these two parts are hybrids,
the result of crossing white with yellow. It is not meant that one can
actually distinguish these two kinds of yellow, the pure yellow and the
hybrid yellow, but the results from planting it show that one third of
the yellow is pure and that the other two thirds transmit white and
yellow in equal ratio.

The main point to notice in all this is that when two individuals having
diverse characteristics are crossed, the characteristics do not fuse and
disappear ultimately, but that the two characteristics are transmitted
in equal ratio, and each will appear in succeeding generations, and will
appear pure, just as if it had not been crossed with something
different. The first offspring resulting from the cross--known as
hybrids--may show either one or the other of the diverse
characteristics, or, when such a thing is possible, even a blending of
the two characteristics. But whatever the actual appearance of the first
generation of offspring resulting from crossing parents having diverse
characteristics, their germ-cells transmit the diverse characteristics
in equal proportion, as explained above.

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 direct