CHAPTER I 



THE FUNDAMENTAL PRINCIPLES OF GENETICS 



THE modern theory of heredity is derived from 

 numerical data obtained by crossing two individ- 

 uals that differ in one or more characters. The 

 theory is primarily concerned with the distribution of 

 units between successive generations of individuals. In 

 the same sense in which the chemist postulates invisible 

 atoms and the physicist electrons, the student of heredity 

 appeals to invisible elements called genes. The essential 

 point in this comparison is that both the chemist and the 

 student of heredity — the sreneticist — have reached their 

 conclusion from numerical and quantitative data. The 

 theories justify themselves in so far as they permit 

 numerical and quantitative prediction of a specific kind. 

 In this essential respect the theory of the gene differs 

 from earlier biological theories that have also postulated 

 invisible units to which were arbitrarily assigned any 

 desired properties. The theory of the gene reverses this 

 order and derives the properties of the genes, so far as it 

 assigns properties to them, from the numerical data 

 alone. 



Mendel's Two Laws. 



We owe to Greffor Mendel the discovery of two of the 

 fundamental laws of heredity on which the modern theory 

 of heredity is based. Later work, clone by others during 

 the present century, has carried us further in the same 

 direction and made possible the elaboration of the theory 

 on a much broader basis. Mendel's discovery may be 

 illustrated by a few familiar examples. 



