The facts of human heredity can more easily be brought home to us by the examination of some actual pedigrees and family histories. We may look at a few representative cases which will serve to bring out some additional aspects of the significance to society of the demonstrated fact of heredity. In the examination of single family histories we should remember that a single pedigree may not accurately illustrate a general law of heredity—again, an individual case may belong to a group of cases without representing them fairly. Even in observing illustrations of Mendel's laws allowance has to be made for the variability due to "chance" meetings of germ cells. It is only when large numbers of individuals are observed that the typical Mendelian fractions and ratios can be strictly observed. It must be borne in mind then that the histories given below illustrate the nature of the facts of heredity rather than the laws of heredity. Some special cautions in the interpretation of certain pedigrees will be suggested in particular cases. Many of the figures are taken from the extremely valuable "Treasury of Human Inheritance," now being published by the Eugenics Laboratory of the University of London. In these figures and some others a uniform series of symbols is used. Successive horizontal lines designated by Roman numerals indicate generations; within a single generation the individuals are numbered consecutively simply for purposes of reference. The meaning of the more common symbols is as shown in Table IV. We may first consider a few pedigrees showing the heredity of physical abnormalities or defects.
HUMAN HEREDITY
Table IV.
Fig. 13.—Family history showing brachydactylism. Farabee's data. (From "Treasury of Human Inheritance.")
Fig. 13 illustrates a family history where brachydactylism (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 mutation to this character, which would be extremely unlikely. Such a pedigree as this illustrates very well how a knowledge of Mendelian heredity may be of the greatest value practically, in determining the fitness or unfitness of marriages in families where an abnormality or defect is known to occur. The course of the inheritance here illustrates the simplest form of Mendelism. We have already indicated that there are many other forms which we have not described and which we cannot undertake to describe here on account of their complexity; in such cases, however, it is still possible to predict with fair accuracy the characters of the offspring of parents whose history is known for one or two generations.
The defect we have just been considering is dominant. Many defects are recessive, i. e., transmitted though not exhibited by a hybrid individual. Viewed from the standpoint of the character of the offspring, mating with such a person would be unfit only when both persons were similarly recessives. Such a chance similarity would be likely only in cases of blood relationship. Here lies the scientific basis for many of the legal restrictions against cousin marriage or the marriage of closer relatives, for here, although both persons may appear normal, the chances for latent ills appearing in the progeny in a pure and permanently fixed condition are greatly increased. Of course the same relation holds for characteristics which are not defects but really valuable traits. Marriage of cousins possessing valuable characters, whether apparent or not, might be allowed or encouraged as a means of rendering permanent a rare and valuable family trait which might otherwise be much less likely to become an established characteristic. Some discrimination should be exercised in the control, legal or otherwise, of such marriages.