i6Section I.R. C. Punnett.
Another set of phenomena which will probably be found of importance
in the heredity of man are those included under the terms “ coupling ”
and “ repulsion.” Characters, each exhibiting simple Mendelian segrega
tion, may become linked together more or less completely in the process of
heredity, or the reverse may occur. Our knowledge of these phenomena
is at present almost completely confined to cases in plants, but evidence is
beginning to be obtained for their occurrence in animals. It is not unlikely
that they will be found to play a considerable part in human heredity. For
one of the most noticeable things about man is the frequency with which
children resemble one or other parent to the seemingly almost complete
exclusion of the other. In view of the mongrelisation of the human race,
the frequency of these cases is very remarkable, and can hardly fail to
suggest that some sort of coupling between characters plays a large part in
human heredity.
Except in very few cases, our knowledge of heredity in man is at present
far too slight and too uncertain to base legislation upon. On the
other hand, experience derived from plants and animals has shewn that
problems of considerable complexity can be unravelled by the experimental
method, and the characters concerned brought under control. Though the
direct method is hardly feasible in man, much may yet be learnt by collecting
accurate pedigrees and comparing them with standard cases worked out in
other animals. But it must be clearly recognised that the collection of such
pedigrees is an arduous undertaking demanding high critical ability, and
only to be carried out satisfactorily by those who have been trained in and
are alive to the trend of genetic research.
