EXPERIMENTAL STUDY OF HEEi;DITY 56 1 



of the same organization of the protoplasm with reference 

 to its character-units, was first developed by Johannsen, 

 of Copenhagen, Denmark, who proposed the term 

 "genes." "The sum total of all the 'genes' in a gamete 

 or zygote," is a genotype. Inheritance is the appearance, 

 in successive generations, of the same genotypical constitu- 

 tion of the protoplasm. Johannsen does not attempt to 

 explain the nature of the genes, "but that the notion 

 'gene' covers a reality is evident from Mendelism." 



This conception of heredity is diametrically opposed 

 to the older and popular conception, but. is much more 

 closely in accord with the facts revealed by recent studies 

 of plant and animal breeding.^ 



4M. Value of Mendel's Discoveries. — The discoveries 

 that, in inheritance, certain characters are dominant 

 over certain others; that a given inheritance {e.g., condi- 

 tions associated with seed-color, odor, eye-color, stature, 

 musical ability, insanity, tendency to some disease) may 

 be carried and transmitted to offspring by an adult 

 who gives no outward signs of carrying the inheritance; 

 that, under certain conditions of breeding, some characters 

 (the recessive ones), whether good or bad, may become 

 permanently lost; that dominant characteristics are 

 certain to appear in some of the offspring — all of these 

 truths, learned by the study of a common garden vegetable, 

 will be recognized at once as of enormous importance to 

 the breeders of plants and animals, and above all to man- 

 kind, in connection with our own heredity. They point 

 the way to the explanation of such enigmas as the pro- 

 verbial bad sons of pious preachers, spendthrift children 



' A discussion of Johannsen's very fruitful method of "pure line" 

 breeding belongs to more advanced studies. 

 36 



