An analysis of the inheritable morphological traits of the ......OF THE MANDIBULAR SECOND PREMOLAR by Fred Ludwig A THESIS submitted to the faculty of the Department of Anthropology

An analysis of the inheritable morphologicaltraits of the mandibular second premolar

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Authors Ludwig, Fred Jerome, 1916-

Publisher The University of Arizona.

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AN ANALYSIS OF THE INHERITABLE MORPHOLOGICAL TRAITS OF THE MANDIBULAR SECOND PREMOLAR

byFred Ludwig

A THESIS submitted to the faculty of the Department of Anthropology

in partial fulfillment of the requirements for the degree ofMASTER OF ARTS

in the Graduate College, University of Arizona

1953

Approved: /(? / # 5 3Director of Thesis D(

^ 9 7 ?/

This thesis has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment is made. Requests for permission for extended quotation from, or reproduction of this manuscript in whole or in part, may be granted by the head of the major department or the dean of the Graduate College when in their judgment the pro­posed use of the material is in the interests of scholar­ship. In all other instances, however, permission must be obtained from the author.

SIGNED:

ill

ACKNOWLEDGMENTS

For the suggestion of this particular thesis problem on certain aspects of the human dentition# the author is deeply ghateful to Dr» Bertram S« Kraus of the Department of Anthropology# University of Arizona, Without the valuable assistance and guidance of Dr, Kraus the path to completion of this study would have been considerably more arduous.The author wishes to make clear beyond question the fact that# although Dre Kraus sketched the broad outlines of the problem# any defections of observation# technique# or in­terpretation are solely the responsibility of the writer.To Dr. Kraus go also the writer's thanks for photographic assistance freely given.

The writer is indebted to Dr. Emil S. Haury# Head of the Department of Anthropology# University Of Arizona# for his consideration and helpfulness in the matter of provid­ing the writer access to the collections of dental and skeletal material maintained by the Laboratory of Physical Anthropology.

To the members of the writer's,thesis committee#Dr. Edward B. Danson and Mr. Daniel Matson# go sincerest thanks for friendly interest and encouragement in this study.

The writer is singularly indebted to Mr. James House­holder# Department of Mathematics# University of Arizona for assistance in the statistical analysis of the data.

TABLE OF COMTENTS

L̂LL̂LSi F Gr-i-lf O O O ' O O O ee o o o o d o o o o o e o - o o %L PERMISSION TO QUOTE iiACKNOWLEDGMENTS » » * . = . . .. . . . illTABLE OF CONTENTS o . » » . Q » . » . » . e » ivLIST OF TABLES o o o o o o o o @ o o © » a © © © © o © vLIST OF ILLUSTRATIONS

FZL^l%I*G S o o o o e o o o o o o o o o © © o O © 0 V IP lSi*fc©S O 0 - 0 O O O 6 0 0 0 O 6 0 0 0 O O « O O O V I 1

CHAPTER I INTRODUCTION © © .. © . © © © . © © . . . 1II REVIEW .OF THE LITERATURE = © . © . © . © © 9III DATA 0 . 0 © 0 © © ■ © - O © © 6 O O © O © © O © 15IV METHODOLOGY © © © . © © © . © © © © © . © 18V THE TRAITS: DEFINITION, ILLUSTRATION,

AND DISTRIBUTION © © © © © © © © © © © © © 23VI TWIN STUDY: STATISTICAL TREATMENT AND

RESULTS © © © © © © © © © © © © © © & © © 39VII DISCUSSION: PART I© DIFFERENTIAL ETHNIC

FREQUENCY © © © © © © © © © © © © ©. © © © 44DISCUSSION: PART II. THE.TWIN STUDY © © 51

VIII CONCLUSIONS © © © © © © © © . © © © © © © 54LITERATURE CITiiD© © © © © © © © © © © © © © © © © © © 55

LIST OF TABLES.■- ■ '* . Page

TABLE I - Trait 1 Relative Frequency by Aspect 24TABLE II - Trait 2 Relative Frequency,by Aspect , ,26TABLE III - Trait; 3 Relative Frequency by Aspect; . .28TABLE IV - Trait 4 • Relative Frequency,by Aspect,, „ 30TABLE V - Trait 5 Relative Frequency by Aspect 32TABLE VI - Trait 6 Relative Frequency by Aspect 1 34

's " * * V - V - v! I " 4 "

TABLE VII - Trait 7 Relative Frequency by Aspect 36TABLE VIII Trait 8 Relative Frequency by Aspect *38TABLE IX - Aspect Frequencies and Expected

Concordances .i,., i e . 0. Q , 39TABLE X - Observed Relative Frequencies of.Con- »

cordance in Monozygotic Twin Pairs 0 • . 4lTABLE XI - Observed Relative Frequencies of, „

Concordance in Dizygotic Twin Pairs 43TABLE XII - Differential Frequency of Trait Aspects

in Four Populations „ . « 0 « V 47

vi

LIST OF ILLUSTRATIONS (FIGURES)

PageFIGURE 1

The Relative Sizes of Mesial and Distal 5ossae & © © © © © © © © © ® © © © © © © © © © 2

FIGURE 2The Occlusal Ridges of The Buccal Cusp © © © © 25

FIGURE 3The Median Occlusal Ridge of the Buccal Cusp © 27

FIGURE 4The Relative Position of The Lingual Cusp » © 29

FIGURE 5 , - f - - • . ,The Number of Lingual Cusps © © © © © © © © © 31

FIGURE 6The Independence of The Lingual Cusp © © © © © 33' •

FIGURE 7The Position of Multiple Lingual Cusps © © © © 35

FIGURE 8The Sagittal Sulcus © © © © © © »© = © © © © 37

vi I

LIST OF ILLUSTRATIONS (PLATES) ,

Following PagePLATE lo

Monozygotic • Twin Pairs of Ma,ndibular '■Second Premolars . . » 0 «, » » : «, 6 » . „ • s » e 53

PLATE 2.Monozygotic Twin Pairs of Mandibular ■Second Premolars . e . 53

PLATE 3- ■Dizygotic Twin Pairs of Mandibular Second Premolars. .. . . @ . . . . . . . . . © © . . 53

PLATE 4<Dizygotic Twin Pairs of Mandibular Second Premolars . © © . . . . © . . . . . . . & ■ 53

CHAPTER I

INTRODUCTION

An examination ̂orf-- the current literature on human tax­onomy discloses in many cases an Inclination toward an attitude of hopeful expectance with regard to any classifi­cation established upon a genetic basis* As representative of this attitude relative to human serological traits Kroeber states % •' 1 . , »

vV *

- With all these genetically definite blood serum types turning up, the prospect improves that we may ultimately be able to distinguish races at least partly by their genetic constitutions as well as by the hitherto customary description of their phenotypes * With only the A-B-O- genetics known, paternity could be determined, or rather eliminated, in only a minority of cases* But with the Ai - Agf the M-N, the eight Rh types, and the secretion genetics also available, the determin­ation is now possible, at least theoretically, in perhaps most cases* Turned around, the same applies to race distinctions*!Ashley-Hontagu generalizes on the application of the

genetic method to.the problem of human variability as fol­lows : !

In recent years a new instrument of Investi­gation has become available in the form qf the genetic method of analysis* The application of such genetic methods to the solution of the prob­lems of human variability has already made some­thing of a beginning, and holds great promise for

1. A* L* Kroeber, 1948, p* l63»

the future o The investigation, and analysis of the mechanism of heredity has steadily yielded valu­able information concerning the individual as well as the race = By the use of genetic methods we may hope to solve the problems of the detailed causes of individual variation and the diversification of the different varieties of man'02Huxley* relative to the subject of developing a genetic

approach to the problems of racial analysis writes:At the moment* accurate gene-mapping could

only be achieved where the genetic basis is simple* e .g,: in cases of dimorph ratio* and then onlywhen we know which of the two genes is recessive* Analysis of blood-groups and the capacity to taste certain chemical substances in man* however* is re­vealing the existence of graded gene-distribution in our own species* though here matters are compli­cated and obscured by migration*

Within the sphere of purely phenotypic inves­tigation* the ideal would be to obtain sufficient data to be able to plot a contour map (phenocontour) instead of one or a few sectional gradients* the contours being isophenes or lines of equal develop­ment of the character measured*

By this means* taxonomy will have at its disposal a new and economical descriptive tool*And further* the data thus amassed will undoubtedly . shed light on problems of the differentiation of subspecies* their range-changes subsequent to differentiation* selection-pressure* the rate of spread of genes* and many other questions of general biological interest«3It is apparent from the foregoing statements that a

genetic approach to the problems of human variability would be of value in understanding the mechanisms of variation from individual to racial levels of the human species as

2* H» F* Ashley-Hontagu* 1951s PPl 6-10= 3= J* S« Huxley* 1939s PP- 516-517-.

3

well as a means of reconstructing the phylogenetic history of man. '

Shortcomings in a- genetic approach .to an understanding of individual and racial differentiation do not lie in the methods employed in obtaining data nor on the observations and conclusions based upon such data. Rather, the weakness appears to exist in the paucity of the available data with which the geneticist must works: These data consist of observable anatomical and physiological variations, the frequencies of which may vary from population to population*

At the present, excluding the pathological syndromes for reasons of their nature and rarity, the list of the human traits whose genetic mechanisms are clearly understood is quite short. Such traits operate by the simple Mendelian method of inheritanoe. Unfortunately for the simplifica­tion of any genetic solution to the problems involved, most human traits do not operate in such a simple manner. Onthe contrary it appears that several different genotypes, or

kcombinations of genes, may produce the same phenotype. In such cases the usefulness of the genetic method is vitiated inasmuch as we have at present no methods for distinguishing these several genotypes.

4. G. Wo Lasker, 1950,. Vol. XV, p. 192.

' The following discrete phenotypic expressions have been demonstrated to operate by the simple Mendelian method according to Race5^ spuhler5,̂ Fox,? Schiff and Sasaki,& and Spuhler and Allen^; ' -

The A, Ag,, B, 0, Blood-groups^The M-N groups^The Anti-S5

The P groups^The Rh Blood groups^The Lutheran Blood groups^The Kell Blood groups^The Lewis Blood groups^The Duffy System^Patterns of the superficial veins of the anterior thorax^ "Peroneus Tertius muscle^lumber of vallate papillae^Taste for PTC?

8The secreting geneqPalmarus Longus muscle^

5« B.6 R, Race, 1950, Vol. XV, p» 20? ff-60 Ho Spuhler, 1950, Vol« XV, pp. 175 ff.7. Ao Lo Fox, 1932, Volo 18, ppo 115-120o80 F. Schiff and B. Sasaki, 1932, Volo 11, pp.- 1426-14299= Jo Ho Spuhler and Go Allen, 1950*

Thusj, in the present state of knowledge we are limited to a classification based upon a dozen or so human charac­teristics o This number is insignificant when compared with estimates of the total number of genic loci for the human species given by several authors. Boyd10'estimates thisnumber at 20^,000 to #2,000, Spuhler's estimate is the same

12as Boyd*s, and Stern estimates a range from 5,000 to 120,000.

notwithstanding this limitation, Boyd is able to state?The genetic differences between different in­

dividuals of the same race can often, even with our present limited knowledge of human genetics, be demonstrated to be as large as ten to twenty genes.A classification based upon characteristics which are

genetically determined should indicate, among other things,some of the evolutionary: processes and history of the humanrace. In this, regard Morant points out?

The main aim of physical anthropology is to unravel the course of human evolution and it may be taken for granted today that the proper study of the natural history of man is concerned essentially with the mode and path of his descent.13And Washburn prefaces a recent article in the same

vein?

W. C.zBoyd, 1952, p. 158.J. N. Spuhler, 19#8, Vol. 108, pp. 279-280. Surt Stern, 19#9, P» 29.G. H= Morant, 1939> Vol. 31, p. 72.

10.11.12.

13.

6

Recently, evolutionary studies have been revitalized and revolutionized by an infusion of genetics into.paleontology and systematics. The change is fundamentally one of point of view, which is made possible by an understanding of the way the genetic constitutions of populations change„ The new systematics is concerned primar­ily with process and with the mechanisms of evolu^ tionary changej whereas, the older point of view was chiefly concerned with sorting the results of evolution. Physical anthropology is now undergoing the same sort of change.1^Any anatomical variations, characteristics, or traits

which may be chosen as suitable for the purpose of con­tributing to an analysis of human genetics must satisfy several criteria. The following criteria are suggested:

1. Such traits should be objective, observable upon inspection of one kind or another.

2. They should be qualitative rather than quantitative.3. Their heritability must be demonstrable.4. They should be conservative characters, or "adapt­

ively neutral“, in the words of Dobzhansky.^5 By this ex­pression Dobzhansky means characters which are slightly, or not at all, modifiable and that they should have no determi­nable adaptive value, and not be subject to high rates of mutation.

The mandibular second premolar has been chosen as the subject of this study because of the fact that it satisfies

14. S. L. Washburn, 1951# Vol. 13# Ho. 7, p. 2 9 8.1 5 . Th. Dobzhansky, 1944, Vol. 2, p. 264.

7

the criteria enumerated» Variations of the pattern of the occlusal surface, may be identified and classified. These variations are qualitative. Data will be presented to demon­strate the heritability of certain of these variations. , The variations of the mandibular second premolar which have been observed are of such nature that the burden of proof must fall,upon those.who would assert that selective advantage is conferred by the possession of one or any combination of such traits. The ontogeny of the occlusal crown surface is such as to exclude the influence of exogenous environmental factors on the hereditary anlage.

The variations of the mandibular second premolar with particular regard to the patterns of its occlusal surface and to the number and arrangement of the cusps thereof appear to satisfy the requirements implied in the following state­ment by Boyd; >

Among the racial characters which we would be tempted to pick out at the present time as non- adaptive# there are certain serological features of the bloodj, such as the genes 0S A# B* H* N# etc^j many other charactersa such as, the direction of hair whorls, general body hairiness (probably), tooth cusp patterns, fingerprint patterns, etc., might be considered..-*-6 •The objectives of the present study are two-fold:

First, to isolate and define the common morphologicaltraits of the -occlusal crown surface of the mandibular

16. W. G. Boyd, 1952, p. 27o

8

second premolar; secondly# to demonstrate the herltabillty' of these traits through a statistical analysis of the dental casts of a number of identical twinse »

This study represents a point which is midway to the ultimate objective of determining genetic mechanisms oper­able in the human species. The necessary steps to gain this ultimate objective seem to be;

1, To isolate and define the traits,2, To demonstrate the herltabillty of the traits,3o To determine the independent or individual variabil­

ity of the traits.4, To determine the genetic mechanism responsible.The third and fourth steps exceed the scope of the

present study.

CHAPTER II

REVIEW OP THE LITERATURE*-

The details of the gross and finer structural modifica­tions of the occlusal crown surface of the mandibular second premolar have been matters of interest to a number of in­vestigators o Most observers note certain constant features in the morphological structures of the crown surface as well as the variability of these structures without describing these variations in sufficient detail for classificatory purposes c In fairness to the authors one should note their descriptions were not formulated with the objective in mind ofJanalyzing the relative frequency of the traits except in a few instanceso , .

Extracts from the literature on the subject of defini­tion of the morphological characteristics of' the lower second premolar will be presented» In each case a brief critique will follow:

Wortman writes:Inferior Premolars 0 Two strong cusps of

which the one is external and the other Internal, occupy the grinding face, and are separated by a deep notch or valley, deepest in the centre<, « * 0 The internal Vertical ridge of the external cusp joins the cingulum hear its central portion, leav­ing a deep pit upon either side „ „ « «, The degree to which this vertical ridge is developed is sub­jected to great variation. It may be almost

10

entirely absent in some individuals or strongly developed in others» „ . . The internal cusp fre­quently shows a tendency to form two„17Mortman refers to the buccal and lingual cusps5 the

sagittal sulcus, the median, ridge of the buccal cusp with its attendant variations, the mesial and distal fossae, and the subdivision of the lingual cusp« All of these struc­tural components are usable in developing a classification of the morphology of the tooth» What is lacking is a pre­cise statement of the explicit relationships which obtain between these various components <,

Black describes the tooth as follows:When the transverse ridge is high, only a

deep pit appears at either side, , The lingualcusp is divided by a sulcate groove, which runs over centrally, or nearly so, to the lingual sur­face, making a three cusped tooth, , , . The cen­tral groove is straight and generally sulcate with a deep pit at each end,18

Black observes the transverse ridge, the mesial and distal fossae, two and three cusps, and the sagittal sulcus both straight and interrupted. These formations and their relationships to each other are not satisfactorily defined for the purposes of the present study,

Hrdlicka, translating Zuckerkandl, writes:The two cusps are connected by a narrow

enamel ridge, , » , The masticatory surface of

17V J, L, Wortman, 1886, pp, 442-443»18, G, V, Black, 1894, p , 55. .

11

the cusps show on each side between the median ridge and the elevated lateral border, a rounded :depression,19 20Zuekerkandl mentions two cusps, the transverse ridge,

and the mesial and distal fossae <> This is an inadequate de­scription and falls far short of expressing the wide range of variation in the tooth. .

Hrdlieka (op. cit.) translating Huhlreiter says:Similarly as in the upper bicuspids, the mas­

ticatory surface of the buccal cusp of the lower bicuspids shows also, as a rule, a prominent and steep median ridge, accompanied fairly frequently .on the distal part of the surface, but only very seldom on the mesial part, by a'secondary ridge.21,n ■Muhlreiter notes the median ridge and the presence of a

secondary occlusal ridge. His description is likewise inade­quate for the present study.

Hrdlieka (op. cit.) observes:The median ridge is straight and vertical,

the laterals may be straight or slightly curved, and proceed distad in a more or less divergent way from the median ridge. . . .Of the lateral ridges one or the other may be completely absent, with the other slightly too well developed. , , . When both the lateral ridges are absent, we.obtain the single median ridge type. . . .Hrdlieka notes the median fidge, accessory mesial and

distal ridges/ and variations of the median ridge. These features by no means exhaust-* the range of variation found in

1 9. A. Hrdlieka, 1921, p. 150°20= E. Zuekerkandl, 1891* P° 49°21. E. Muhlreiter, 1891* 2nd and 3rd Editions, p. 5 8 .

12

the tooth but here again one finds reference-to certaintraits commonly observed by other Investigators=

The median occlusal ridge of the buccal cusp is one ofthe structures with which this study is concerned. In fact^because of its constant occurrence it has been chosen as apoint of reference in defining other traits. Hrdlieka'sremarks concerning the ridge are of interest? ■

The median of the intermarginal ridges of a crown or a cusp is without exception the best de­veloped and most frequent. .It is the axial ridge of the "protocone11 or "main loben> and the main strengthening ridge of.the crown.or a cusp as a -whole.22 -

'OQPedersen describes the lower second premolar of East Greenland Eskimos as consisting of two types— one with two main cusps, the other with three. Pedersen (op. cit.) also mentions other investigators who have observed the incidenceof two or three cusps among various populations. These inves-

oil ' oA ovtigators are Becker, Hjelmann, de Terra, and Kajava. ‘28 , .Be arohge-Cohen • recognizes two basic forms, of the

; •22. A. Hrdlieka,.1921, p. l67«23« P. Oo Pedersen, 19^9s P» 1 6 2.24. H. Becker, 1936, pp. 359-373. . :v25. G. Hjelmann, 1928, pp. 1-132.26. M. de Terra, 1 9 0 5.2 7= Y.KaJava, 1912, pp. 1-64.28. Th. de Jonge-Cohen, 1 9 2 0.

13

lower second premolar, one sexltubercular# the other qulntltubercular<, His article, though preoccupied with morphogenetic implications5 considers accessory lingual cusps j, symmetrical and asymmetrical mesial and distal halves of the tooths and variations in the pattern of the sagittal sulcus. ■

In the material quoted from the authors above one finds repeated references to certain characteristics of the oc­clusal crown surface which are among the traits considered in this study. . The differences of interpretation and con­clusions drawn by other investigators cannot be assailed here with advantage. However, it appears that the results of the present study and others of like nature will have as one common result the urging, of caution upon those who might be inclined to generalize upon the problems of dental morphology in the absence of considerable data and intensive scrutiny of that data.

As will be noted, the descriptions of the various struc­tural modifications of the crown surface pattern as given by the authors above are not intended for, nor are they in most cases, amenable to statistical interpretation. Thus, in the interest of statistical expediency it is necessary to de­scribe these characteristics in terms which are unambiguous.

CHAPTER III

DATA

All of the casts and teeth upon which the observations of this study are based are from the dental collection of the Laboratory of Physical Anthropology of the University of Arizona. The casts in this collection were obtained by Dr. Bertram S. Kraus, of the Department of Anthropology, and by students working under his direction. Many of the casts are those of the dentitions of children in the Tucson Public Schools. Others represent family pedigrees. Some of the casts were taken in the San Xavier Papago Indian Reservation others, from the Yaqui sample, were taken in the village of Pascua.

The. dental impressions were taken with "Elastic Impres­sion Cream,H an alginate, made by Dental. Perfection Company, Glendale, California. Although the use of an alginate for impressions has been criticized, the criticisms have been levelled at the shrinkage of this material. For this work the alginate proved sufficiently accurate since no metrical observations were made. The material used for casting was "Kerr’s Rapid Stone," manufactured by Kerr Manufacturing Com­pany, Detroit, Michigan. The technique employed in making impressions and casts followed standard practice among dental

%2> O

15

technicians, but in.many, cases had to be modified to meet the exigencies of circumstance.

The.composition by. ethnic group of the sample is as fol­lows; (with regard to casts and teeth used in this study)

Ethnic Group

Unrelated Caucasoid Dizygotic Twins Monozygotic Twins Papago Chinese .Negroid Mexican Mexican-Yaqui Prehistoric Indian

No, of Teeth

694?57M2436428548

Total. 452

The prehistoric Indian material is from the catalogued collection of the Department of Anthropology, University of Arizona, and represents material from various prehistoric sites throughout Arizona,

Although a large number of casts, in excess of 800, are available for study in the Laboratory of Physical Anthropol­ogy, the actual nhmber used as data in this study is con­siderably smaller. The total of teeth used for observation was 452, as indicated above. This was due to several factors:

16

A number of the easts not usable were those of deciduous teeth; other casts in some.Instances lacked the mandibular second premolar for various reasons5 i,e6j, teeth may not have erupted, or might have been extracted, or the patterns may have been obscured by dental work* Another factor reduc­ing the sample to its present limits was that of wear. Par­tial or complete wear of the occlusal surface in many eases obscured the traits to such a degree that it was necessary to reject the cast.

The mandibular second premolar, according to Schour and Massler,^— erupts between the eleventh and the twelfth year of life, and except in cases of severe attrition due to malocclusion or coarse diet, the first signs of wear, should not appear until the beginnings of adult life, or possibly around twenty years of age. The major portion of the sample falls between the ages of ten and twenty years. Many cases would also be found in which the age of the individual would be considerably in excess of the higher figure. This may be due, at least in part, to the fact that the mandibular second premolars do not have an exclusively grinding func­tion. Rather, their function appears to be intermediate with regard to either grinding or cutting. Also, in occlusion, the tooth presents chiefly the peripheral surfaces of its occlusal area for attrition. In each sample the only

2 9 . I. Schour and M. Massler, 194-1, pp. 1153-1160 =

17

consideration was whether or not the tooth presented clearly the traits in questione Those which did not were rejected*

In line with other investigations of dental traits sexes were not distinguished in this study*

' ; • " METHODOLOGY \ ‘ V': ; : . :

-Lasker anticipates certalri of the difficulties in de­termining what constitutes a hental trait and the manner in ' which observations tonst be arranged, as follows 1

; ’ . Most; obvious to one who would make such a. study , o is the problem of determining what ; constitutes a traity and of clustering;the- obser­vations made Into a class if ioatory system that j

■ has some meaning beyond convenience in presenting '. the datac-50 - , ; : - v. : '̂' ■ ;The morphology of the mandibular second premolar is ex­

tremely varied, To gain an idea of the range of variation and to discern the possible relationships which might obtain between components of the occlusal crown surface of the tooth an initial inspectional survey was made<> The objective was to Isolate certain variations whlch5 thought common» might: -indicate varying degrees ofloecurrenCe, : ■- " '

After this preliminary survey was made and literature relative to the problem studied, certain relationships and discrete features began’ to emerge and to look rather promising for further study< 1 , i;-; ' : i ^

An outstanding morphologleal feature of each tooth was the median occlusal ridge of, the buccal cusp, This more or

d»':W«: Lasker> : 1950, p. 191-

less steep median ridge In almost every case dominates the ■occlusal' area of the tooth= Since this feature is constant it was chosen as the reference point for a number of varia­tions in the occlusal,.patterns which will be described in some detail latero The form of the median occlusal ridge of the buccal cusp Was itself•observed to be subject to a number,of variations. Two of the most common were chosen as traits and will be discussed in Some detail>

Other distinctive features of the occlusal surface ap­peared with high frequency in most of the samples studied, Accessory occlusal ■ ridges parallel to the course of the median occlusal ridge of the buccal cusp were revealed. These were : either absent or present and if present bore certain defin­ite relationships td the median occlusal ridge.

Each tooth bears two more or less distinct fossae. One of these lies mesial to the median occlusal ridge of the :buccal cusp* The other lies distal.thereto ahd in almost every case at a distinctly lower level. This' feature^ as

- has been pointed out. by Wortman^1 and later authors # forms a valuable guide in determining the mesial and distal surfaces of the extracted tooth and the side:of the jaw to which it belongs,• This feature is so constant in its appearance and demonstrates so little variation. that it was. excluded from consideration. It was observed^ however/ that the mesial 1

61, J» L. Wortmanj. 1886# p. 442.

20

and distal fossae, did vary in their occlusal dimensions6 in some cases the fossae: appeared equal in mesio-buceal and disto^lingual diameters® Howevers in most cases the mesial fossa was smaller in its maximum occlusal diameters than was: the distal» ̂ \ i ̂ ' "'u

The sulcus which,runs in a mesio-distal direction was found to: be subject to a number of variations s two of which were chosen as distinctive patterns with high frequency of occurrence c ■ ' V i :: v / .

The lingual lobe was observed to exist in a number of relationships to the;long axis of the'median occlusal ridge :df the buccal cusp These variatipns are described in' some • detail later. /, '

A further characteristic of the lingual cusp was its :division in a number of cases into double, or leSs frequently, triple cusps« These variations and their definition are dis­cussed later® In the: case of the lingual cusp being sub- •:divided, a special set of relationships^ holds between their disposition and:the long:axis of. the median ridge of the buccal d u s p o . : ; i : ■■ :t::: t-- - i

These several variations rather loosely described above all represent fairly common and discrete patterns which exist on the occlusal surface of the tooth alone« Other character­istics were noted relative to:the .grooving of the lingual and buccal vertical surfaces of the tooth.but.theseare not included in the present study due in some cases to their

21

rarity and In others to the difficulty involved In observing them on dental casts»

Following the preliminary survey a list was made of those traits which were easily observed and which exhibited variations in frequency» A card file was set up listing each tooth which would be usable for Observatione Each tooth was identified as to the ethnic group to which it belonged» The teeth were then inspected and those traits noted were checked on the cards» After the .entire sample had been checked in this manner with information on each tooth on a separate card it was possible to set up sheets covering each ethnic group and to plot the occurrence of the specific traits within that particular group«,

With regard to the manner in which ethnic groups were distinguished the consideration was borne in mind that these ethnic groups actually represent breeding groups and as such would possibly reflect in the distribution of the traits certain frequencies which might be diagnostic» The actual breakdown of the groups is based upon an earlier study con­cerned with traits of the mandibular first premolar by Kraus

32and Furr.The ethnic groups were set up as follows?The Caucasoid, including unrelated samples as well as

dizygotic and monozygotic twins„ The monozygotic twins were

32. B. S. Kraus' and M. Furr, (in Press).

22

msed to increase the size of the sample« However, only one individual of a pair was used to avoid biasing the sample.

The Mongoloid, including Papago, Prehistoric Indian, and Chinese populations. ,

The Negroid.The Caueaso-Mongoloid, including Mexican and Mexican-

Yaqui populations.

CHAPTER V

THE TRAITS: DEFINITION, ILLUSTRATION,AND DISTRIBUTION

Trait 1. - The Relative Sizes of Mesial and Distal Fossae

An inspection of the occlusal surface reveals a sepa­ration of the area into two pits, or fossae, one mesial and one distal. These depressions are separated by the junction of the median ridge of the buccal cusp with that of the lin­gual cusp,. The linguo-buccal position of the junction de­termines the relative sizes of the fossae0 The fossae may be of unequal or equal sizes „ Hence, three aspects of the trait are noted:

1-a - The two fossae are of equal size. (Figure a)A medial junction of the ridges of the buccal and lingual cusps divides the mesial and distal moieties in such a manner that the maximum diameters of the fossae appear equal.

1-b - The mesial is larger than the distal. (Figure b) In this case the junction of the ridges of the buccal and lingual cusps is distal or distally inclined.

1-c - The mesial fossa is smaller than the distal. (Figure e) In this case the junction of the ridges is mesial or mesially inclined.

24

Figure I.

a b c

TABLE I.

Trait 1. - Relative Frequency by Aspect

Ethnic Group 1 —a Pet. 1-bPet. 1-c Pet.Caucasoid 15-9 6.2 77.9

Unrelated 11.6 11.6 76.8Dizygotic 27.7 .0 72.3Monozygotic 7.1 3.6 89.3

Mongoloid 21.6 4.3 74.1Papago 2Q.5 2-3 68.2Prehistoric Indian 18.7 8.3 73.0Chinese 12.5 .0 87.5

Negroid .0 .0 100.0Caucaso-Mongoloid 18.1 1.6 80.3

Mexican 9.5 .0 90.5Mexican-Yaqui 22.4 2.3 75.3

The above Table showsi a frequency distribution of thevarious aspects of Trait 1, which is similar in all the ethnic groups except the Negroid. In the Negroid sample each tooth examined showed Trait 1-c, (the mesial fossa smaller than the distal).

25

Trait 2, - The Occlusal Ridges of the Buccal Cusp

Ordinarily the buccal cusp shows a prominent and steep median ridge on its occlusal surface. However, many of the teeth examined have an accessory ridge or ridges which follow a course roughly parallel to the median ridge, though con­siderably shorter in length. If there is only one additional ridge, it may lie either mesial or distal to the median ridge. In those instances where two additional ridges are present one is mesial to, and the other distal to the median ridge.The trait consequently has four distinct aspects:

2-a - Median occlusal buccal ridge only. (Figure a)2-b - Median occlusal buccal ridge, plus one distal

ridge. (Figure b)2-c - Median occlusal buccal ridge, plus one mesial

ridge. (Figure c)2-d - Median occlusal buccal ridge, plus mesial and

distal ridges. (Figure d)

Figure 2.

a b e d

26

TABLE II>

Trait 2. - Relative Frequency by Aspect

Ethnic Broup let. 2rb ■' Pot. 2rCPet. 2-dPet.Caucasoid 49*6 36.4 1.4 12.6

Unrelated . 43.5 39*1 2.9 14.5Dizygotic S 55*3 34 s 0 .0 10.7Monozygotic 55*6 33*3 .0 11.1Mongoloid 7*0 39*5 8.8 44*7

Papago 7*1 28*6 7*1 57*2Prehistoric Indian 8.3 33*3 10*4 48.0Chinese 4.2 70.8 8*3 1 6 . 7

Negroid 33*3 36.1 5*6 2 5.OCaucaso-Mongoloid 41.7 38.6 .0 19*7

Mexican 42.9 50.0 *0 7*1Mexican-Yaqui 41.2 32*9 .0 25*9

The aspects of Trait 2 show a distribution pattern which has certain interesting features* In the case of 2-as (the median ridge only) frequencies in the Caucasoid, Negroid, and Caucaso-Mongoloid groups range from 33*3 to 49*6 per cent. Among the Mongoloids the frequency is 7.0 per cent. Also in the Mongoloids the frequency of g-d is considerably greater than that in other groups®

27

Trait 3» - The Median Occlusal Ridge of the Buccal Cusp

All mandibular second premolars examined, had, on their occlusal surface, a more or less prominent median ridge running from the apex to the base of the buccal cusp. In sagittal cross-section this ridge shows a considerable variety of form. The commonest condition was that of the ridge having the same general form throughout its course. Less commonly, the form of the ridge changed in the area near its base. In these forms the lower part of the ridge diverged forming two or more secondary ridges. Thus, two aspects of the trait are noted:

3-a - The median ridge is single. (Figure a)3-b - The median ridge is divergent.(Figure b)

Figure 3«

a b

ai

28

TABLE III.

Trait 3» - Relative Frequency by Aspect

Ethnic Group 3-aPCte 3-bPCteCaucasoid 86 e 8 13.2

Unrelated 87.0 13.0Dizygotic 91.5 8.5Monozygotic 78 o 5 21*5

Mongoloid 82*8 1 7 . 2

Papago 95.5 4.5,Prehistoric Indian 81*3 18*7Chinese 62*5 37.5Negroid 77.8 ro ro ro

Caucaso-Mongoloid, 81*1 18.9Mexican 85.7 14*3Mexican-Yaqui 78.8 21*2

The distribution pattern for Traits 3-a and 3-b is similar for all groups* with the exception of the Chinese in which the frequency of 3-a is considerably lower* and that of 3-b considerably higher* than that in other groups«

29

Trait 4, - The Relative Position of the Lingual Cusp

In those instances in which there is a single lingual cusp the greater volume of its mass viewed from its occlusal aspect may be related in one of three ways to the long axis of the median occlusal ridge of the buccal cusp. Hence, the trait presents three aspects:

4-a - If the larger portion of the lingual cusp is medial to the long axis of the median ridge of the buccal cusp, the position is described as medial. (Figure a)

4-b - If the larger portion of the lingual cusp is mesial to the long axis of the median ridge, the position is mesial. (Figure b)

4-c - If the larger portion of the lingual cusp is dis­tal to the long axis of the median ridge, the position is described as distal. (Figure c)

In order to observe this trait accurately it is necessary to orient the lingual aspect of the tooth in such a manner that it is in direct opposition to, or at a 90° angle to, the mesio-distal plane of the tooth.

Figure 4.

a b

30

TABLE IV.

Trait 40 - Relative Frequency by Aspect

Ethnic Group 4-a 4-b 4-cPet. Pet. z Pet.Caucasoid 37.6 54.1 8.3

Unrelated 25.0 64.6 10.4Dizygotic 5 0 . 0 42.3 7.7Monozygotic 62.5 37.5 .0

Mongoloid 54.6 43.4 2.0Papago 45.0 * 52.5 2.5Prehistoric Indian 59.5 38.1 2.4Chinese 64.7 35.3 .0

Negroid 21.0 79.0 .0Caucaso-Mongoloid 37.0 6lo6 1.4

Mexican 20.7 75.9 3.4Mexican-Yaqui 47.7 52.3 .0

Trait 4-a has a lower frequency in the Negroid, sample than among the others. Trait 4-b varies in its distribution likewise5 and finds its highest frequency in the Negroids„ Trait 4-c was not found among the Negroids# nor in several other groups. In all groups 4-c showed low frequency.

31

Trait 5» - The Number of Lingual Cusps

In accordance with a practice established by Kraus, a cusp is defined as having an independent apex, however slight. The apex represents an elevation which may be pal­pated. One lingual cusp was the commonest condition ob­served. Less commonly there were two. Three lingual cusps were relatively rare. Four lingual cusps were not observed.

The trait thus presents three aspects:5-a - One lingual cusp. (Figure a)5-b - Two lingual cusps. (Figure b)5-c - Three lingual cusps. (Figure c)

Figure 5.

33- B. S. Kraus, 1951, p. 353

32

TABLE Vo

Trait 5» - Relative Frequency by Aspect

Ethnic Group 5-a Pet« . 52b Pet. 5-ePet.

Caucasoid 60.1 39.2 .7Unrelated 69»6 30.4 .0Dizygotic 55«3 44.7 .0Monozygotic / 42.9 51.8. 5.3

Mongoloid 9 1 . 0 9.0 .0Papago 97.6 2.4 .0Prehistoric Indian 95.7 4.3 .0 (Chinese 7 0 .8 2 9 .2 .0

Negroid 5 2 .8 41.7 5 . 5Caucaso-Mongoloid 6 5 .2 34.8 .0

Mexican 76.3 23.7 .0Mexican^-Yaqui 59.5 40.5 .0

The distribution pattern for Trait 5-a is similar among all groups with the exception of the Mongoloid in whose case the frequency is considerably higher=

Trait 5-b also is dissimilar in its distribution in the Mongoloids where the frequency is lows

Trait 5“C# three lingual cusps, rarely occurs among any of the groupsI

Trait 6. - The Independence the Lingual Cusp

The morphological relationships existing between the buc­cal cusp and the lingual cusp are varied as will be noted in the several conditions outlined in descriptions of other traits. In most cases the lingual cusp possesses an apex which is independent of the buccal cusp. However, when the lingual cusp is joined or fused with the central occlusal ridge of the buccal cusp, certain variations are manifest.The tooth, in such instances, presents a cuspoid appearance. The sagittal sulcus is interrupted and the apex of the lin­gual cusp cannot be palpated. The area of the fused lingual cusp, which would mark the occlusal ridge of that cusp, is continuous with the median ridge of the buccal cusp. Thus, the median occlusal ridge of the buccal cusp runs in an un­broken line to the junction of the occlusal and lingual areas of the tooth. Hence, either of two conditions may occur:

6-a - The lingual cusp is independent. (Figure a)6-b - The lingual cusp is fused with the median oc­

clusal ridge of the buccal cusp. (Figure b)

Figure 6.

34

TABLE VI.

Trait 6. - Relative Frequency by Aspeqt

Ethnic Group 6—a Pet. 6-bPet.Caucasoid 1 0 0 .0 .0

Unrelated 1 0 0 .0 .0Dizygotic 1 0 0 .0 .0Monozygotic 1 0 0 .0 .0

Mongoloid 9 6 .6 3 . 4

Papago 93° 2 6.8Prehistoric Indian 97=9 2.1Chinese 100.0 .0

Negroid 100.0 .0Caucaso-Mongoloid 1 0 0.0 . 5-5

Mexican 100.0 .0Mexican-Yaqui 9 1 .8 . 8 . 2

The range of Traits 6-a and 6-b is similar in all groups. However5 some occurrence of 6-b is noted in the Mongoloid and Caucaso-Mongoloid groups.

35

Trait 7 * - The position of Multiple Lingual Cusps

In cases where double or triple lingual cusps occur sev­eral relationships may exist between the buccal cusp and the lingual cusps. These relationships may be described in terms of the position of the lingual cusps with respect to the long axis of the median occlusal ridge of the buccal cusp. The lingual cusps converge toward a point either directly opposed to the base of the median occlusal ridge of the buccal cusp or to a point either mesial or distal there­to. Thus, three aspects occur:

7-a - The lingual cusps are medial to the median oc­clusal ridge of the buccal cusp. In this case a horizontal extension of the long axis of the median ridge would bisect the total mass of the lingual cusps. (Figure a)

7-b - The greater part of the total mass of the lingual cusps is mesial to the median occlusal ridge of the buccal cusp. (Figure b)

7-c - The greater part of the total mass of the lingual cusps is distal to the median occlusal ridge of the buccal cusp. (Figure c)

Figure 7.

36

TABLE VII.

Trait 7« - Relative Frequency by Aspect

Ethnic Group 7—a Pet 0 7-bPet. 7-gPet.Caucasoid 44.6 5-4 5 0 . 0

Unrelated 47.6 4.8 4 7 . 6Dizygotics 57*1 4.9 38.0Monozygotics 21.5 7.1 71-4Mongoloid 20.0 .0 80.0

Papago .0 .0 100.0Prehistoric Indian 5 0.0 .0 5 0 . 0Chinese 14.3 .0 85-7Negroid .0 2 6 .7 73-3Caucaso-Mongoloid 28.2 2 0 .5 51-3

Mexican 44.4 11.2 44.4Mexican-Yaqui 23 0 3 23.3 53-4

Trait 7**a occupies a wide range in all groups exceptthe Negroid in which case it is completely absent. Trait7-b does not occur in. the Mongoloid group, but does occurto some extent among the others Trait 7-c shows high fre-quencies among the Mongoloids and Negroids. Lower fre­quencies are found among the Caucasoid and Caucaso-Mongo1old groupso '

37

Trait 8 . - The Sagittal Sulcus

The sagittal sulcus follows a mesio-distal course be­tween the buccal cusp and the lingual cusp. Although a number of individual variations relating to the patterns formed by the sulcus might be described as factors distin­guishing one tooth from another, for the purpose of this analysis, two aspects of the general pattern of the sulcus are considered:

8-a - The mesio-distal course of the sulcus is not in­terrupted by any enamel extensions from the occlusal medial ridges of buccal or lingual cusps. (Figure a)

8-b - The sulcus may be interrupted in its course by elevations or by enamel extensions projecting from the oc­clusal surfaces of the buccal and/or the lingual cusps. In other cases there may be a banking up of the medial ridges of either or both the buccal and lingual cusps. The result presents an arrangement in which the mesial and distal fossae are not joined in one continuous sulcus but are two distinct depressions. (Figure b)

Figure 8■ ■ ■ ■

38

TABLE VIII o

Trait 8» - Relative Frequency by Aspect

Ethnic Group 8-aPet. 8-b Pet.Caucasoid 68.7 31.3

Unrelated 6 5 . 2 34.8Dizygotics 61.7 38.3Monozygotlcs 89 = 3 10.7Mongoloid 16.4 83.6

Papago 6.8 93.2Prehistoric Indian 6.2 93.8Chinese 54.2 45.8Negroid 7 2 .2 2 7 .8

Caucaso-Mongoloid 45.7 54.3Mexican 45.2 54.8Me±iean-Yaqui 45=9 54.1

The distribution pattern of Trait 8-a is similar in all groups with the exception of the Mongoloids0 Trait 8-b like­wise varies distinctly from the other groups in Mongoloids»

CHAPTER VI

TWIN STUDY: STATISTICALTREATMENT AND RESULTS

The statistical procedures employed in the twin study have been devised by Householder and K r a u s *3^- The validity of the method has been demonstrated previously* Any defects of interpretation or errors of calculation are solely the responsibility of the writer.

In a sample of 145 Caucasoid teeth the relative fre- quencies of the various aspects of the traits defined are as shown in TABLE IX below;

TABLE IX

Aspect Frequencies and Expected ConcordancesI

Traita

IIObserved Frequency b c d

mExp = of Concordances

la 15*9 6.2 : 77*9 - 63 = 62o 49 = 6 36=4 1=4 12=6 39*53, 86=8 13 = 2 - - 77=14o 37-6 54 = 1 8 = 3 — 44 = 15 c 60 = 1 39*2 = 7 - 51=56o 100 = 0 = 0 - - -7 c 44=6 5=4 50=0 - 45 = 28. 68 = 7 31 = 3 - - 57=0

34 = James Householder and B= S. Kraus=

4o

Column III Is calculated on the principle that if the probability of an event is p5 then the probability of the event occurring twice in two trials is p^, or pXp. The prob­ability or relative frequency of any two of the teeth being alike in any of the various aspects of a given trait is the sum of the squares of the relative frequencies of the re­spective aspects o Such a similarity is called a concordance in this study5 and the expected relative frequency of such concordances for each trait are shown in Column.Ill of Table IX o

A randomly chosen sample of 17 pairs of monozygotic twinSj, from the same population as that from which the above sample of 145 teeth was taken, has been observed for con­cordances between siblings for the teeth involved. Two com­parisons, one on the fight and one on the left, were possible with each pair. Not all teeth were valid for observation, however, and in consequence there were not 34 events or trials for concordance, but 30, in the monozygotic series, Each of these trials was either a concordance for a given trait or it was noto '

If the probability of a concordance for a given trait is p, then the probability of its non-concordance is q _ 1-P, Since the situation described is precisely that of a binomial

juristprobability distribution, function, the standard deviation of the relative frequency for a sample of 30 eases is;.

\ I P q\ W

41

The observed.' relative frequency of concordance for the various traits are shown below In TABLE X, along with their standard deviations, computed as described above, and using for p the values shown In Column II of TABLE IX.

Column IV In TABLE X Is - the difference between the ex­pected per cent of concordances In TABLE IX, Column III and the observed per cent of concordances In TABLE X, Column II, with the difference, expressed in multiples of the

.V ~ xh (X \ •'j t i u fv<gjb_v"standard deviation as shown in TABLE X, Column III.

TABLE X ■

Observed Relative Frequencies of Concordance In Monozygotic Twin Pairs

ITrait IIObs. Rel. Freq. in Mono. Twin

Pairs

IIIStandardDeviation

—Difference in Multiples of Sd

1. 9606 8 .7 8 3.422. #2.9 8.93 5.983. 100.0 7.67 2.994. 92.3 9.07 5.315. 96.5 ‘ 9.13 4.936. 100.0 - -7® 8.7.5 9.09 4.078. 93.1 9.04 3.99

From Column IV in TABLE X it may be observed that all, ' " - ; ■ <

of the differences between the representative sample per­centages and the monozygotic sample percentages are

42

significant with the exception of Trait 6, the occurrence of one aspect of which is constant in the representative Caucasoid population. In the biological sciences thesignificance level is generally agreed to be any value in

AhuZAkcV}excess of 2 (Sd. from the mean. Such significance has been exceeded considerably in each of the traits observed in the monozygotic sample. This indicates either the occurrence of a rare event or the presence of a significant difference of structure in the two samples being compared.

A random sample of 12 pairs of dizygotic twins has been selected from the same population as were the two samples described above and the same comparisons were made for this dizygotic series. The results of the comparison in the dizygotic sample are shown in TABLE XI, where Column III of TABLE IX has been reproduced as Column II, and Column III of TABLE X as Column IV for convenience.

43

TABLE XI

Observed, Relative Frequencies of Concordance in Dizygotic Twin Pairs

ITrait

IIEXP a ^ OfConcordance

• IIIOb8 a $ ofConcordance

IVStandardDeviation

VDiff, in Multiples of Sd.

1 o 63»6 60.9 10 a 04 .272a ■39*5 61.0 10.19 2.113. 77.1 82.6 8.76 .634. 44.1 41.2 10.35 .285. 51.5 65.2 10.43 ' 1.316. 100.0 100 = 0 - -7» 45.2 28.6 10.39 I .6 0

8 a 57.0 73.9 1 0 .3 2 1.64

All of the traits except Trait 2, show no significant difference from the representative sample since all the differences are less than the value of two standard devia­tions o Trait 2 may be considered to be on the borderline between significance and non-significance.

CHAPTER VII

DISCUSSION; PART I DIFFERENTIAL ETHNIC FREQUENCY

In CHAPTER V, Tables 1 through 8 present frequencies of the traits defined in this study. Certain differences in frequencies among the four populations' may be noted. In some instances the distribution patterns of the several trait aspects varied to such an extent as to suggest differ­entiation, Some of these.differences will be discussed.

Trait aspects 1-a and 1-b showed similar frequency among all groups. However, trait aspect l-o (the mesial fossa smaller than the distal) occurred in all Negroid teeth. In other groups the frequency for this aspect did not exceed 8 0 ,0 per cent.

Trait aspect 2-a (the median ridge only) had a low fre-. quency in the Mongoloids as compared with that of other groups. Trait aspects 2-b and 2-c showed no striking fre­quency differentials among the e'thnic groups, Trait aspect 2—d (the median ridge plus two lateral ridges) appeared in the Mongoloids with greater frequency than in other groups.

Trait aspects 3-3- and 3-b, concerning the median oc­clusal ridge of the buccal cusp, did not vary in frequencies to any unusual extent among the several groups and would, therefore, be of little diagnostic value.

45

The various aspects of Trait 4 refer to the position of the lingual cusp. Trait aspect 4-a (the medial position) showed a distinctly lower frequency for the Negroid popula­tion, running from 16.0 to 33»0 per cent less. Trait aspect 4-b "(the mesial position) in the Negroid group, as follows from the distribution of 4-a, showed the frequency pattern to be unlike that of the other groups. Trait aspect 4-b (the distal position), though not of high frequency in,any group, was completely absent in the Negroids. Larger samples might indicate this aspect to be more common than shown by this study. A point of departure for further study is sug­gested. '

Trait.5 (the number of lingual cusps) has a similar fre­quency distribution for all groups with the exception of aspect 5-a. (one lingual cusp). The Mongoloids showed a high frequency of 5-a and a resultant low frequency of 5-b (two lingual cusps). The third aspect of Trait 5 (three lingual cusps) occurred rarely in any of the groups and would be of questionable differential value.

Both aspects of Trait 6 (independence or fusion of the lingual cusp) are of doubtful differential value.

Trait aspect 7-a (medial multiple lingual cusps) was absent only in the Negroid group. Trait aspect 7-b (mesial multiple lingual cusps) did not occur in the Mongoloid group. The, aspect may be of diagnostic value. Trait aspect 7-c (distal multiple lingual cusps) showed its highest frequency

46

in the Mongoloids (80 = 0 per cent) and in the Negroids (72=3 per cent). In the Gaucasoid and the Caucaso-Mongoloids frequencies were 50,0 and 51*3 per cent respectively. The differences between the two ranges suggest differential value,

Trait aspect 8-a.(unbroken sagittal sulcus) showed a fre­quency of 16,4 per cent in.Mongoloids, This figure appears to be diagnostic when compared with frequencies ranging from 45*7 to 72,2 per cent in the other groups, In consequence of the low frequency of 8-»a, Trait aspect 8-b, the inter­rupted sagittal sulcus 3 showed high frequency in the Mongo­loids which is again highly suggestive,

Larger samples might indicate variable frequencies con­sistent with those observed in the present study and thereby provide tools for use in determining ethnic affinities. In all probability it would not be possible to determine affin­ity on the basis of one aspect alone, but such an objective might be gained if the aspect formed a part of a complexeof Trait aspects which differed in their frequencies for par­ticular groups,

In line with such an objective the following table shows the findings of the present study arranged in a manner which might by further development make possible the distinction of ethnic affinity based upon relative frequencies of the Trait aspects.

4?

TABLE XII

Differential Frequency of Trait Aspects in Four Populations

Gaueaso-Caueasoid. Mongoloid Negroid . Mongoloid Trait High Low High Low High Low High Low

1 ' ' ■ . ■■ ' ■ ; a bc ' X

=a Xbc Xd X .

3 :■ 'ab

4a • Xb Xc X

5a Xb Xc • '6ab7a Xb Xe X8a Xb X

48

The foregoing scheme is not to be construed as a device of immediate practical usefulness» Its purpose is solely torepresent in schematic form some of the considerations whichmay be drawn on the basis of relative frequency of the trait aspects alone« To be of practical ethnic diagnostic value the data should be analyzed statistically and ranges ex­pressed. in terms of standard deviations or equivalent de­vices, Larger samples and further refinement of the tech­nique would be required.

In the light of the results of the present study, a field of research is indicated, the investigation of which might prove of considerable value toward providing a solution to some of the problems of the variation of the human species.

Dahlberg, in discussing human dentition with particular regard to the form and character of the premolars, writes:

On the whole the premolars of the Indians arenot pertinently different from other populations, being moderate in size and having a somewhat larger development of the lingual cusp on the lower firstpremolars.36The foregoing statement represents the type of categor­

izing conclusion which may be successfully refuted by further work in comparative odontology. The present study shows that discrete dental characteristics vary in relative frequency among populations. It must follow that thinking on the ethnic or racial affinities of mankind must be in terms of

36. A. A. Dahlberg, 1949* p. 149«

49

the relative frequency of morphological characteristics among different populations and not in terms of individual differences» Only in this manner may a human taxonomy be developed which will provide some of the answers to the problems of individual and racial differentiation and be of value beyond mere convenience in presenting the data.

Univ. of Arizona Library

50

SUMMARY

The trait aspects or patterns of the occlusal surface of the mandibular second premolar are similar in form in four ethnic groups.

The frequencies with which the trait aspects occurred were compared for the four ethnic groups,

Differences in frequency for various trait aspects in the four ethnic groups suggest the possibility of using such differential frequencies to determine ethnic affinity=

■ ' ■ . *

: ■ ̂ • ; .. 51

DISCUSSION; PART II THE TWIN STUDY

The primary objective of this part of the study is to provide a demonstration of the results of the operation of hereditary agencies within the human organism, specifically within the dentition. No attempt will be made to arrive at an understanding of the hereditary mechanisms which express themselves in the manner which has been observed. This exceeds the scope of the present study.

Eight traits, embracing twenty-two aspects, were iso­lated and defined. All of these traits with the exception of one whose high incidence precluded its use, were demon­strated to be heritable to a degree which significantly exceeds what chance alone would permit,

The question may arise concerning the lack of complete and perfect concordance among the monozygotic twin pairs.This.question frequently arises in regard to studies in­volving the twin method of genetic analysis, Spuhler in discussion on the data on mode and inheritance of normal morphological variations says:

The twin method of genetic analysis is of prime importance in determining the heritability of human characters. The basic concept is that monozygous twins are reproduced by equational (mitotic) cell divisions and therefore possess identical gene pairs at each locus. Thus, lack of phenotypical concor­dance between monozygous twins must be due to non- genetic factors (the genotypes being constant),35

35° J. N, Spuhler, 1949, p, 179°

52

If the hypothesis of heritability for the traits defined in this study is valid,, we can expect the monozygotic pairs to differ significantly from the total representative Cauca­soid population. That they do has been demonstrated, Like­wise, if the hypothesis is valid, we should not expect the dizygotic pairs to differ significantly from the total repre­sentative Caucasoid population. This was demonstrated to be the ease with the exception of one borderline trait. That such should be the case follows from the concept that the genes of each of a dizygotic pair are inherited independently of the other.

Plates I and II illustrate monozygotic twin pairs of man­dibular second premolars. Figures a and b of Plate I show left and right teeth of one individual of a pair. Figures cand d of Plate I show left and right teeth of the other ofthe pair. In this pair, as in that depicted in Plate II, gen­eral agreement in the pattern of the occlusal surface may be noted. The sulcus, as well as the other characteristics, show essentially the same pattern. The agreement, in fact, in these two pairs is so striking that one might easily con­fuse the teeth of one with those of the other.

Plates III and IV illustrate dizygotic twin pairs. In these there is little agreement. The pattern of the surface and the general arrangement of the cusps vary to such an ex­tent that there would be no danger of confusing the teeth because of their similarity.

53

SUMMARY

An analysis of the traits herein defined with regard to their concordance in seventeen pairs of monozygotic twins shows' them to be concordant to a degree surpassing what chance alone would permit=

An analysis of the traits with regard to their con­cordance in twelve pairs of dizygotic twins shows them to be concordant only to a degree which would be found among the total representative sample=

PLATE I

MONOZYGOTIC TWIN PAIRS OFMANDIBULAR SECOND PREMOLARS

PLATE II

MONOZYGOTIC TWIN PAIRS OF MANDIBULAR SECOND PREMOLARS

PLATE III

DIZYGOTIC TWIN PAIRS OFMANDIBULAR SECOND PREMOLARS

PLATE IV

DIZYGOTIC TWIN PAIRS OFMANDIBULAR SECOND PREMOLARS

L D3R

D4L T D4I

CHAPTER VIII

CONCLUSIONS

The objectives of the present study are to Isolate and define the common morphological traits of the occlusal crown surface of the mandibular second premolar and to demonstrate the heritability of these traits=

Eight morphological traits embracing twenty-two aspects have been defined. These traits occurred in four ethnic groups: Caubasoid, Mongoloid, Negroid, and Caucaso-Mongoloid. • /

■ ■ ' . /Although the traits5 and most of the aspects of thetraits, appeared in each ethnic group, the frequency of their appearance varied from group to group.

The varying frequencies suggest the possibility of determining ethnic affinity upon the basis of relative frequencies of trait aspects.

Heritability of the traits has been demonstrated by an analysis of seventeen monozygotic twin pairs. Results of the analysis indicate a significantly higher concordance, or similarity, in these pairs than found in the total repre­sentative Caucasoid population.

The same traits when compared in twelve dizygotic twin pairs did not differ significantly in their concordance from that,,which would occur in the total representative samples.

LITERATURE CITED

Becker, Ho1936 Ora Kronorna hos tmderkakens praemolarer, deras storlek

och form sarat omsesldiga storleks-och formforhallende» Odontologisk Tidskroft, (Gothenburg) 44:359-373=

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Philadelphia, 1891* also 3rd Edition, 1194,Boyd, W. Co1952 Genetics and The Races of Man, Little, Brown and Com-

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