- Email: [email protected]
Heredity and the orthodontist—A study in confusion
HEREDITY
WENDELL
AND
L. WYLIE,
THE
ORTHODONTIST-A CONFUSION
STUDY
IN
D.D.S., M.S.,* SAN FRANCISCO, CALIF.
T
HE subject I am to discuss is confusing as it is intriguing, and no small part of the confusion which the subject of heredity holds for the uninitiated is due to the fact that we use, in describing the transmittal of biologic entities, the same word “inheritance” to chronicle the transfer, en famille, of goods and chattels. Since every one of us has to make the long climb to the adult state from nothing more than a fertilized egg, an object slightly smaller than one of the periods on this printed page, it must be evident that the attributes we observe in our fellows are not inherited per se, but only the tendency to produce them. Yet it is a fact that the tiny egg and the still smaller sperm cell are the only cells having any continuity in the genealogical tree. This will be no lesson in genetics, but instead an examination of the dental and related literature, in an effort to discover to what extent the dentist has considered the long-discussed matter of nature vs. nurture, or heredity vs. environment. The first fact we discover is that not only dentists but a good many others as well, are, if not actually opposed to the idea of considering heredity in relation to health, quite willing to let the subject die of neglect. This is shown in the almost universal search for environmental factors to explain the misfortunes which beset human flesh: dietary sins of omission and commission from the natal day onward, long forgotten bouts of illness in childhood, which, nevertheless, are said to leave their mark as dentofacial anomalies, and various and sundry other factors which take no account of inherent potentialities of the organism. It is as if we wish to extend our commendable political doctrine that every little boy born in the United States of America has the right to become president, and make it mean that every little boy begins at birth with the potential for having thirty-two evenly aligned teeth which he may keep as long as he keeps the spark of life. It is unfortunate that such a noble concept should have so little to support it. Before considering the main topic of heredity and facial form, we should turn to a related subject, the matter of constitution, variously known as body build or type. It is important because it is tied up with the matter of disproportion between various parts, something frequently argued in dental circles. The matter of type is pertinent here because every student of type or constitution must face the problem of consistency or harmony in one individual of Palm
Read before the Third Annual Seminar for the Springs, Calif., Oct. IO, 1946. *University of California, College of Dentistry,
154
Study San
and Francisco,
Practice Calif.
of
Dental
Medicine.
Heredity
and the Orthodontist
155
a given type. We can readily recognize a person as being lean and linear, or muscular and well built, or round and chubby, and we may choose to describe him as leptosomatic, a,thletic, or pyknic, if we remember our college textbooks that well. Can we, however, take that individual feature for feature, dimension for dimension, and say that he is consistently leptosomatic, athletic, or pyknic in every respect “2 Actually, we find that many investigators of body build report a large degree of this sort of disharmony in constitution. In the recent work this disharmony is called dysplasia, and while of Sheldon and Tucker,l they wisely avoid any premature conclusions regarding inheritance, they do provide what they call their “D-index, ” a quantitative measure of the amount of dysplasia found in one individual. This reluctance on the part of Sheldon and Tucker to associate immediately the phenomenon of dysplasia with inheritance is not universal. Thirty years ago, Davenport? sought to interpret anthropometric data in the light of genetic principles, and wrote on the “intermingling of races,” which he viewed with alarm. His definition of race was a narrower one than most: “A more or less purebred ‘group ’ of individuals that differs from other groups by at least one character, or, strictly, a genetically connected group whose germ plasm is characterized by a difference, in one or more genes, from other groups. Thus blue-eyed Scotchmen belong to a different race from some of the dark Scotch." It is evident that when Davenport frowned on race-crossing, he did not mean marriage between Caucasians and Mongoloids, but merely the crossing of dissimilar individuals. These objections to race-crossing were on the basis of the extreme dysplasia and maladjustment consequent to it. Certain animal experiments led him to dire conclusions concerning human intermarriage. He crossed Leghorn chickens with Brahmas, with interesting results. As you know, the Leghorn is a “career girl”-she lays an egg a day quite consistently, but has no great inclination to hatch them. The Brahma, on the other hand, is a homebody that thinks about raising a family when a few eggs have been laid. When these chickens were crossed, Down on the farm, she was called “broody.” the first generation produced fairly good layers who would hatch eggs, but abandon the chicks early, having lost interest. The next generation, however, were complete failures as either layers or mothers. To quote Davenport : “The instincts and functions of the hybrids were not harmoniously adjusted to each other. ’ ’ In the light of modern psychology, they were emot,ionally maladjusted, and should be psychoanalyzed. On the basis of the experimental evidence just cited, he reasoned that a cross of Scotch people, having tall constitutions, with adequate viscera for their size, with Southern Italians, with adequate viscera for their size, would yield gross disharmonies. That is, he mentioned offspring whose stature would be large, and whose internal organs would be insufficient to serve the large frame, or the reverse condition where there would be an excess of circulatory apparatus for a small body. Dealing with a problem explained differently by most orthois more striking than the regular dental arcades dontists, he observed : “Nothing commonly seen in the skulls of inbred native races and the irregular dentitions of many children of the tremendously hybridized American.” Davenport later
Wendell
L. Wylie
studied actual physical dysplasias; with Steggerdas he published the excellent monograph, ‘ ‘ Race-Crossing in Jamaica, ” which reported disproportion between limb length and body length in crosses of whites, blacks, and mulattoes. The dentist who reads the above might be confused if he turned to one of his own journals to find a geneticist4 quoting his colleague, Castle: “Crosses made between the largest and smallest known races of rabbits, one weighing three times as much as the other, produced crossbreeds which were remarkably vigorous and prolific, intermediate in size between the two parent races. The inheritance of size depends on factors which are general, affecting all parts of the body. A rabbit of large weight also had a long skull, long ears, and long leg bones to match the great weight, and a rabbit with short ears was invariably of small general size. We may dismiss as groundless the fear that a mixed human race will necessarily contain physical features of unbalanced proportions.” Hrdlicka,5 the anthropologist, had much the same point of view, for he wrote : “The extremes of hybridism occur among animals, especially domesticated animals. Yet the results of unions between the horse and the ass, and those of widely different cattle, dogs, cats, and poultry are not conspicuous for malocclusions. And the same is true of extreme human hybridism, as that of the white and the Negro. One is forced to assume that so far as the jaws and teeth are concerned, they are inherited in all these cases not in dissociation, but as a compound unit character.” By now, our dentist, embarked in his hit-or-miss reading program, turns to a familiar dental name. J. Leon Williams is known to dentists through the pamphlets of supply houses, and from lectures in freshman prosthetics. The general impression is that Williams believed in the closest sort of harmony between fa.ce form and tooth form, so that even if one never saw the natural teeth of a denture patient, one could be guided by the form of the face in selecting teeth which would have a form identical with the lost natural teeth. What he actually established was a plan for the selection of artificial teeth which would fashion an harmonious blending of artificial teeth and the face, a unity which might, in some cases, be more harmonious with respect to form and line than the natural teeth. Let Williams explain it himself6 : “Such a basis of procedure produces artificial dentures more nearly in perfect harmony with the face in many cases, than were the natural teeth of the individual. The laws of heredity as exhibited in mixed races rarely permit a perfect harmony in the different features of the body . . . the oft-repeated statement that nature always produces teeth in harmony with face and feature is a mistake due to lack of careful observation . . . is there anything more evident to us as we walk up and down the streets. . . than the incongruities of nature? ” This quotation from Williams was written in 1914; it is particularly interestin g when it is read along with articles he published in 1911. At that earlier date he was getting ready to formulate his system; he unmistakably declared that there was harmony in facial lines and tooth form, and stated that he had experiments underway to demonstrate it. Three years later he was obliged by the force of evidence collected on skulls and human material, to change his point of view.
Heredity
and the Orthodontist
157
So much for the consideration of type. It is apparent that most investigators have recognized a certain amount of disharmony or dysplasia in the morphology of human faces. The orthodontic field has produced some contributions on hereditary dental conditions : Ashley-Montagu, Baker, Kelberg, Lunstrom, and many others have reported isolated cases of twins showing remarkable similarity of tooth arch form, occlusion, etc. Korkllaus,7 Bachrach and Young,* Goldberg,Q and Kantorowicz have reported on their various larger groups of twins, again reporting strong resemblances. In some cases, both twins had local factors in common. In other twins, where mirror-imaging occurred in other parts of the body, mirror-imaging also occurred in dental conditions. Kantorowicz, Terwoe, and Korkhaus suggest that the midline diastema is a Mendelian dominant. Korkhaus says that Angle’s Class III is a Mendelian dominant; Iwagaki,‘O on the other hand, finds that it is a Mendelian recessive. Case I1 the pioneer orthodontist, believed in the hereditary disproportion between booth material and the size of the supporting bases; the “bi-maxillary protrusions” he described were thought to be occasioned by the forcing of largesized teeth into the arch mesial to the ramus, causing an anterior tipping of the teeth. DeweyI said it was impossible to accept the idea of large teeth from one parent and small jaws from another; Angle dismissed it as absurd. To my knowledge, no one has ever provided conclusive proof that such a thing could not happen; at the same time, it does not cover enough of the cases, and it ignores too many possible etiological agents of equal probability. There is a far more acceptable reason for prejudice against the easy acceptance of heredity as a cause of malocclusion. I refer to the tendency of some to confuse the term “hereditary” with the concept of “too difficult to treat.” An otherwise excellent investigation on identical twins done some time ago includes in the discussion this philosophy: since the investigator had proved to his own satisfaction, at least, that heredity is the principal determining factor in malocclusion, we could accordingly expect complete success in few cases. The fact that genetic constitution has established a deformity in no way demonstrates that tooth movement shall be difficult, or that relapse is inevitable. If heredity is the determining factor in so many cases, and it in itself is an obstacle to successful treatment, how can our friend explain the fact that some operators get preponderately good results, while others do so uniformly poor work. The dazzling sun of heredity shines as brightly on one side of the town as it does on the other. Heredity cannot be used as a convenient wastebasket, into which we may drop all the numerous problems which come our way. If we are interested in entirely objective investigations of heredity, supported by a plausible amount of evidence, we may turn to animal or plant experimentation. One of the most outstanding contributions in the field of animal biology was reported in 1941 by Stockard.13 Stockard, anatomist, geneticist, and dog lover, gathered together on an experimental farm in New York State
158
Wendell
L. Wylie
thousands of dollars worth of pedigreed dogs, all registered with the American To work with him he inKennel Club, and each one with unsullied ancestry. vited physiologists, geneticists, histologists, and an orthodontist. When a thorough study of the purebreds was completed, the most diverse sorts of crosses were made, and the progeny of several generations were examined, Salukis, long-haired dogs with the same body build as the more familiar greyhound, were mated to dachshunds; massive St. Bernards were crossed with ridiculous lap dogs, and backcrosses of all these strange combinations were made. Certain abnormalities of dogs which are known to be inherited : the achondroplastic splaylegs of the dachshund and the basset hound, and the defective cranial base of bulldogs, were examined for Mendelian behavior. In a great variety of ways it was clearly demonstrated that in crosses of extreme types of dogs, marked disharmonies could be produced. Excess amounts of skin tissue, and disproportionately large tongues were seen to occur, with marked crowding of teeth in a great Dane-Pekinese cross. Such manifestations of the operation of heredity led Stockard to these conclusions : 1. In size and form the cranium and the face are independent. 2. There is lack of correlation between the upper face and the mandible. A short palate does not necessarily signify a short mandible, nor does a short mandible signify a short palate. 3. The size of the dental arch shows a far greater fluctuation than does the size of the teeth. (Bony supporting structures seem to be Where the snout is short, the teeth are too modified more than teeth.) large for the jaw-crowding, rotation. 4. In these hybridization studies, it made no difference which parent was male or female. There appears to be no sex-linkage in the characters with which we are concerned. 5. The majority of the characters of the bulldog skull are undoubtedly recessive in crosses with long-snouted dogs. No matter to what degree I admire the resourcefulness and thoroughness which mark this study by Stockard and his associates, my own knowledge of orthodontic anomalies will not permit me to accept the interpretations made ,with regard to human malocclusion. Another comparison which was made was the bulldog and the human-class III malocclusion. The bulldog has an inherited chondrodystrophy of the anterior cranial base, which prevents the normal forward growth of the upper face. The cranial base of the Class III patient is normal in its morphology. A rare pathologic condition known as craniofacial dysostosis, or “lower skull, ” is more analogous, for the same area is affected. It is accompanied by orthodontic complications of an extreme sort, but they do not resemble the Class III case, and One may grant the superthey are plainly incidental to the cranial pathology. ficial resemblance in both the Class II and the Class III, but no more than that. In my previous paper, I mentioned that it was fallacious immediately to assume that a dentofacial anomaly is caused by some condition frequently found with it. This may well be true, but no one can be criticized for wanting an explanation as to why such associations may be observed.
Hereclity
and the Orthodofztist
159
A phenomenon which geneticists call linkage is known to account for the frequent association of certain characters in experimental animals, and while in the strictest sense it has never been proved as a mechanism in human inheritance, it is certainly not unreasonable to assume that it operates in man, and that if he were not so refractory a subject of investigation, that proof would be possible. Linkage may be cxplaincd in this way: The individual units involved in t,he inheritance of characters are called genes and are strung on the chromosome something like beads on a string. Genes closest together are least likely to become separated in the distribution of chromatin material, and consequently characters determined by those genes t,end t,o be observed in association. If we keep in mind t,hat positive proof of the points has yet to be produced, we may speculate as to how t,raits observed in association might be satisfactorily explained without resortin g to the more obvious cause and effect relationship. For instance, two physical traits freqncntly seen together might be said to be associated through gcnet,ie linkage. Iq’urthermore, susceptibility to two or more physical ailments might be found in one individual through the operation of the same mechanism, or a physical trait might be associated with a specific disease susceptibility on the same basis. One of the most interesting esplorations in the relationship between morphologic charact,crs and disease is the series of studies conducted by DraperI and his associates at the Presbyterian Hospital in New York. They have established that the Ion g and lean among us are more susceptible to pernicious anemia and to tuberculosis, while the stockier t,ypes are more commonly laid low by gall bladder trouble and cardiovascular involvements. Those who love to find causative relationships could, I suppose, produce a very plausible tie-up bctwecn the ailment and t,hc physical trait. I defy them to explain, however, how a gonial angle of little obtusity can produce gall bladder disease, or how gall bladder disease may render any gonial angle less obtuse. It is, nevertheIcss, a fact that not only :I characteristic angle of 111~jaw but a typical dental arch form is found in gall bladder patients. It is not unreasonable to assume, but fairly difficult to prove, that the findings of Draper may be explained on the basis of linkage. Are those who are so quick to provide the etiological basis for t,he various jaw anomalies sufficiently aware that the alleged etiolopical factor and the anomaly it is said to produce may be related only in the germ plasm? I am afraid they are not. Instances where the causes of dentofacial anomalies are alleged but never quite proved are legion. In order to document my case, I shall have to commit a few heresies. First of all, I can cite several rcfcrcnces wherein are stated that respiratory allergies and other stock items on the rhinologist’s shelf cause what the orthodontist,s call Class II malocclusion and what the public calls “Andy Gump accept and find greatly interesting the reports that chin. ’ ’ I wholeheartedly this type of malocclusion is found in patients with those afflictions, although I must inform you that there are many individuals who require the attentions of the otorhinolaryngologist who have never found it necessary to consult the orthodontist across the hall. A sufficient amount of unimpeachable research has been done on normal and abnormal facial growth to make it clear that
160
Wendell
L. Wylie
ordinary ill health will not affect the facial proportions during postnatal life, although it may affect the ultimate size of the face. Accordingly, I submit once again that linkage is the most plausible explanation for the association between the retruded mandible and respiratory involvement. With the passage of each year, the importance of inheritance in human welfare is more clearly recognized. The fact that allergies clearly run in families has been documented. Stammering and left-handedness, both of which follow family lines, are known to be associated, although how much of this is’ linkage and how much is neurophysiology remains to be shown. It has also been found that stammering affects pairs of identical twins nearly twice as often as it affects pairs of fraternal twins. Identical twins are affected similarly by tuberculosis from three to six times as frequently as are fraternal twins, although environmental factors are identical. As long ago as 1921, Hooton15 pointed out that the hard parts of the human body, the bones and teeth, are relatively stable as compared with the soft parts, and therefore excellent subject matter for the investigation of inherited traits. This is a challenge which dentists might well accept. There are some technical difficulties which complicate the task, all too well known to me, which need not be enumerated here. There is one consummation devoutly to be wished, however, for which I call. This is a somewhat more sophisticated attitude on the part of our profession, involving the following points: 1. The realization that “Nature” has no particular plan for the individual organism, and therapy consists of more than restoring the derailed patient to the track. HugheP has said this before me, but it bears repeating. 2. The realization that many of the things which we seek to remedy are not pathologies, but merely individual variations which we may not happen to like. This in no way militates against our treating these conditions if we are able to do so. 3. The realization that inheritance governs trivial things as well as gross conditions. 4. The realization that “congenital” and “inherited” are not synonymous terms. 5. The recognition that a patient is a part of his family-emotionally, economically, and as a creature subject to disease and morphologic modification. In the realms of medicine and dentistry, no man stands alone. REFERENCES The Varieties of Human Physique, New York, 1. Sheldon, W. H., and Tucker, W. B.: 1940, Harper & Brothers. Proc. Am. Phil. Sot. p. 364, 2. Davenport, Chas. B.: The Effects of Race Intermingling, 1917. No. 395 3. Davenport, C. B., and Steggerda, M.: Race-Crossing in Jamaica, Publication of the Carnegie Inst. of Wash., 1929. 4. Guyer, M. F.: Abnormalities of the Teeth and Jaws From Standpoint of Inheritance, INT. J. ORTHODONTIA 10: 323-332, 1924. 5. Hrdlicka, Ales.: The Causes of Malocclusion (Heredity and Other Causes), Dental Cosmos 64: 489, 1922. Temperamental Selection of Artificial Teeth, Dental Digest 20: 6. Williams, J. Leon: 63, 125, 185, 243, 305, 1914.
Heredity
and the Orthodontist
161
Anthropologic and Odontologic Studies of Twins, INT. J. ORTHODONTIA 7. Rorkhaus, G.: 16: 640, 1030. 8. Bachrach, Henriette and Young, Matthew: A Comparison of the Degree of Resemblance of Dental Characters Shown in Pairs of Twins of Identical and Fraternal Types,. Dental Cosmos 70: 463, 1928. 9. Goldberg, S.: Brometrics of Identical Twins From the Dental Viewpoint, J. Dent. Research 9: 363, 1929. 10. Iwagaki, H.: Hereditary Influence of Malocclusion. I. Statistical Studies on the Heredity of Progenia, INT. J. ORTHODONTIA 24: 3ZS, 1038. 11. Case, Calvin S.: Laws of Biology Regarded as Etiological Factors in Malocclusion, INT. J. ORTHODONTIA 7: 212, 1921. 12. Dewey, Martin: Practical Orthodontia, ed. 3, St. Louis, 1917, C. V. Mosby Co. 13. Stockard, Chas. R.: The Genetic and Endocrinic Basis for Differences in Form and Behavior, Philadelphia, 1941, Wistar Institute. 14. Draper, Geo.: The Relation of Face, Jaws, and Teeth to Human Constitution and Its Bearing on Disease, Dental Cosmos 77: 820-840, 1935. 15. Hooton, E. A.: Scientific Papers of the Second International Congress of Eugenics, 1921, 2: 64, 1923. 16. Hughes, B. 0.: Nature’s Plan and Orthodontics, A&r. J. ORTHODONTICS AND ORAL BURG. 31: 3G0, 1013.
WENDELL
AND
L. WYLIE,
THE
ORTHODONTIST-A CONFUSION
STUDY
IN
D.D.S., M.S.,* SAN FRANCISCO, CALIF.
T
HE subject I am to discuss is confusing as it is intriguing, and no small part of the confusion which the subject of heredity holds for the uninitiated is due to the fact that we use, in describing the transmittal of biologic entities, the same word “inheritance” to chronicle the transfer, en famille, of goods and chattels. Since every one of us has to make the long climb to the adult state from nothing more than a fertilized egg, an object slightly smaller than one of the periods on this printed page, it must be evident that the attributes we observe in our fellows are not inherited per se, but only the tendency to produce them. Yet it is a fact that the tiny egg and the still smaller sperm cell are the only cells having any continuity in the genealogical tree. This will be no lesson in genetics, but instead an examination of the dental and related literature, in an effort to discover to what extent the dentist has considered the long-discussed matter of nature vs. nurture, or heredity vs. environment. The first fact we discover is that not only dentists but a good many others as well, are, if not actually opposed to the idea of considering heredity in relation to health, quite willing to let the subject die of neglect. This is shown in the almost universal search for environmental factors to explain the misfortunes which beset human flesh: dietary sins of omission and commission from the natal day onward, long forgotten bouts of illness in childhood, which, nevertheless, are said to leave their mark as dentofacial anomalies, and various and sundry other factors which take no account of inherent potentialities of the organism. It is as if we wish to extend our commendable political doctrine that every little boy born in the United States of America has the right to become president, and make it mean that every little boy begins at birth with the potential for having thirty-two evenly aligned teeth which he may keep as long as he keeps the spark of life. It is unfortunate that such a noble concept should have so little to support it. Before considering the main topic of heredity and facial form, we should turn to a related subject, the matter of constitution, variously known as body build or type. It is important because it is tied up with the matter of disproportion between various parts, something frequently argued in dental circles. The matter of type is pertinent here because every student of type or constitution must face the problem of consistency or harmony in one individual of Palm
Read before the Third Annual Seminar for the Springs, Calif., Oct. IO, 1946. *University of California, College of Dentistry,
154
Study San
and Francisco,
Practice Calif.
of
Dental
Medicine.
Heredity
and the Orthodontist
155
a given type. We can readily recognize a person as being lean and linear, or muscular and well built, or round and chubby, and we may choose to describe him as leptosomatic, a,thletic, or pyknic, if we remember our college textbooks that well. Can we, however, take that individual feature for feature, dimension for dimension, and say that he is consistently leptosomatic, athletic, or pyknic in every respect “2 Actually, we find that many investigators of body build report a large degree of this sort of disharmony in constitution. In the recent work this disharmony is called dysplasia, and while of Sheldon and Tucker,l they wisely avoid any premature conclusions regarding inheritance, they do provide what they call their “D-index, ” a quantitative measure of the amount of dysplasia found in one individual. This reluctance on the part of Sheldon and Tucker to associate immediately the phenomenon of dysplasia with inheritance is not universal. Thirty years ago, Davenport? sought to interpret anthropometric data in the light of genetic principles, and wrote on the “intermingling of races,” which he viewed with alarm. His definition of race was a narrower one than most: “A more or less purebred ‘group ’ of individuals that differs from other groups by at least one character, or, strictly, a genetically connected group whose germ plasm is characterized by a difference, in one or more genes, from other groups. Thus blue-eyed Scotchmen belong to a different race from some of the dark Scotch." It is evident that when Davenport frowned on race-crossing, he did not mean marriage between Caucasians and Mongoloids, but merely the crossing of dissimilar individuals. These objections to race-crossing were on the basis of the extreme dysplasia and maladjustment consequent to it. Certain animal experiments led him to dire conclusions concerning human intermarriage. He crossed Leghorn chickens with Brahmas, with interesting results. As you know, the Leghorn is a “career girl”-she lays an egg a day quite consistently, but has no great inclination to hatch them. The Brahma, on the other hand, is a homebody that thinks about raising a family when a few eggs have been laid. When these chickens were crossed, Down on the farm, she was called “broody.” the first generation produced fairly good layers who would hatch eggs, but abandon the chicks early, having lost interest. The next generation, however, were complete failures as either layers or mothers. To quote Davenport : “The instincts and functions of the hybrids were not harmoniously adjusted to each other. ’ ’ In the light of modern psychology, they were emot,ionally maladjusted, and should be psychoanalyzed. On the basis of the experimental evidence just cited, he reasoned that a cross of Scotch people, having tall constitutions, with adequate viscera for their size, with Southern Italians, with adequate viscera for their size, would yield gross disharmonies. That is, he mentioned offspring whose stature would be large, and whose internal organs would be insufficient to serve the large frame, or the reverse condition where there would be an excess of circulatory apparatus for a small body. Dealing with a problem explained differently by most orthois more striking than the regular dental arcades dontists, he observed : “Nothing commonly seen in the skulls of inbred native races and the irregular dentitions of many children of the tremendously hybridized American.” Davenport later
Wendell
L. Wylie
studied actual physical dysplasias; with Steggerdas he published the excellent monograph, ‘ ‘ Race-Crossing in Jamaica, ” which reported disproportion between limb length and body length in crosses of whites, blacks, and mulattoes. The dentist who reads the above might be confused if he turned to one of his own journals to find a geneticist4 quoting his colleague, Castle: “Crosses made between the largest and smallest known races of rabbits, one weighing three times as much as the other, produced crossbreeds which were remarkably vigorous and prolific, intermediate in size between the two parent races. The inheritance of size depends on factors which are general, affecting all parts of the body. A rabbit of large weight also had a long skull, long ears, and long leg bones to match the great weight, and a rabbit with short ears was invariably of small general size. We may dismiss as groundless the fear that a mixed human race will necessarily contain physical features of unbalanced proportions.” Hrdlicka,5 the anthropologist, had much the same point of view, for he wrote : “The extremes of hybridism occur among animals, especially domesticated animals. Yet the results of unions between the horse and the ass, and those of widely different cattle, dogs, cats, and poultry are not conspicuous for malocclusions. And the same is true of extreme human hybridism, as that of the white and the Negro. One is forced to assume that so far as the jaws and teeth are concerned, they are inherited in all these cases not in dissociation, but as a compound unit character.” By now, our dentist, embarked in his hit-or-miss reading program, turns to a familiar dental name. J. Leon Williams is known to dentists through the pamphlets of supply houses, and from lectures in freshman prosthetics. The general impression is that Williams believed in the closest sort of harmony between fa.ce form and tooth form, so that even if one never saw the natural teeth of a denture patient, one could be guided by the form of the face in selecting teeth which would have a form identical with the lost natural teeth. What he actually established was a plan for the selection of artificial teeth which would fashion an harmonious blending of artificial teeth and the face, a unity which might, in some cases, be more harmonious with respect to form and line than the natural teeth. Let Williams explain it himself6 : “Such a basis of procedure produces artificial dentures more nearly in perfect harmony with the face in many cases, than were the natural teeth of the individual. The laws of heredity as exhibited in mixed races rarely permit a perfect harmony in the different features of the body . . . the oft-repeated statement that nature always produces teeth in harmony with face and feature is a mistake due to lack of careful observation . . . is there anything more evident to us as we walk up and down the streets. . . than the incongruities of nature? ” This quotation from Williams was written in 1914; it is particularly interestin g when it is read along with articles he published in 1911. At that earlier date he was getting ready to formulate his system; he unmistakably declared that there was harmony in facial lines and tooth form, and stated that he had experiments underway to demonstrate it. Three years later he was obliged by the force of evidence collected on skulls and human material, to change his point of view.
Heredity
and the Orthodontist
157
So much for the consideration of type. It is apparent that most investigators have recognized a certain amount of disharmony or dysplasia in the morphology of human faces. The orthodontic field has produced some contributions on hereditary dental conditions : Ashley-Montagu, Baker, Kelberg, Lunstrom, and many others have reported isolated cases of twins showing remarkable similarity of tooth arch form, occlusion, etc. Korkllaus,7 Bachrach and Young,* Goldberg,Q and Kantorowicz have reported on their various larger groups of twins, again reporting strong resemblances. In some cases, both twins had local factors in common. In other twins, where mirror-imaging occurred in other parts of the body, mirror-imaging also occurred in dental conditions. Kantorowicz, Terwoe, and Korkhaus suggest that the midline diastema is a Mendelian dominant. Korkhaus says that Angle’s Class III is a Mendelian dominant; Iwagaki,‘O on the other hand, finds that it is a Mendelian recessive. Case I1 the pioneer orthodontist, believed in the hereditary disproportion between booth material and the size of the supporting bases; the “bi-maxillary protrusions” he described were thought to be occasioned by the forcing of largesized teeth into the arch mesial to the ramus, causing an anterior tipping of the teeth. DeweyI said it was impossible to accept the idea of large teeth from one parent and small jaws from another; Angle dismissed it as absurd. To my knowledge, no one has ever provided conclusive proof that such a thing could not happen; at the same time, it does not cover enough of the cases, and it ignores too many possible etiological agents of equal probability. There is a far more acceptable reason for prejudice against the easy acceptance of heredity as a cause of malocclusion. I refer to the tendency of some to confuse the term “hereditary” with the concept of “too difficult to treat.” An otherwise excellent investigation on identical twins done some time ago includes in the discussion this philosophy: since the investigator had proved to his own satisfaction, at least, that heredity is the principal determining factor in malocclusion, we could accordingly expect complete success in few cases. The fact that genetic constitution has established a deformity in no way demonstrates that tooth movement shall be difficult, or that relapse is inevitable. If heredity is the determining factor in so many cases, and it in itself is an obstacle to successful treatment, how can our friend explain the fact that some operators get preponderately good results, while others do so uniformly poor work. The dazzling sun of heredity shines as brightly on one side of the town as it does on the other. Heredity cannot be used as a convenient wastebasket, into which we may drop all the numerous problems which come our way. If we are interested in entirely objective investigations of heredity, supported by a plausible amount of evidence, we may turn to animal or plant experimentation. One of the most outstanding contributions in the field of animal biology was reported in 1941 by Stockard.13 Stockard, anatomist, geneticist, and dog lover, gathered together on an experimental farm in New York State
158
Wendell
L. Wylie
thousands of dollars worth of pedigreed dogs, all registered with the American To work with him he inKennel Club, and each one with unsullied ancestry. vited physiologists, geneticists, histologists, and an orthodontist. When a thorough study of the purebreds was completed, the most diverse sorts of crosses were made, and the progeny of several generations were examined, Salukis, long-haired dogs with the same body build as the more familiar greyhound, were mated to dachshunds; massive St. Bernards were crossed with ridiculous lap dogs, and backcrosses of all these strange combinations were made. Certain abnormalities of dogs which are known to be inherited : the achondroplastic splaylegs of the dachshund and the basset hound, and the defective cranial base of bulldogs, were examined for Mendelian behavior. In a great variety of ways it was clearly demonstrated that in crosses of extreme types of dogs, marked disharmonies could be produced. Excess amounts of skin tissue, and disproportionately large tongues were seen to occur, with marked crowding of teeth in a great Dane-Pekinese cross. Such manifestations of the operation of heredity led Stockard to these conclusions : 1. In size and form the cranium and the face are independent. 2. There is lack of correlation between the upper face and the mandible. A short palate does not necessarily signify a short mandible, nor does a short mandible signify a short palate. 3. The size of the dental arch shows a far greater fluctuation than does the size of the teeth. (Bony supporting structures seem to be Where the snout is short, the teeth are too modified more than teeth.) large for the jaw-crowding, rotation. 4. In these hybridization studies, it made no difference which parent was male or female. There appears to be no sex-linkage in the characters with which we are concerned. 5. The majority of the characters of the bulldog skull are undoubtedly recessive in crosses with long-snouted dogs. No matter to what degree I admire the resourcefulness and thoroughness which mark this study by Stockard and his associates, my own knowledge of orthodontic anomalies will not permit me to accept the interpretations made ,with regard to human malocclusion. Another comparison which was made was the bulldog and the human-class III malocclusion. The bulldog has an inherited chondrodystrophy of the anterior cranial base, which prevents the normal forward growth of the upper face. The cranial base of the Class III patient is normal in its morphology. A rare pathologic condition known as craniofacial dysostosis, or “lower skull, ” is more analogous, for the same area is affected. It is accompanied by orthodontic complications of an extreme sort, but they do not resemble the Class III case, and One may grant the superthey are plainly incidental to the cranial pathology. ficial resemblance in both the Class II and the Class III, but no more than that. In my previous paper, I mentioned that it was fallacious immediately to assume that a dentofacial anomaly is caused by some condition frequently found with it. This may well be true, but no one can be criticized for wanting an explanation as to why such associations may be observed.
Hereclity
and the Orthodofztist
159
A phenomenon which geneticists call linkage is known to account for the frequent association of certain characters in experimental animals, and while in the strictest sense it has never been proved as a mechanism in human inheritance, it is certainly not unreasonable to assume that it operates in man, and that if he were not so refractory a subject of investigation, that proof would be possible. Linkage may be cxplaincd in this way: The individual units involved in t,he inheritance of characters are called genes and are strung on the chromosome something like beads on a string. Genes closest together are least likely to become separated in the distribution of chromatin material, and consequently characters determined by those genes t,end t,o be observed in association. If we keep in mind t,hat positive proof of the points has yet to be produced, we may speculate as to how t,raits observed in association might be satisfactorily explained without resortin g to the more obvious cause and effect relationship. For instance, two physical traits freqncntly seen together might be said to be associated through gcnet,ie linkage. Iq’urthermore, susceptibility to two or more physical ailments might be found in one individual through the operation of the same mechanism, or a physical trait might be associated with a specific disease susceptibility on the same basis. One of the most interesting esplorations in the relationship between morphologic charact,crs and disease is the series of studies conducted by DraperI and his associates at the Presbyterian Hospital in New York. They have established that the Ion g and lean among us are more susceptible to pernicious anemia and to tuberculosis, while the stockier t,ypes are more commonly laid low by gall bladder trouble and cardiovascular involvements. Those who love to find causative relationships could, I suppose, produce a very plausible tie-up bctwecn the ailment and t,hc physical trait. I defy them to explain, however, how a gonial angle of little obtusity can produce gall bladder disease, or how gall bladder disease may render any gonial angle less obtuse. It is, nevertheIcss, a fact that not only :I characteristic angle of 111~jaw but a typical dental arch form is found in gall bladder patients. It is not unreasonable to assume, but fairly difficult to prove, that the findings of Draper may be explained on the basis of linkage. Are those who are so quick to provide the etiological basis for t,he various jaw anomalies sufficiently aware that the alleged etiolopical factor and the anomaly it is said to produce may be related only in the germ plasm? I am afraid they are not. Instances where the causes of dentofacial anomalies are alleged but never quite proved are legion. In order to document my case, I shall have to commit a few heresies. First of all, I can cite several rcfcrcnces wherein are stated that respiratory allergies and other stock items on the rhinologist’s shelf cause what the orthodontist,s call Class II malocclusion and what the public calls “Andy Gump accept and find greatly interesting the reports that chin. ’ ’ I wholeheartedly this type of malocclusion is found in patients with those afflictions, although I must inform you that there are many individuals who require the attentions of the otorhinolaryngologist who have never found it necessary to consult the orthodontist across the hall. A sufficient amount of unimpeachable research has been done on normal and abnormal facial growth to make it clear that
160
Wendell
L. Wylie
ordinary ill health will not affect the facial proportions during postnatal life, although it may affect the ultimate size of the face. Accordingly, I submit once again that linkage is the most plausible explanation for the association between the retruded mandible and respiratory involvement. With the passage of each year, the importance of inheritance in human welfare is more clearly recognized. The fact that allergies clearly run in families has been documented. Stammering and left-handedness, both of which follow family lines, are known to be associated, although how much of this is’ linkage and how much is neurophysiology remains to be shown. It has also been found that stammering affects pairs of identical twins nearly twice as often as it affects pairs of fraternal twins. Identical twins are affected similarly by tuberculosis from three to six times as frequently as are fraternal twins, although environmental factors are identical. As long ago as 1921, Hooton15 pointed out that the hard parts of the human body, the bones and teeth, are relatively stable as compared with the soft parts, and therefore excellent subject matter for the investigation of inherited traits. This is a challenge which dentists might well accept. There are some technical difficulties which complicate the task, all too well known to me, which need not be enumerated here. There is one consummation devoutly to be wished, however, for which I call. This is a somewhat more sophisticated attitude on the part of our profession, involving the following points: 1. The realization that “Nature” has no particular plan for the individual organism, and therapy consists of more than restoring the derailed patient to the track. HugheP has said this before me, but it bears repeating. 2. The realization that many of the things which we seek to remedy are not pathologies, but merely individual variations which we may not happen to like. This in no way militates against our treating these conditions if we are able to do so. 3. The realization that inheritance governs trivial things as well as gross conditions. 4. The realization that “congenital” and “inherited” are not synonymous terms. 5. The recognition that a patient is a part of his family-emotionally, economically, and as a creature subject to disease and morphologic modification. In the realms of medicine and dentistry, no man stands alone. REFERENCES The Varieties of Human Physique, New York, 1. Sheldon, W. H., and Tucker, W. B.: 1940, Harper & Brothers. Proc. Am. Phil. Sot. p. 364, 2. Davenport, Chas. B.: The Effects of Race Intermingling, 1917. No. 395 3. Davenport, C. B., and Steggerda, M.: Race-Crossing in Jamaica, Publication of the Carnegie Inst. of Wash., 1929. 4. Guyer, M. F.: Abnormalities of the Teeth and Jaws From Standpoint of Inheritance, INT. J. ORTHODONTIA 10: 323-332, 1924. 5. Hrdlicka, Ales.: The Causes of Malocclusion (Heredity and Other Causes), Dental Cosmos 64: 489, 1922. Temperamental Selection of Artificial Teeth, Dental Digest 20: 6. Williams, J. Leon: 63, 125, 185, 243, 305, 1914.
Heredity
and the Orthodontist
161
Anthropologic and Odontologic Studies of Twins, INT. J. ORTHODONTIA 7. Rorkhaus, G.: 16: 640, 1030. 8. Bachrach, Henriette and Young, Matthew: A Comparison of the Degree of Resemblance of Dental Characters Shown in Pairs of Twins of Identical and Fraternal Types,. Dental Cosmos 70: 463, 1928. 9. Goldberg, S.: Brometrics of Identical Twins From the Dental Viewpoint, J. Dent. Research 9: 363, 1929. 10. Iwagaki, H.: Hereditary Influence of Malocclusion. I. Statistical Studies on the Heredity of Progenia, INT. J. ORTHODONTIA 24: 3ZS, 1038. 11. Case, Calvin S.: Laws of Biology Regarded as Etiological Factors in Malocclusion, INT. J. ORTHODONTIA 7: 212, 1921. 12. Dewey, Martin: Practical Orthodontia, ed. 3, St. Louis, 1917, C. V. Mosby Co. 13. Stockard, Chas. R.: The Genetic and Endocrinic Basis for Differences in Form and Behavior, Philadelphia, 1941, Wistar Institute. 14. Draper, Geo.: The Relation of Face, Jaws, and Teeth to Human Constitution and Its Bearing on Disease, Dental Cosmos 77: 820-840, 1935. 15. Hooton, E. A.: Scientific Papers of the Second International Congress of Eugenics, 1921, 2: 64, 1923. 16. Hughes, B. 0.: Nature’s Plan and Orthodontics, A&r. J. ORTHODONTICS AND ORAL BURG. 31: 3G0, 1013.