hydactylism (an
abnormality of the digits commonly called shortfingeredness, due to
the lack of one joint in each digit) is present and frequently
associated with dwarfism. We may describe this case rather fully
because it illustrates nicely the heredity of a trait according to the
Mendelian formula. The parentage of the affected female (II, 1) who
started this line is uncertain. The marriage was with a normal male
whose parentage is unknown but evidently normal. This pair produced 11
children, the character of 8 of whom is known; 4 were affected, 4
unaffected, a Mendelian ratio resulting from the mating of a normal
with a hybrid individual, the observed character dominating (i. e.,
the abnormality appearing in the hybrid individuals). According to
Mendelian laws, the normal offspring of affected hybrids when mated
with normals should produce all normal offspring; this result is shown
clearly through generations IV-VI, where no affected individuals are
produced by two normal parents, although one or two of the
grandparents were affected. Marriage of a normal person with one
affected parent is fit because this individual is wholly without
germinal determiners for this character. Marriage between a normal and
an affected person is unfit (or it would be if the observed character
were a serious defect) because approximately one half their offspring
will be affected like the one parent. Thus in IV, 7-21, we see 12
children from one such marriage, 7 of whom are affected, 5 unaffected.
All of the 11 children of the 5 unaffected are normal, while of the 16
children of the affected persons, all of whom that married at all
married normal individuals, 9 were affected, 7 unaffected. Similar
relations are found in generation VI, where the 9 affected persons in
V married normals, producing 33 children, 15 of whom were affected, 18
unaffected. Taking all the offspring of marriages between unaffected
and affected (hybrid) persons through the four generations III-VI, we
find 35 affected and 33 unaffected, with the condition of 3 unknown.
There is no instance in this pedigree of the marriage of two affected
persons, but such a marriage would be highly unfit (again in the case
of a serious defect) because we know that all their offspring would be
affected. Mating of two unaffected persons, even though each had one
affected parent, would be fit because the offspring would all be
unaffected, barring the possibility of a new variation or muta