tion to
this character, which would be extremely unlikely. Such a pedigree as
this illustrates very well how a knowledge of Mendelian heredity may
be of the greatest value practically, in determining the fitness or
unfitness of marriages in families where an abnormality or defect is
known to occur. The course of the inheritance here illustrates the
simplest form of Mendelism. We have already indicated that there are
many other forms which we have not described and which we cannot
undertake to describe here on account of their complexity; in such
cases, however, it is still possible to predict with fair accuracy
the characters of the offspring of parents whose history is known for
one or two generations.

[Illustration: FIG. 13.--Family history showing
brachydactylism. Farabee's data. (From "Treasury of Human
Inheritance.")]

The defect we have just been considering is dominant. Many defects are
recessive, i. e., transmitted though not exhibited by a hybrid
individual. Viewed from the standpoint of the character of the
offspring, mating with such a person would be unfit only when both
persons were similarly recessives. Such a chance similarity would be
likely only in cases of blood relationship. Here lies the scientific
basis for many of the legal restrictions against cousin marriage or
the marriage of closer relatives, for here, although both persons may
appear normal, the chances for latent ills appearing in the progeny in
a pure and permanently fixed condition are greatly increased. Of
course the same relation holds for characteristics which are not
defects but really valuable traits. Marriage of cousins possessing
valuable characters, whether apparent or not, might be allowed or
encouraged as a means of rendering permanent a rare and valuable
family trait which might otherwise be much less likely to become an
established characteristic. Some discrimination should be exercised
in the control, legal or otherwise, of such marriages.

[Illustration: FIG. 14.--Family history showing
polydactylism. (From "Treasury of Human Inheritance.")]

Fig. 14 gives a brief pedigree of a family in which polydactylism
occurs. This is a condition in which one or more additional or
supernumerary fingers or toes are present in the extremities. The
Mendelian character of the heredity of this defect is less clear than
in the preceding, yet there are many indications that this is really
an illustration of a complex M