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 factors and is said to be nulliplex. Thus the union of gametes AB (♀) 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, 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
Fig. 56. Results of Crossing White-flowered and Red-flowered Races of Mirabilis Jalapa (Four O'clocks), giving a pink hybrid in F1, which when inbred gives in F2 1 white, 2 pink, 1 red. (From Morgan, after Correns.)
flowered four o'clocks which are nulliplex 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. 56).
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 inheritance discovered by Mendel, but in the main Mendel's conclusions have been confirmed
