endelian formula. Probably if the
parentage of the individuals marrying into this family were known we
should be able to give a complete formula. At any rate the pedigree
illustrates the unfit character of the matings with affected persons,
for in no instance has such a marriage resulted in the production of
fewer than one half affected offspring.

Fig. 15 illustrates a form of what is known as "split hand" or
"lobster claw," where certain digits may be absent in the hands and
feet. In this case all the digits are absent except the fifth. This is
frequently associated with syndactylism or the fusion of the remaining
digits into one or two groups. When present this usually affects all
four extremities. Two pedigrees of this defect are illustrated in Fig.
16. Here again we have a defect whose inheritance follows quite
closely the Mendelian formula, although the character of the matings
is not fully known; it is unnecessary to describe the details--the
histories speak for themselves.

[Illustration: FIG. 15.--Mother and two daughters showing
"split hand." (From Pearson.)]

Fig. 17 illustrates a pedigree of congenital cataract. This history is
less satisfactory because the matings are given in only three
instances. It is known from other data that this defect follows simple
Mendelian laws. Normal individuals produce only normals, while
affected persons produce one half or all affected offspring according
to the character of the mating.

Fig. 18 illustrates the heredity of another defect of the eye called
night blindness. This is a retinal defect, the affected being able to
see only in strong illumination. The particular form of the disease in
this family resulted in total blindness later in life. Little is known
definitely concerning the character of the matings; no mating is known
to have been with an affected person and some are known to have been
with unaffected. Of the 42 descendants of the first affected person
only 6 are known to have been unaffected. Can there be any doubt
regarding the unfitness of these matings? In generation III a single
mating led to a family of 10 children _all_ affected by this serious
defect, rendering them dependents.

One of the most complete pedigrees of a defect on record is given in
condensed form in Fig. 19. This summarizes the extraordinarily
complete data of Nettleship covering nine, and in one branch ten,
consecutive generations. The defect is another form of night blind