PHENOMENA OF INHERITANCE 249 



of Mendelian inheritance are more simply 

 stated in terms of presence or absence of 

 single characters than in terms of contrasting 

 characters. 



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 fac- 

 tors, the zygote receives only negative factors 

 and is said to be nulliplex. Thus the union of 

 gametes AB (?) and AB (<?) yields zygote 

 A ABB, 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, that is, when it is duplex; if derived 

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

 diluted in appearance and is intermediate be- 

 tween the two parents. For example, when 

 white-flowered four o'clocks which are nulli- 

 plex are crossed with red-flowered ones which 



