CHAPTER XVIII 



STABILITY OF THE GENE 



IN what has been said, so far, it has been implied that 

 the gene is a stable element in heredity, but whether 

 it is stable in the sense that a chemical molecule is 

 stable, or whether it is stable only because it fluctuates 

 quantitatively about a persistent standard, is a question 

 of theoretical and perhaps of fundamental importance. 



Since the gene cannot be studied directly by physical 

 or chemical methods, our conclusions concerning its sta- 

 bility must rest on deductions from its effects. 



Mendel's theory of heredity postulates that the gene 

 is stable. It assumes that the gene that each parent con- 

 tributes to the hybrid remains intact in its new environ- 

 ment in the hybrid. A few examples will serve to recall 

 the nature of the evidence for this conclusion. 



The Andalusian race of poultry has white, black, and 

 blue individuals. If a white bird is mated to a black one, 

 the offspring are slate-colored or blue. If two of these 

 blue-colored birds are mated, the offspring fall into three 

 classes, black, blue, and white, in the proportion of 1 : 2 : 1. 

 The gene for white and the gene for black separate in the 

 blue hybrid. Half the mature germ-cells come to carry 

 the black-producing element and half the white-produc- 

 ing element. Chance fertilization of any egg by any sperm 

 will give the observed proportions 1:2:1 in the second 

 filial generation. 



The test of the correctness of the assumption that the 

 germ-cells of the hybrids are of two kinds is as follows. 

 If a blue hybrid is back-crossed to a pure white bird, half 



