Normal and brachydactylous hands placed together for comparison. (From Drinkwater.)
Radiograph of a brachydactylous hand.
Though the interest attaching to heredity in man is more widespread than in other animals, it is far more difficult to obtain evidence that is both complete and accurate. The species is one in which the differentiating characters separating individual from individual are very numerous, while the number of the offspring is comparatively few, and the generations are far between. For these reasons, even if it were possible, direct experimental work with man would be likely to prove both tedious and expensive. There is, however, another method besides the direct one from which something can be learned. This consists in collecting all the evidence possible, arranging it in the form of pedigrees, and comparing it with standard cases already worked out in animals and plants. In this way it has been possible 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
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
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
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.
Pedigree of Drinkwater's brachydactylous family. The affected members are indicated by black and the normals by light circles.