theory 

 of chance, are assumed to be represented by genes in the same chromo- 

 some. The argument from linkage has been elaborated by a long 

 series of important discoveries in the last decade to such a degree that 

 it now amounts almost to proof that the chromosomes contain the 

 genes. For this evidence, however, the student should consult works 

 on genetics. 



What the genes are is largely a matter of conjecture. Since the 

 chromosomes are presumably composed of very complex proteins, it is 

 not unnatural to suppose that the genes are proteins. Some have held 

 that they might be, not distinct substances, but merely the side-chains 

 of protein molecules in the chromosomes. Others have suggested that 

 they are enzymes. But there is little evidence on which to base a theory 

 of the nature of the genes, and further discussion of the matter would be 

 chiefly speculation. 



How the genes produce the characters they represent is likewise 

 little understood. It is evident that they influence a long series of 

 chemical reactions (metabolic processes) in the development of the organ- 

 ism. Since the cytoplasm must participate in many of these reactions, 

 objections have occasionally been raised against the view that the genes 

 determine the characters. However, with the exception of the plastids 

 the cytoplasm appears not to exercise any specific influence^ in producin g 

 the characters of the adult; it is merely a medium in which the gl|iei acT, 

 and material out of which the genes produce their characters. Although 

 the cytoplasm exercises no specific influence on the genes, it is probable 

 that interactions among the genes themselves regularly occur. A gene 

 by itself, even if it were in the usual kind of cytoplasm, would doubtless 

 produce nothing. The absence of only one or two of the genes ordinarily 

 present is known, in many cases, to change entirely the characters pro- 

 duced by those that remain. The genes are perhaps, then, not as dis- 

 tinct and independent as the student was led to suppose from the 

 somewhat mathematical accounts of the phenomena of heredity given 

 earlier in the chapter, but are members of a highly organized system. 



1 There is room to debate the influence of the cytoplasm in heredity, but such a 

 debate would introduce a whole new group of phenomena many of which are not 

 clear. It seems unwise to discuss ihese phenomena here. 



