98 Heredity and Environtnent 



recessive characters are not due to the absence of factors for 

 dominant characters; there are many genetical and philosophical 

 objections to such a view, which leads logically to some strange 

 conclusions, such as Bateson's speculations on evolution (p. 282). 

 Morgan and his associates have found that a given dominant 

 character may have several different kinds of recessive contrast- 

 ing characters or allelomorphs ; thus the dominant eye color of the 

 wild pomace fly, Drosophila melanogaster, is red, but, instead of 

 a single contrasting recessive character, eleven have been found, 

 viz., apricot, blood, buff, cherry, coral, ecru, eosin, ivory, tinged, 

 wine, and white. Such a condition is known as "multiple allelo- 

 morphism." If the red color is due to the presence of a certain 

 factor, all these other allelomorphic colors cannot be due to its 

 absence, since, there can be only one kind of absence. Each of 

 these recessive colors must be due to the presence of a differentia) 

 factor. Therefore the presence-absence hypothesis must be aban- 

 doned. 



When both gametes carry similar dominant factors the zygote 

 has a "double dose" of such factors and is said to be duplex \ 

 when only one of the gametes carries such a factor the zygote 

 has a "single dose" and is simplex, when neither gamete carries 

 a positive factor or factors, the zygote receives only negative fac- 

 tors and is said to be nulliplex. Thus the union of gametes 

 AB ( 9 ) and AB ( $ ) yields zygote AABB, which is duplex in 

 constitution ; gametes Ab ( $ ) and aB ( $ ) yield zygote AaBb, 

 which is simplex ; gametes ab ( $ ) and ab ( $ ) yield zygote aabb, 

 which is nulliplex. 



In some instances a character comes to full expression only 

 when it is derived from both parents, thaj: is, when it is duplex; 

 if derived from one parent only, that is, if simplex, it is diluted 

 in appearance and is intermediate between the two parents. For 

 example, when white-flowered "four o'clocks" which are nulli- 

 plex are crossed with red-flowered ones which are duplex 

 the progeny, which are simplex, bear pink flowers; in this case 

 red flowers are produced only when the factor for red is derived 



