We can not help feeling that in most
cases heredity in man is being made to appear much simpler than it
really is; and that particularly in mental characters, analysis of
traits has by no means reached the bottom.
If we were asked to make out a list of characters, as to the Mendelian
inheritance of which there could be little doubt, we would hardly be
able to go farther than the following:
The sex-linked characters (one kind of color-blindness, hemophilia, one
kind of night-blindness, atrophy of the optic nerve, and a few other
rare abnormalities).
Albinism. This appears to be a recessive, but probably involves multiple
allelomorphs in man, as in other animals.
Brachydactyly, apparently a dominant. This is so much cited in
text-books on Mendelism that the student might think it is a common
character. As a fact, it is extremely rare, being found in only a few
families. The similar trait of orthodactyly or symphalangism, which
likewise appears to be a good Mendelian dominant, seems to exist in only
one family. Traits like these, which are easily defined and occur very
rarely, make up a large part of the cases of probably Mendelian
heredity. They are little more than curiosities, their rarity and
abnormal nature depriving them of evolutionary significance other than
to demonstrate that Mendelian heredity does operate in man.
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