As a first illustration I may take the case of human eye color. The iris is made up of a trestle-work of fibers, in which are suspended particles that give the blue color. In addition, in many eyes much brown pigment is formed which may be small in amount and gathered around the pupil or so extensive as to suffuse the entire iris and make it all brown. It is seen, then, that the brown iris is formed by something additional to the blue. And brown iris may be spoken of as a _positive_ character, depending on a determiner for brown pigment; and blue as a _negative_ character, depending on the absence of the determiner for brown.
Now when both parents have brown eyes and come from an ancestry with brown eyes, it is probable that all of their germ cells contain the determiner for brown iris pigmentation. So when these germ cells, both carrying the determiner, unite, all of the progeny will receive the determiner from both sides of the house; consequently the determiners are double in their bodies and the resulting iris pigmentation may be said to be _duplex_. When a character is duplex in an individual, that means that when the germ cells ripen in the body of that individual each contains a determiner. So that individual is capable, so far as he is concerned, of transmitting his trait in undiminished intensity.
If a parent has pure blue eyes, that is evidence that in neither of the united germ cells from which he arose was there a determiner for iris pigmentation; consequently in respect to brown iris pigmentation such a person may be said to be _nulliplex_. If, now, such a person marry an individual duplex in eye color, in whom all of the germ cells contain the determiner, each child will receive the determiner for iris pigmentation from one side of the house only. This determiner will, of course, induce pigmentation, but the pigmentation is simplex, being induced by one determiner only. Consequently, the pigmentation is apt to be weak. When a person whose pigment determiners have come from one side of the house forms germ cells, half will have and half will lack the determiner. If such a person marry a consort all of whose germ cells contain the determiner for iris pigmentation, all of the children will, of course, receive the iris pigmentation, but in half it will be duplex and in the other half it will be simplex. If the two parents both be simplex, so that, in each, half of the germ cells possess and half lack the determiner in the union of germ cells, there are four events that are equally apt to occur: (1) an egg _with_ the determiner unites with a sperm _with_ the determiner; (2) an egg _with_ the determiner unites with a sperm _without_ the determiner; (3) an egg _without_ the determiner unites with a sperm _with_ the determiner; (4) an egg _without_ the determiner unites with a sperm _without_ the determiner. Thus the character is duplex in one case, simplex in two cases, and nulliplex in one case; that is, one in four will have no brown pigment, or will be blue eyed. If one parent be simplex, so that the germ cells are equally with and without the determiner, while the other be nulliplex, then half of the children will be simplex and half nulliplex in eye pigment. Finally, if both parents be nulliplex in eye pigmentation (that is, blue eyed), then none of their germ cells will have the determiner, and all children will be nulliplex, or blue eyed. The inheritance of eye color serves as a paradigm of the method of inheritance of any unit-character.