sible to demonstrate in man the existence of several
characters showing simple Mendelian inheritance. As few besides medical men
have hitherto been concerned practically with heredity, such records as
exist are, for the most part, records of deformity or of disease. So it
happens that most of the {171} pedigrees at present available deal with
characters which are usually classed as abnormal. In some of these the
inheritance is clearly Mendelian. One of the cases which has been most
fully worked out is that of a deformity known as brachydactyly. In
brachydactylous people the {172} whole of the body is much stunted, and the
fingers and toes appear to have two joints only instead of three (cf. Figs.
32 and 33). The inheritance of this peculiarity has been carefully
investigated by Dr. Drinkwater, who collected all the data he was able to
find among the members of a large family in which it occurred. The result
is the pedigree shown on p. 173. It is assumed that all who are recorded as
having offspring were married to normals. Examination of the pedigree
brings out the facts (1) that all affected individuals have an affected
parent; (2) that none of the unaffected individuals, though sprung from the
affected, ever have descendants who are affected, and (3) that in families
where both affected and unaffected {173} occur, the numbers of the two
classes are, on the average, equal. (The sum of such families in the
complete pedigree is thirty-nine affected and thirty-six normals.) It is
obvious that these are the conditions which are fulfilled in a simple
Mendelian case, and there is nothing in this pedigree to contradict the
assertion that brachydactyly, whatever it may be due to, behaves as a
simple dominant to the normal form, _i.e._ that it depends upon a factor
which the normal does not contain. The recessive normals cannot transmit
the affected condition whatever their ancestry. Once free they are always
free, and can marry other normals with full confidence that none of their
children will show the deformity.

[Illustration: FIG. 34.

Pedigree of Drinkwater's brachydactylous family. The affected members are
indicated by black and the normals by light circles.]

{174}

The evidence available from pedigrees has revealed the simplest form of
Mendelian inheritance in several human defects and diseases, among which
may be mentioned presenile cataract of the eyes, an abnormal form of skin
thickening in the palms of the hands