When one of the diverse characteristics appears in the first generation
of offspring and the other does not appear, or is not apparent, the one
that appears is said to be _dominant_, while the one not appearing is
said to be _recessive_. In our example of the yellow and white corn,
yellow is dominant and white recessive. And it must be remembered that
the white corn that appears in the second generation will breed true
just as if it had never been crossed with the yellow corn. One third of
the yellow of the second generation would also breed true if it could be
separated from the other two thirds.

It is not here claimed that Mendelism is a universal principle, that all
characteristics are transmitted in this way. However, the results of the
numerous experiments in heredity lead one to expect this to be the case.
Most of the experiments have been with lower animals and with plants,
but recent experiments and statistical studies show that Mendelism is an
important factor in human heredity, in such characteristics as color of
hair and eyes and skin, partial color blindness, defects of eye, ear,
and other important organs.

The studies that have been made of human heredity have been, for the
most part, studies of the transmission of physical characteristics. Very
little has been done that bears directly upon the transmission of mental
characteristics. But our knowledge of the dependence of mind upon body
should prepare us to infer mental heredity from physical heredity. Such
studies as throw light on the question bear us out in making such an
inference.

The studies that have been more directly concerned with mental heredity
are those dealing with the resemblances of twins, studies of heredity in