Weismann's
prediction, that the chromosomes are the carriers of heredity, came to
be looked on as a fact, by many biologists.
But when so much of Professor Weismann's system was accepted, other
parts of it went along, including a hypothetical system of "determiners"
in the chromosome, which were believed to determine the development of
characters in the organism. Every trait of an animal or plant, it was
supposed, must be represented in the germ-plasm by its own determiner;
one trait, one determiner. Did a notch in the ear run through a
pedigree? Then it must be due to a determiner for a notch in the ear in
the germ-plasm. Was mathematical ability hereditary? Then there must be
a determiner, the expression of which was mathematical ability.
For a while, this hypothesis was of service in the development of
genetics; some students even began to forget that it was a hypothesis,
and to talk as if it were a fact. But the exhaustive tests of
experimental breeding of plants and animals have long caused most of the
advanced students of genetics to drop this simple hypothesis.
In its place stands the factorial hypothesis, evolved by workers in
America, England, and France at about the same time. As explained in
Chapter V, this hypothesis carries the assumption that every visible
character is due to the effects of not one but many factors in the
germ-cell.
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