PRINCIPLES OF GENETICS 25 



The Theory of the Gene. 



We are now in a position to formulate the theory of the 

 gene. The theory states that the characters of the indi- 

 vidual are referable to paired elements {genes) in the 

 germinal material that are held together in a definite 

 number of linkage groups; it states that the members of 

 each pair of genes separate when the germ-cells mature 

 in accordance ivith Mendel's first laiv, and in consequence 

 each germ-cell comes to contain one set only; it states 

 that the members belonging to different linkage groups 

 assort independently in accordance with Mendel's second 

 law; it states that an orderly interchange — crossing-over 

 — also takes place, at times, between the elements in 

 corresponding linkage groups; and it states that the fre- 

 quency of crossing-over furnishes evidence of the linear 

 order of the elements in each linkage group and of the 

 relative position of the elements with respect to each 

 other. 



These principles, which, taken together, I have ventured 

 to call the theory of the gene, enable us to handle prob- 

 lems of genetics on a strictly numerical basis, and allow 

 us to predict, with a great deal of precision, what will 

 occur in any given situation. In these respects the theory 

 fulfills the requirements of a scientific theory in the full- 

 est sense. 



