erm _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.

Let us now consider some of the physical traits of man that follow the
same law as brown eye color, traits that are clearly positive, and due
to a definite determiner in the germ plasm.

Hair color is due either to a golden-brown pigment that looks black in
masses, or else to a red pigment. The lighter tints differ from the
darker by the absence of some pigment granules. If neither parent has
the capacity of producing a large quantity of pigment granules in the
hair, the children cannot have that capacity, that is, two flaxen-haired
parents have only flaxen-haired children. But a dark-haired parent may
be either simplex or duplex; and so two such parents _may_ produce
children with light hair; but not more than one out of four. In general,
the hair color of the children tends not to be darker than that of the
darker parent. Skin pigment follows a similar rule. It is really one of
the surprises of modern studies that skin pigment should be found to
follow the ordinary law of heredity; it was commonly thought to blend.
The inheritance of skin color is not dependent on race; two blonds never
have brunette offspring, but brunettes may have blondes. The extreme
case is that of albinos with no pigment in skin, hair, and iris. Two
albinos have only albino children, but albinos may come from two
pigmented parents.

Similarly, straight-haired parents lack curliness, and two such have
only straight-haired children. Also two tall parents have only tall
children. _Shortness_ is the trait: tallness is a negative character.
Also