238 PRINCIPLES OF ANIMAL BIOLOGY 



Since the inheritance of Hnked characters is still called Mendelian, 

 it would be better if the statement of Mendel's law could also be liberal- 

 ized. A better formulation would be: The fundamental units of heredity 

 are distributed hy means of the chromosomes. This would exclude' plastids. 

 Also, to understand the law it would be necessary to know a good deal 

 about chromosomes. 



The Nature of Genes. — It is practically certain that the genes are 

 chemical substances and that it is through their chemical properties 

 that they control the development of the characters they represent. 

 Presumably they are protein in nature. One reason for considering them 

 protein is that the chromosomes give protein reactions, and the genes 

 make up a fraction of the chromosomes. Moreover, genes are highly 

 specific in their action; that is, they do certain definite things with con- 

 siderable precision, and not other things. Highly specific reactions are 

 characteristic of proteins in general, which would help to explain the 

 functioning of genes if these be protein. 



Moreover, genes are subject to change. Although any mechanism of 

 heredity must have some degree of permanence — otherwise there would 

 be no heredity — genes do not remain forever the same. One of the genes 

 for red eye in Drosophila changed, and the eye color was then brown. 

 A gene for gray body color in the same species changed, and yellow body 

 resulted. A gene for uniform color in mice changed, and the mice in 

 succeeding generations were spotted. Changes of this sort are known 

 as mutations. They must be chemical changes of the genes, which would 

 be not only possible but probable if the genes were proteins. The chemi- 

 cal structure of proteins is very complex, and occasional permanent 

 change is more likely in complex substances than in simple ones. 



It is a current concept that a gene may be a single protein molecule. 

 One reason for so believing is the suddenness with which gene mutations 

 occur. If a gene were composed of several molecules, any change in 

 chemical structure would presumably, just as a matter of chance, affect 

 only one of them. The argument is that, with a number of molecules to 

 change, mutation might tend to be a gradual process. With only one 

 molecule, any structural change must affect the whole gene at once. 



Practical Applications. — Knowledge of heredity has been used for 

 centuries to improve the economic situation of the human race. The 

 classical field in which that has been done is the in^provement of crops and 

 farm animals. The knowledge upon which this improvement rested 

 was, until comparatively recent times, little more than a knowledge that 

 heredity existed. Its laws have been fairly well understood by breeders 

 only in the present century, but by the year 1900 most of the develop- 

 ment of domestic races had already been accomplished. The reason for 

 this great success of the early breeders is that their method was practically 



