least seven generations. It is an excellent
example of an inherited defect due to a single Mendelian factor.]

One of the best attested single characters in human heredity is
brachydactyly, "short-fingerness," which results in a reduction in the
length of the fingers by the dropping out of one joint. If one lumps
together all the cases where any effect of this sort is found, it is
evident that normals never transmit it to their posterity, that affected
persons always do, and that in a mating between a normal and an affected
person, all the offspring will show the abnormality. It is a good
example of a unit character.

But its effect is by no means confined to the fingers. It tends to
affect the entire skeleton, and in a family where one child is markedly
brachydactylous, that child is generally shorter than the others. The
factor for brachydactyly evidently produces its primary effect on the
bones of the hand, but it also produces a secondary effect on all the
bones of the body.

Moreover, it will be found, if a number of brachydactylous persons are
examined, that no two of them are affected to exactly the same degree.
In some cases only one finger will be abnormal; in other cases there
will be a slight effect in all the fingers; in other cases all the
fingers will be highly affected. Why is there such variation in the
results produced by a unit character? Because, presumably, in each
individual there is a different set of modifying factors or else a
variation in the factor. It has been found that an abnormality quite
like brachydactyly is produced by abnormality in the pituitary gland. It
is then fair to suppose that the factor which produces brachydactyly
does so by affecting the pituitary gland in some way. But there must be
many other factors which also affect the pituitary and in some cases
probably favor its development, rather than hindering it. Then if the
factor for brachydactyly is depressing the pituitary, but if some other
factors are at the same time stimulating that gland, the effect shown in
the subject's fingers will be much less marked than if a group of
modifying factors were present which acted in the same direction as the
brachydactyly factor,--to perturb the action of the pituitary gland.

This illustration is largely hypothetical; but there is no room for
doubt that every factor produces more than a single effect. A white
blaze in the hair, for example, is a well-proved unit factor in man; the