Phenomena of Inheritance 97 



many genetical and philosophical objections to such a view, which 

 leads logically to some strange conclusions, such as Bateson's 

 speculations on evolution (p. 268). 



When both gametes carry similar positive 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 ( 2 ) and AB (S) yields zygote AABB, which is duplex in 

 constitution; gametes Ab (9) and aB (S) yield zygote AaBb, 

 which is simplex ; gametes ab ( 2 ) 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, that 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 

 from both parents, pink flowers when it is derived from one parent, 

 white flowers when it is derived from neither parent (Fig. 28). 



5. Summary of Mendelian Principles. — Since the rediscovery 

 in 1900 of Mendel's work many investigators have carried out 

 similar experiments on many species of animals and plants and 

 have greatly extended our knowledge of the principles of inheri- 

 tance discovered by Mendel, but in the main Mendel's conclu- 

 sions have been confirmed again and again, so that there is no 

 doubt that they constitute an important rule of inheritance among 

 • all organisms. 



In brief the "Mendelian Law of Alternative Inheritance" or of 

 hereditary "splitting" consists of the following principles : 



(a) The Principle of Unit Characters. — The heritage of an 



