STRUCTURE OF THE GERM PLASM ii 



the development of a complete individual w^ith all its struc- 

 tures and functions. That is, the new individual is double as 

 to his genes; he carries two complete sets/ each containing all 

 that is necessary for producing an individual. For every gene 

 in the maternal string there is a corresponding gene in the 

 paternal string (there are certain exceptions in some organ- 

 isms), so that the two strings are throughout parallel in 

 structure and action. Thus every kind of gene is represented 

 twice — by one example from the mother, the other from the 

 father. So the genes are in pairs — one gene maternal in origin, 

 the other paternal in origin. Each function, in development, 

 each effect, has two genes, either of which may perform it. 

 This doubleness of organisms in respect to their genes is one 

 of their most important features; it must never be forgotten. 



But the two genes of a pair, though they have in general the 

 same function, may, and often do, perform that function in 

 a different manner. One of them, from the mother, tends to 

 produce eyes of a certain color — in man, for example, brown 

 eyes; while the one from the other parent may tend to pro- 

 duce eyes of another color — ^blue eyes, perhaps. One of a pair 

 may tend to produce a rapidly growing, vigorous body, the 

 other a slow-growing, weak body. One of a pair may tend to 

 produce acute hearing, the other defective hearing. One may 

 tend to produce blood of a certain type of the several possible 

 blood types, the other another blood type. And so of all their 

 effects. 



In such cases, where the two corresponding genes differ, 

 usually one of the two prevails over the other. If one tends to 

 produce brown eyes, the other blue, the brown-eye gene pre- 

 vails and the individual has brown eyes; these have more 

 pigment than blue. If with respect to any function or struc- 

 ture one gene tends to produce a vigorous or normal individ- 

 ual, the other a weak or defective one, the one that tends to 



