Morphological traits of the maxillary first premolar

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Authors Wasser, Roger Gifford, 1930-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/551219 MORPHOLOGICAL TRAITS OP THE MAXILLARY FIRST PREMOLAR

by

Roger G. Wasser

A Thesis

Submitted to the faculty of the

Department of Anthropology

In partial fulfillment of the requirements for the degree of

MASTER OF ARTS

In the Graduate College, University of Arizona

1953

Approved: IMv. of Arizona Library

' 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 Library0 Brief quotations

from this thesis are allowable without special permission/ provided that accurate acknowledgment of source is made*

Requests for permission for extended quotation from or fco­ production 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 scholarships In all qther instances s however? peraission must be obtained from the1 author, . : ‘ /

SIGHEDi ' ' , ACKNOWLEDGMENTS ‘ -

The Writer wishes to express his gratitude to

Dr, Bo S a Kraus for his close, cooperation and many helpful suggestions in the preparation of this manuscript and to

Mr, J„ E, Householder of the Mathematics Department at the

University of Arizona for the statistical method used in this study.

i TABLE OF CONTENTS Section- - v : : " ' Page

©.. ©' © © e INTS0DUCT10H s : > « © o • © • © © © © © i . © ” e , ... i" •

. . Genet l o Bas i s . of'Classification. » '* © ■ a . © & e © •O' 4 . . ' i

The Problem ® e a © , e. © ©; © © ' o . a. A e- © 4 ; ©

' : Definition of Terms ® «. ® e e. * * * : 0 " © e © , e- • 0 \:x

LITERATURE' » e ® . ®- e e • © ' ©• © © • e ' © • © © © •. 9 '0 : 6 '

Descriptions of the Maxillary First Premolar a 'o ;; © -' 6:3-

Seme tie Possibilities of the Bentitlon a » «■ «. « : 10:.

' ' , Bentitlon fop Ethnic Studies v, > 6. ■«, » « .« e, .® ® ® , 11

DEFINITION OF TRAITS A ® . V A « « H \

Methods of Trait Determination ® . ,»■ «, ® ® ® » ® » 13

Trait I® Sagittal Snlons ® i.- ... 0: V ® e. e; ® : e.: ® ® . 13

Trait lie 3 Oeelusal Paraeone .Ridges-.® a « . >. ® ..®,-.® ;i6. '

: Mesial Oeelusal1 Paraeone Ridge » e.. ® ® : ®.- . ® ® ,i- ,17 MM>Mesial Oeelusal Paraeone Ridge 1.® ® ® « » 17 Median Ocelus dl Paraeone Ridge » ®., . .s 9:* ® ® ® { 17 Mid-Distal Oeelusal Paraeone Ridge 6: 1.® ® ® ® 18 ■ Distal Occlusal Paraeone Ridge s ® * s . s ® ® 18

Trait III.® Oeeiusal Pro too one Ridges ® ® ® ® ® ® ® lo

Mesial Ode Ins al Protoeohe Ridge ® s &. ®. ® 19 " Median Oeelusal Proto cone Ridge #. ® * ®. <, ® s. 20 Mid-Distal Occlusal Protoeone Ridge' V e ® ; ® 20 Distal Oeelusal Protoeone Ridge ® ® ® ® 21

Trait IV® Mesio-Buccal Paraeone Parastyle ® ® ®. ® 21

Trait V®. Marginal Occlusal Ridges ® ® ® ® ® .22

Mesial Marginal Paraeone Ridge ® e ® ® . ® ®.-. P. 22 Mesial Marginal. Protoeone Ridge .® e. , o ® . 22 ;

11 Section Page Distal Marginal Paracone Ridge © e • © 23; Distal Marginal Pro toe one Ridge © © 23

STATISTICitL PRBATMElf « > » » •:*.»* » © - © @ @ % Determination of Ooncordance Prequencies a 2]^ - . '■ " ' - . . - -- - V- . ■ - . ■ Significance of Concordance Prequeneies s « «, > »: > 26 POSSIBLE ETERI0 VARIATIOH 11 ASPECT EREQEEHGY■. >' = > 30

- The Ethnic Group s 0 © ■ o , © .© © 30

Observed Variation © © e o e e ; © " © © © - © . © 31 - ■■■ . _ . - ’ SUMMARY' AMD 'COICLUSIOIS & 6 © , © - o ©. 6 6 © 36

BIBLIOGRAPHY '.. ®' » s ; ©- 0 © © © o- "o >-s A. © © © © 38 ELATES

M m b e r ’i Page

I* Illustration of Traits » , # • „ . • Foil owing Page; 2 3 :

II» Illustration of Traits l .« e ® , Following Page , 23

III© Dizygotic Twin Pair © , » Following Page 29

IE© Monozygotic Twin Pair © © Following Page 29

TABLES

1© Differences between Expected and Observed Concordance Frequencies Expressed in Terms of the Standard'" Errors of Expected Ooneordanee . Frequencies in Relation to .Dizygotic

, 28

20 S'iffiWnees between Esspeoted and • Observed Concordance Frequencies . Eipf^Ssed in Terms of the Standard Errors of, Expected Goneordanoe . Frequencies in Belatiori to Monozygotic • Twin Pairs @ & e » ©/ 0 0 o ® @ ® v « ® © © © © © ^9

3© Aspect Frequencies in Ethnic Groups,: , v Traits I and II © © © ©. - © . v - « - » ©: © ©. © 33

li-® Aspect Frequencies in Ethnic Groups 1 .Traits 111 ano. IE . © © » © ■© © © © © © . © © © © 3^1

;5. Aspect Frequencies in Ethnic Groups; Trait E o o @ d a 35 IHTRODtlOHOli

Uhtil recentlyj, human taxonomy has been based entirely

upon" phenotypic morphological traits. ¥lth this system;it is

impossible to separate actual heritable variations from en­

vironmental and. cultural manif est ationstof a trait * Conse­

quently confusion has arisen as to the taxonomic position of many human groups $ and f the re fore s. ■ many classifications have

beeh 'devised'. Recently the genetic basiS' of classification

has ' shown considerable promise;, in huriian' t axonomy . If has the.

advantage of using traits for which the genetics are known* -

The range of phenotypic, variation in the trait can be estab­

lished excluding, any environmental and cultural effects0

At present only a dozeh or so traits have been worked out

genetically* This is due to the difficulty of establishing precise genetic mechanisms for a trait in a system where perhaps

50*000 genes operate. Traits to be useful for Such a considera­

tion must be largely non-adaptive* show a low rate of mutation, be easily discernible, and be shown to be heritable*

* Dental traits show many, possibilities for this type of study* The tooth forms while it. is beneath the gum and does not erupt until the crown has completely developed* External

environmental factors., therefore* take no part in the formation

of their structures* ^Attrition, caries and other environmental factors altering the crown after eruption are readily discernible.

1 2 1 • In his studies of twins Newman states that mono zygotic

twin pairs show great similarity in dental ehar act eristics

indicating that their structure is heritable. In a recent

study of the morphological structures of the mandibular first

premplar•V the heritability of these traits-has been demon-

SWatedo'' ■..- ' " .* : / . J': . .

"The maxillary first premolar was chosen for this study, for • ; - . ' x ' several.reasons® The crown surface of the tooth showed a re- ' ' ' ' . : ? : - l.atlvely large number of discernible structures® . The crown

was; sufficiently simple in general morphology (only two cusps

being'present) to limit the size of . the problem for. .a study

of this nature® -i The study will extend the work already begun o . - ■ oh- the premolars by Kraus, and. Furr-h - ; ■ . ; . . ! » ■ t . : ' - ' ; ' . r' . w . . Gasts had to be relied upon in the determination' and

isolation'of morphological traitsy as well as in the statis-

tical treatments' since there was ho certain phenotypic method . ■ VF ' ’ " . 1 ' ' . ' - . ■ . :' ■ " ' .: for -discerning a maxillary first from a maxillary second pre- .v molar® Generally the lingual cusp of the:f irst, is'of; lesser

dimehs ions; than the buccal, while that ref the second is of more1 nearly the same proportions as the buccal® Bht-from

observations of the casts, it ’was found that this relationship

does not always hold true'. ' Since the structures of both the

first and second maxillary premolar crowhs are very' similar,

^ ^ ' ' . - _ . .. . | : '' ' _ ; - -v - ^ / r • ... • i -- ' - % r - v-;- • -: ' - --

1 ® Newman , 19l)h, bp® l31-i3C '

2 ® Kraus, Householder, Jones, in manuscript0 ' - ■ ■ ' • : 31 Kraus, Furr, in press® i

dnd sinee■ the roots on both may be fused or separated9 the ,

use of randomly extracted teeth was_z#gated@

: - The scope pf this study was two-fold^. Traits on the

crown surface of the maxillary first: premolar were Isolated

and defined, A statistical comparison between 17 Caucasoid

monozygotic and* 13 Caucasoid dizygotic -twin pairs ini relation

to a random Caucasoid population represented by 8I4- teeth was

then ■ use d to de termine the herit ability-of these trait s» It

yet Remains to show that these t raits1 ;.ahej independently

variable^ and what the genetic mechanism is controlling thel#

appearance's :v-"* "h - v

The casts of. ^ S teeth representing various ethnic divi­

sions (Mongoloid, Caucasoid,'Negroid®-. Gaucaso-Mongolold), were/bbserved to determine possible^ ethnic variations, in the

defined traits and in aspect frequencies of, these traits0

' 'Cusp nomenclature s to be consistent with, previous ..litera­

ture *on the subject;, follows that used.'by Gregory^"; but no evolutionary concepts are implied in^the terms as used in this study.,:..: The buccal, cusp, is termed the para-metacone,h: the lingual the proto cone« Since the paracorie was supposedly the original cuspg the para-metacone will be referred to as the paracones : " ■ ■■ ■’ ■ -- ■ ■ '-4

As an aid to the readers the following technical terms * are defined: '' f' ' v‘ ■. ■ - . ..v 1 a • Mono zygotic twins--(Monos, -one ) (Zygote 5, a fart ili zed egg

, i|-®' Gregory> 1922, pp« : ix-x6 . - - .; -. - ; - - - - ■ • - ' . li­ re suiting from the union of two gametes of opposite kind,

ovum and sperm)» Single egg twins formed by division of the

embryo0. Such twins are referred to as :identical, howevers

they are. never truly identical, but-differ more or less with

respect to all their traits® This is due- to both pre - and

post-natal environment® But since both members of a pair . / , : ' . y ■ ■ . ; - v *' - ■ result from the division of s-single;'fertilized egg at some

point in the embryonic stage of developments each; will possess

the same genotypeo .. ; : ; 1

20 Dizygotic twins--(Pis two)® Twins formed by the-fertiliza

tion of two individual ova released atv approximately the same

time® They are? therefore, the same as regular sibs® , They

need not have the same father®. Occasionally these fraternal

twins will be quite similara . This occurs when the parents

are genotypically similar®' - : -- - ." 1; ;

3® . Buccal cusp — (Bucca, cheek) = That cusp which is nearest the cheeko It is referred to. as the Paracone® v-;

ij.®.. Lingual cusp - - (Lingua, tongue) ? That cusp which is near­

est the tongue® It is .referred to as the Protocone©. : v.

5® Mesial— Anterior© The front part of the tooth®. That portion which is closest to the face© ' ; . V?

6® .Pistal-.-Posterior©: The back part. of the tpothe. .That per- tion which faces' toward the - back© ' ; ‘ '

7® Concordance-->Jhen the right tooth of one Individual of a pair is compared to the right tooth of the other individual ,

With relation to d specific trait, and the expression of the trait is the same on both teeth, then the two show a concord-; 5

anee* The same applies to the lefts, there being two possi­

ble concordances for a pair*

8-a Occluss-l surface>--That surface on the teeth where the

Tippers, .and,, lowers articulate«, . It has also been referred to

as the masticatory surfaces

9a . Grown--That portion of the tooth projecting above the gum.

The enamels ; LITEBATOKE

It has long been recognized from studies o f monozygotic

and dizygotic twins that dental characters were heritable e .

But these studies were concentrated primarily on similarities

in .the shape of-the dental arches S l in eruption and occlusion

patterns3 in relative positions of carless in pathological

conditions and In gross measurements with little or no regard

for the structures of individual teeth,'• . f

Structures of the crown surface of the maxillary first

premolar have never been adephately defined. Ales. Hrdlicka

has conveniently brought together the important .literature oh

the subject prior to 1921» ' The following quotations are ob­

tained from this s cure e @ - : /

An •American investigator, G-, B, Black s gives a relatively

cotip lets description: - ^ •

The buccal cusp is larger, and forms the terminal point of the buccal surface. From the point of this cusp, four ridges lead away at right angles. Two of these form cutting edges 9 which slope away me si ally and distally to ihe mesio- buceo-occlusal and disto-buccorocclusal angles, where they join the .mesial and distal marginal ridges. The central buccal ridge leads away centrally on the ,buccal surface ofV the gingival line, forming the convexity of this surface® The triangular ridge slopes down to the central part of the crC#i and joins a similar ridge from the lingual cusp to form the transverse ridge or ends in a central sulcate groove , ,

, 1, Hrdlicka, 1921, pp^ ll|2-1^9e

2,' Ibid,, p p » lliii-l^l, ; • / : • L, ' -•* -- ^ V ■ V .1. 'i. .

\ ' - ■i" ■. ■ ■ ■■ P P 6 .' -Lingual cusp— the lingual triangular ridge „leads down from the central point of the cusp to the central groove9 to join its" fellows' from the buccal cusp in the formation of the transverse ridge, or is divided from it by a deep auleate groove„ Ibis ridge is seldom prominent9 ¥e;ry frequently the central ‘incline ’; of the lingual cusp is a plain surface* - •

The mesial and distal marginal ridges are strong ridges of enamel which rise in the mesial and distal terminations of the- cutting edges of the buccal cusp s and form the 'mesial, and distal' margins of the occlusal surfaces They - join with the - : ridge forming the lingual cusp; or they are Usually, divided from'the latter by the mesial and distal grooves; though these' grooves are often indistincty especially in the-teeth that have been somewhat worno - - . ” ' .■ -1 . {X ;:V': ' V ' . , ' ."V. , > The occlusal surface Of the bicuspids have five develop - mental grooves; the central s mesial, distal^ .'mesial, triangu­ lar! ' • and distal triangular^ The central groove is deeply sulcata! and divides the triangular ridges, or passes over; their junction in a shallow; line s arid - sinks into a; tr-iangular pit at either end® The mesial and' distal grooves fare really continuations of the central, .which pass over’ the marginal ridges as - ye ry. fine.,. 1 inea' or ' as' mere:' delihitgroove^, and - mark , the; boundary of the lingual lobe o. They -are ' rarely fishured; while ' the central groove is frequently: fi.ssured throughout its course® The : triangular, grooves, - mesial and distal, , run from the mesial and distal-:.pits toward, the mesio- buc;,e al and dis to -buccal angles-! dividing the marginal: ridges from the triangular® They are occasionally sulc ate.-in the ' first part of-their] course and. are .generally lost = toward the meslo-T:, and disto-bue.cal" angles' by becoming shallower;-but in young, ,unwom teeth they can often be . followed as a-fine line running ever the cutting edges of the huecal cusp hear the . angles, and leading, into the bucoal grboves® I These 'are the marks; of cenfluence 6f : the jaeslal and distal lobes, with the middle lobe®. . In the o e M r a l ' incline of the llngual ousp, siipplemental-g-rooves a.^ often , seen-meetingithe^etriaaigular grooves of the buccal side",. ' 'The triangular groovesare ' OCc asionally fis sured "for a short distance with the .-central ° ° - . .- - - - - ’ . -- ’ - ’ : r in summing up the work’ of. other English and 'American in- vestigators Hrdlieka'5istates: :; lei; : : -r . > . dope, Tomes/nOSborrie, ; Scbtt!;Gidley, Gregory, Woodward 'and othefs 0 e o; haxre cohcentrated pn the gross forms, and; on the of 1 homologies of the various Cusbb, rather than on the finer modeling. 0 ® ® d y _ ; ; v':

3® Ibid® most part'W gross forms as has been Indicated for the English ' " ' ■ : ' . . if. ^ and American workers^ Muhlreiter and Zuckerkandl9 however^

have- given the- finer features some - consideration, i •

Zuckerkandl^' has treated the upper premolars; together 0

The translation^, after Hrdllckag reads: ’ i ■ i ^ \ ’ :■ ■' ■ ' ;The mastioatory surface of the buccal cusp is strongly convex and branching frail the transverse furrow shows not • seldom two short lateral depressions^ through which the sur­ face of the cusp is subdivided into several ridges. The middle of.these,is usually the steutesto ; Theeorreapending surface of the lingual cusp top,- is strongly convex,, and here and there' will also show secondary furrows, ; o ■ / ; 5 ■. ' ' ■ ' . Muhlreiter -. treats the upper premolars in the same manner ^ -."i. • - ^ as-'-;was done by : Zuekerkandl, but he- ment ions - ■relatively more of

finer detalls» ' " ‘-'l ■ ■ .. - - v 3. 'o. n/, ^ . -/Ml '"■. v if ' "-'/■ .,/ : . 'The'mastle:atory "Shrf ace;bi* f M. b d e p a l - cusp is frequently provided,especially near the ppint,’ with a prominent- median . ridge V A." Similar. ridge,.. but generally less marked, is seen in^ corresponding location; on the lingual cusp« Besides this y « i not seldom there appears on both sides of the masticatory surface also- several secondary ridges, s%) arated from the stouter median ridge by small secondary- groo ves radiating from the main furrow between the two. cusps» This, is an expression of the same folding which appears With such predilection on the lingual surf ace of the -upper front: teeth » »

. In individual - cases'the' folding extends also to the lin­ gual'cusp and the wholb masticatory surf ace of the teeth appears then ridged, with grooves radiating between the r! rldggs . ■ , ■ ! ■ . V - . - /- :y . -/.._ ;// _/■ / -: \. / ... V y ' - ':Adloff^ treats the upper first premolar as follows:

& ibid», pp ,; ilj^-i50o:'" / / - ’ /. ■ / 5-o ibid0 > p / 'i520 :/ / ‘ ■ - . - " - - ' 6a . Ibidb ,. p-o l58c :ii; ''': ’ - y ' / yi 9 On the labial cusp a broad ridge that 'extends from the neck pf the tooth to the point of the cusps is found with a shallow groove on each side' of it ^ r so that the cusp is plain­ ly divided,into three parts® This division of the labial surface o f the anterior premolar is almost always distinctly recognizable 0 ®

Hrdlickaj aft eh'conducting: a study of ridges and de­ pressions Of the dent it ion r came to the conclusion, that:

The’ridges and depression on the Crowns and"also on the „ cusps of the teeth are fixed? not incidental characteristics' and furthers

The ridged condition of the crowns and cusps is so generalized that it compels the assumption of its constituting another of the fundamental characters of the teeth ® .

However? he makes no definition of these characters on the maxillary first premolar® -

. later investigators have not added to G-,Blackts desi eription of the crown surfacess In reviewing, the first upper premolar Pedersen7 compares relative stoutness of the crown with that of the upper second p remolarQ Breadth-length , V indices suffice for ei’oWn form® Root form and frequency of root bifurcation are'dealt with at length as is the relative ' . . ' po size of the pulp cavity® In. discussing the cusps Pedersen states: ' " " ”-:

All P, supp o examined in the akulls and on the casts from East Greenland? have two cusps of which the facial is

7» Hrdlicka? 1 9 2 1 ? pp® l 6 0 -l6io

8 * Ibid®j p 0 1 6 1 ®

9* Pedersen? 1949? pp® 1 5 8 -1 6 1 ® H O e Ibido? p® 159® constantly men© stout than the lingual® In the skulls the occlusal cusp pattern was well preserved on thirteen teeth, only® One of these exhibited a cusplet mesially® Ho occlusal pearls were seen in either group of material® ,

In a study by Kraus and Purr^, a systematic description

has' been established for the crown of. the mandibular first

premolars. with individual structures on the crown being

treated' as distinct heritable units® ; «; . . t ; ■■■ ' ' ■ ■ ; .• ■ ; . ;■ : ^ Until this study was madeP genetic studibs of the teeth

were confined to a description of the similarities Of the

... . : - gross features in twin, groups® Many authors have agreed,

howevers that a study of the dentition is very promising fbom

a genetic ^standpoint ® In establishing their crltera for

differentiating monozygotic from dizygotic twin pairs lawman, :i ■ " 12 . Freeman, and Hoizinger states

They must have, essentially the same types" of'teeth, the same irregularities =• in dentition® (Casts of the dentition of most1 of the'pairs were made by a dentist and Subsequently used to check earlier, diagnoses)o .

Dr® Goldberg^^ made this study ‘and his conclusions were

based entirely on the resemblances of the dental arches in : monozygotic twin pairs® '

A® A® Dahlberg^ states that:

11® Kraus, Furr, in manuscript® . j . F " - : ■ . ' . ; ;• ' " . !. " ' 12® lewma.n, Ffeeman, Hoi zinger, 1937 # P*^ 35*

13® 'Goldberg, 1929, pp® 363-^09*

. llj-® Dahlberg, 19l{-9» p,« 114-0® All human dentitions are basloally the same® The differ ences between Individuals are in the number and extent of the primary and secondary character of the tooth groupss which in turn are the reflections of the genetic constitution of the individualo . .

He does nothowever» list any specific characters for the

maxillary first premolar ^ 0 ■

Ashley-Mont agu^ s in. studying Monozygotic twins with

respect to a particular pathological condition, states thati

Apart from the indistinguishable likeness of the form of the teeth, their crown patterns, groovess ridges, orientation, relations, state of. eruption, and calcification, a pathologi­ cal resorption of the upper right second deciduous molar ‘is described together with an invasion of the resorbed area by the first pemahent molar--a series of conditions which have taken place in precisely the same way in each twin®.

These conditions are considered to be entirely genotypic, affording a remarkable confirmation of the unitary nature of . the developmental processes which are. operative in uniovular (as these twins .undoubtedly are) identical:-twins® f

The crown patterns mentioned above aremot outlined in the

article®.;.;-. . >: ;■

- - * : Morkhaus ; also hag recognized-thelgreat similarity in - ■ • (1 ,. - . ■ . . . the crowns of teeth of monozygotic twins® Ho- specific traits

are mentioned® Bachrach. and Young have shown the same great resemblances in monozygotic twin pairs ih relation toicariess

eruption, and hypoplasia® :..r'

Because these'traits::!afe 'heritable, they become very 1 : . . - r : : ; ;.u.: .. - "

lg®? Ibid® ,:'pp9 :1^8-11^9®

16® Ashley-Montagu, 1933g P « 6^4»

.17® Korkhaus, 19303 ppo 394 ff o

18® Bachrach, Young," 1927,pp.. 1293 ff 12

important in the study of race'0 Birdsell gives a list of

genetic and phenotypic traits suggested.for the solution of

new world origins» Dental charaeters .are listed as‘promising phenotypic traits„ About these phenotypic characters he says j

1 The phenotypic characters here listed can be traced in their heredity in living family groups9 and hence may in time yield gene frequency analyses on cranial r emains®

Boyd'' confiims this opinion.,,. He states s :

Without genetic analysis: of a characteristic, there is always, the. possibility that .two groups of men exhibiting' the about the same visible or external- char act eristi cs- may never­ theless differ genetically a good deal,.for, as has already been pointed-outl different genes may sometimes produce prac­ tically the same end effect, . A good example of this is furnished by the: several different mechanisms by which sup - pression of the eruption of the upper lateral incisors is effected, 'Drawing conclusions, as.-to.,the racial affinities from the study of similar characteristics not Inherited in the same manners,! or for that matter^ similar characteristics the mode of whose Inheritenoe is not known3 can be very mis­ leading In physical anthropology, ■ :

'• Before the traits of the maxillary first premol.ar, or of any tooths, can be. of true value; in ethnic studies, the genetics of their inheritance mush be shown, , To establish genetic-mechanisms, the defined traits must be shown to be heritable. The purpose of this paper is to show that traits which will be defined for the crown surface of the maxillary first premolar are heritable„ A future study may determine the genetic mechanisms® .1 - - .

19:» Birdsell, 1949. p . 56®,

2 0 ® Boyd, 1950. pp, 317-318® DEFINITION OF TRAITS

To determine morphological traits on the orown surface

of the maxillary first premolar9 dental casts from the collec­

tion of the Laboratory of Physical Anthropology were observed®

Detailed pictures of the crown surface were drawn® By this

process individual structures oh the crot-m became manifesto

These structures were singled out for further c oi sider at ion®

From a sample of fifty casts picked at random quick dla

gramatic sketches were drawn for each of the chosen structures

the expected range of variation of each structure being deter­

mined in this manner® Structures observed to have very low

frequencies of occurrence were omitted in favor of those which

appeared more frequently® V

After dete.mining the generally occurring structures^ a ' " . . ' . trait list was formed^ and aspects of each trait were defined^

Cards listing each trait and its aspects were mimeographed®

The entire collection of casts was examined with relation to the defined traitsy each tooth being described on one of the cards according to the aspect of the, traits it possessed®

Photographs of casts illustrating,, each trait were taken using sr-Cine Exact a fitted with an extension tube® Settings were f/ 2 2 at one half of a second®

Trait I® Sagittal Sulcus

The sagittal sulcus is a deep fissure lying between the paracone and the protocone, and running in a mesio-distal

13 direction the length of the occlusal surface» It c an he

divided into three portions# the centra.!, mesial and distal*.

The central portion is deeply sulcate«,/ separating the central

occlusal ridges of the paracone from the protocone» The

mesial and distal portions are much: shallotrer and' are. gener­

ally forked, at least one’hr ^ e h of each fork crossing; the

occlusal margins of ttis - tooth*, 'The. other either crosses the ,

occlusal margin or veers toward the mesio- and disto-huccal

angles respectively of the paracone® Occasionally the distal

portion of. the sulcus, will ..not be forked, but continues as a

single groove toward the disto-buccal angle of the paracone®

When both branches of the fofkt cross the occlusal margin#1 a

lobe of enamel is formed between them® Two such lobes may be

• present on the occlusal surface9 a mesial sagittal lobe and a

distal sagittal lobe.® - \ / - -. . ;

The protocone may have grooves cor reap ohding In position

•to the branches "of the mesial or distal sagittal sulcus

which run to the mesio- or disto-buccal angles of the paraeohe®

These grooves are considered as separations for the mesial and

yi"distal occlusal protocone ridges and not as branches of the

sulcus® ; v

Six aspects; of the sagittal;sulcus pattern have been

observed® ■ . ' -y - - : •

- „ • " - . - ' ' Aspects i ..

a® Both branches of the mesial fork cross the occlusal

margin® The distdl portion is not forked and bends' toward

the disto-buccal angle® (Plate II, 1®) 1? b. One branch of the mesial fork crosses the occlusal margin, the other runs toward the mesio-buccal angle. The

distal portion is not forked and bends toward the disto- buccal angle. (Plate I, a) c. The branches of both the mesial and distal forks cross the occlusal margins. (Plate II, j) d. One branch of the mesial fork crosses the occlusal margin, the other runs toward the mesio-buccal angle. Both branches of the distal fork cross the occlusal margin. (Plate I, e) e. One branch of both the mesial and distal forks crosses the occlusal margin, the other runs toward the buccal paracone angles. (Plate I, f) f. Both branches of the mesial fork cross the occlusal margin. One branch of the distal fork crosses the occlusal margin, the other runs toward the disto-buccal angle.

(Plate II, k) (Plate I, b)

Illustrations of each aspect: mesial

buccal <— 16

Trait IIo Qcoius.al Paracone Ridges

' Five ridges may be present on the occlusal surface of

the paracone. From mesial to distal they are:

lo Mesial Occlusal Paracone Ridge • *

2, Mid-Mesial Occlusal Paracone. Ridge

,3V Median Occlusal Paracone Ridge

il*a, Mid-Distal Occlusal Paracone Ridge

5V Distal Occlusal Paracone Ridge

The mid-mesial j, median, and mid-distal make- up the central

occlusal paracone ridges. All five ridges run from the buccal

margin of the occlusal surface of the paracone to thei sagittal

sulcuss meeting the sulcus at approximately right angles.

Occasionally enamel bridges (Plate II5 g) are formed with

corresponding ridges of the protocone s obscuring somewhat the

path, of the sulcus a Usually the ridges terminate at the

sulcus o - . I

. The ridges may occur in any numerical ccmbination with

the exception that the median ridge is neanly always present.

Each , ridge may possess a variety of shapes ranging •'"from -slight

convexity to sharply defined ridgeSj, and the amount of area

that each covers may vary. These differences of degree are probably heritable factorss but the sample of teeth examined was of Insufficient size to accurately prepare such a classi­

fication, Therefore, each ridge, in this study is considered

as having only two aspects, presence or absence, the degree of presence being omitted. ■ . ' \ ... 17 Trait II* 1* Mesial Occlusal Paracone Rid^e

The mesial occlusal paracone ridge extends from the

mesiorbuecal aagle to the sagittal sulcusIts distal border

may be formed by the branch of the mesial fork of the sagit-

ttal sulcus which veers toward the mesio-buccal angle a If

both branches of the mesial fork cross the mesial occlusal •

margin? a secondary: groove may be present to form the distal

border® - , . : '' '

■ Aspects: ' 1 ■ ; ' ' -1/ - - • ■

a ® ■ Absent (Plate lis k) \ •

b 0 Present (Plate l s e and f)

Trajt IIo 2a Mid-mesial Occlusal Paracone Ridge

The mid-mesial occlusal paracone ridge lies between the

mesial and median occlusal paracone ridges® Its-mesial

border coincides with the distal border of the mesial ridge

when present and its distal border coincides with the mesial

border of: the median ridge®

Aspects ~

a® Absent (Plate II? 1) v : .

' b,: Present: (Plate I-, f) ■ vy . .

Trait IIs 3 q Median Occlusal Paracone' Ridge

The median occlusal paracone ridge extends from the apex

of the paracone on the buccal margin to the central portion

of the sagittal sulcus® It is the most prominent of the

paracone occlusal ridges and is almost always present® It

was found to be absent in only one case (Plate !«, c) out of . ; . , 18 the whole sample0 This ridge also shows the greatest range

of variation of all the occlusal paracone ridgesa

Aspects %

ae Absent (Plate iy c)

b 6 Present (Plate iy d) •

Trait II& k-a Mid-distal Occlusal Paracone Ridge

The mid-distal occlusal paracohe ridge lies between the .

median and distal occlusal paracone ridges* Its mesial bor­

der coincides with the distal border of the median ridge and

its distal border coincides with the mesial border of the

distal ridge when present»

Aspects: : -

a* Absent (Plate II9 k)

• . Present (Plate 1I9 ti)

Trait Ila $ g Distal Occlusal Paracone Ridge • '

The distal occlusal paracone ridge runs from the disto- buccal angle of the paracone to the sagittal sulcus* Its mesial border may be formed by the branch of the distal fork of the sagittal sulcus which veers toward- the disto-buceal

angle* If both branches of the distal fork cross theddistal occlusal border, a secondary groove may be present to form the mesial border^

Aspects:

a* Absent (Plate II, i)

b 0 Present (Plate Iy a and e)

Trait III* Occiusal Protocone Ridges

As many as four ridges may be present on the occlusal ■ ■ ■ ■ . 19 surface of the protocone® From mesial to distal they ares

lo Mesial Occlusal Protocone Ridge

2 0 Median Occlusal Protocone Ridge

3o Mid--distal Occlusal Protoeone,.Ridge

4.0 Distal Occlusal Protoeone Ridge

The apex of the protoeone oh the lingual occlusal margin is

always raesially- located in respect to the apex of the paracone6.

Generally the protoeone is:smaller and the occlusal Surface

less rugged than the paracone (Plate l, f)s the ridges being :

less distinct0 In some cases9 howevers it is as large as the

paracone (Plate II* g ) 0

As with the occlusal paraeone ridges* only presence or

absence will be dealt with^ the configurations of each ridge

being too varied to be accurately classified with the limited

sample observed. These ridges may occur in any^.numerical

combination with the exception that the.median occlusal ridge

is usually present, :i v: '

There are no lingual angles of: the protoeone corresponding

to the buccal angles of the paracone. The protoeone curves

from the mesial and distal ends of the sagittal sulcus to the

apex of the cusp * forming a p arabola in outline „

Trait III0 1, Mesial Occlusal Protoeone Ridge ,

The mesial occlusal protoeone ridge extends along the mesial border of the occlusal surface of the protoeone from the sagittal sulcus, it is usually separated from the median

occlusal protoeone ridge by a shallow depression (Plate I* a) *. 20

a distinct, groove being rare®

Aspects g , ,

a e' Absent (Plate Is f) ;

b e Present (Plate I, a) ,

Trait III® 2 6 Median Occlusal Protocone Ridge — — —— -— :— ———— •—:— — ------: -- ~ The median occlusal protocdhe ridge extends from the

apex of the protocone on the lingual occlusal margin to the

central portion of the sagittal sulcus® It is the most

prominent of the protoeone occlusal ridges and is usually

present (howevers- it is absent more often than the median

occlusal paracone ridge--Plate II ^ !)<, Like the median

occlusal paracone ridges :it shows a great degree of variation when present e -.ul A: s Aspects'i .

a* Absent (Plate II, 1) : <

b s Present (Plate I, d)

Trait H I 3 « Mid-distal Occlusal Protoeone Ridge

The mid-distal occlusal protoeone ridge lies between the median and distal occlusal protoeone ridges and is separated from them by grooves or depressions at the medial and distal marginso It extends from the lingual occlusal margin to the sagittal sulcus0 '

Aspects:

as Absent (Plate I, d)

b 0 Present (Plate II, j) 21 Trait III, Distal .Obelusal Protocone Ridge

The distal occlusal protocone ridge extends along the

distal margin of the occlusal surface of the protocone from

the sagittal sulcus s It is separated me si ally from its ad­

jacent ridge by either a groove or .a depression*

Aspects? . ' v ' : ' - J;' J " ' T"\' - :

ae Absent-. (Plate I# f) =

.. b. Present (Plate 11,, g ) ' , . . . ■ ■ ■

Trait IV0 Me sio-buccal paracone Paras tyle

Often at the mesio-buccal aspect of the paracone a para­

style is observed, A variety of expressions ranging from

Slight bulging to a distinct tubercle is embraced under the t e m parastyle o' It is present most commonly as a bulge. The bulging may concentrate at the buccal end of the mesial occlusal paracone ridg.e.|,.. or it may , center at the buccal ends of both the mesial and pid-mesial ridgeso A slight groove or depression running vertically along, the. mesial buccal surf ace ie of the crown designates, the reparation of the parastyle from ' the main-body of the cusp8 In some cases the groove separa­ ting the mesial from the mid-mesial occlusal paracone. ridges crosses the buccal margin, of the paracone causing the, para­ style to he distinctly separated from the main body of the cusp

(Plate I, e). Any classification of the many varieties of expression of the trait-would be purely .arbitrary, hence, only presence or absence is considered, although the particular type of variation may also be heritable* ■' 22

A disto-buccal paracone parastyle is sometimes presents

corresponding to the mesio-buccal paracone parastyle* The

trait is too rare to treat in this study*

Aspects:

a* Absent (Plate Is £) . .

b 0 Present (Plate II$ h) .

Trait 7* Marginal Occlusal Ridges -

A fine ridge of enamel may follow the occlusal borders

of both the paracone and ,prof be one a. producing the marginal

occlusal ridge* This ridge is usually broken at the sagittal

sulcus and at the apices of the two cuspss hencej, it can be

considered as four separate units0: The ridges may be present

in any of the possible numerical arrangements, however, the

marginal paracone ridges .are usually present if protocone

marginal ridges are manifesto -

Trait Vo I* Mesial Marginal Paracone Ridge

..The mesial marginal paracone ridge follows the. occlusal

border of the paracone from the apex of the cusp mesially to

' the s agittal sulcus e 1 -

Aspects i' ' ’

aQ Absent (Plate II, -h.} . , , : . .

bo Present (Plate II, 1)

Trait V* 2 0 Mesial Marginal Protocone Ridge.

The mesial marginal prbtocone ridge extends from the apex

of the protocone mesially along the occlusal border to the

sagittal sulcus* ■ -Aspects;

a0- Absent (Plate II9 k) .

bo Present (Plate II,.’. 1} - ;,

Trait Vp 3„ Distal Marginal Paracone Ridge

The distal marginal paracone ridge extends from the apex of the paracone distally to the sagittal sulcus, follow- ing the occlusal border of the cusp»

Aspects; •; :

a0 Absent (Plate I, b)

bo Present (Plate II, 1)

Trait Vo ko Distal Marginal Profocpne Ridge

The distal marginal protocone ridge extends from the , apex of the protocone distally to the sagittal sulcus, follow­ ing the occlusal border cof the cusp»

Aspect's; ’ . .

a. Absent (Plate I, f ) •

b 8 Present Plate I, e) Plate I. Illustration of Traits Plate II. Illustration of Traits .STATISTICAL TREATMENT

If morphological traits of the maxillary first premolar

crown are basically heritables they should be concordant in

monozygotic twin pairs? that is, the right tooth of one twin

of a pair will show the same aspects of a trait as the right

tooth of the other" twin and the same is true of the lefts.

(Elate I-Vs e s fs g s h) 0 Therefore, there are two possible

concordances in-a twin pair®. One hundred percent concordance

cannot be expected for, each trait.;: In mono zygotic twin; pairs because of mirror imaging and possible environmental varia­ tions caused by the twinning processes but very high'con­ cordances should be observed® 'When compared to the expected frequency of concordarice vof a population taken in random pairs, the monozygotic twin pairs’ sEouid show a significantly higher degree of concordances to indicate that something .

Other than chance was opefating in the appearance of the - traits® When dizygotic twin pairs are compared to the random popul at ion; in a s imil ar manne r, they ; shoul d show no signifi­ cant difference in Gohcordance® (Plate ill, a, b, c, d ) 6

' . To ascertain the expected degree of concordance of a

Specific trait for a population taken in random pairs, a sample of 8If teeth were examined," - The sample consisted of

I® Newman, Freeman, Hoizinger, 1937, PP, 36-31 < ' ■ ■ 25 Gaueasoid easts from tlie colleetion of the Laboratory of

.Ehysical Anthropology,, i; .. . ,

Each trait,has at least two aspects.. The teeth.were

arranged according to the aspect of the trait that they

possessed© The number of teeth under each aspect divided by

the total number of teeth observed for the trait gave the

frequency of the teeth expected to fall within each aspect ©

Concordance in each aspect is the condition of any pair taken at random being similar, disicordance is the condition

of any pair taken at random being different© By squaring the frequencies of a given trait and summing these squares, the frequency of expected concordances forthe given- trait is obtained, For example Trait IV has two aspects, a and b, with frequencies A .and, B respectively. The probability of a concordance under aspect.; a. is represented by A , The probability of a concordance under aspect b is represented 2 ■ 2. ' -2’ - : by B o The sum of A and B gives the total frequency of expected concordancesfor the traito The expected frequency of discordance in,the trait isgiven by 1-(A^ plus B?}, since total- concordance plus total discordance equals l, ?

For ascertaining the degree of observed concordance of a specific trait in twins, a sample of 1 ? monozygotic and 13 dizygotic twin pairs were examined® The sample consisted of

Caucasoid casts from the collection of the Laboratory of

Physical Anthropology,

For each twin there are two possible comparisons, the . . ' : 2 6

' „ "" right teeth and the left teeth* Kaeh comparison is treated

as a separate event making a total of 3 ij. possible concordances

for monozygotic and 26 for dizygotic twin pairs* The size

of the sample is not uni form for all traits because of wear

or caries . obliterating parts of the crown of the teeth*

The observed frequency of concordances in the monozygotic

and dizygotic twin pairs was computed for each trait* The

number of observed concordances divided by the number of

possible concordances for each trait gave the frequency of

observed concordance in the sample

In determining the expected and observed frequencies of

concordance Traits II0 3 * and III* 2 * were omitted* Since

both traits showed almost exclusively aspect b (presence),

the expected frequencies of concordance would be the same as

the observed frequencies in dizygotic.and monozygotic twin

pairs* '

The monozygotic twin pairs show a higher degree of fre­

quency of concordance than do either the dizygotic twin pairs

or the population taken in random pairs* To. test for signi­

ficance in the differences between expected and observed

frequencies, the standard error of the expected concordance

frequencies was computed and the differences expressed’in

terms of these standard errors* The formula

v V was used to compute the standard errors, letting sigma be

the standard error, the concordance frequency, ^ the ■ disGordanee frequencys and n the maximum number of concordances

possible•for the trait9

If the differences between the expected and observed v

frequencies of concordance divided by the standard errors■

of the expected frequencies of concordance were less than

twdg that is9 within two standard errors of the mean,-,

chance can be said to be operating in the determination of the concordance frequencies* Table 1 shows this comparison „ for dizygotic twin pairs in relation to the population taken in random pairs» In eight of the thirteen traits the dizygotie twins fell within two standard errors of the expected mean® .

In four traits they were within @1|_6 of a standard, erf brief the nonsignificance limit@ Trait II*. 2e showed a variation of li-067 standard errors from the meang the observed eon-. cordanee frequency being much less than the expected ; concordance frequency6 In general, however g the dizygotic twin pairs seem to fall within the range of expected chance0

Table 2 shows a similar comparlson for monozygotic twin pairs 0 "In all cases the monozygotic twins are more than two V

Standard errors from the mean* The .frequencies of concordance ares therefore, not governed purely, by change in this group, and heredity must play a dominant role in the determination of the structures 6 " . 28

Table 1

Differences between Expected and Observed Concordance

Frequencies Expressed in Terras of the Standard ErrbrsJ . .':

of Expected Concordance Frequencies in relation to Dizygotic

Twin Pairs

a b • v • a; D

Trait Expected Observed Standard 1,A - B l concordance concordance j Errors. r u.; q— frequencies frequencies

lo =237 = 174 = 0656 0=97 I I o lo - 0678 =500 - o0721 ' 2=46

. - *208 . _ _=D765. 4° 67 . - h *507 o375 =0771 1.71 5 = ,458 =0765 ' 1,39

111=1= 0625 =750 = 0747 1 = 67 ; 3 = 069! =580 *0713 1,56 lj-o = 580 = 750 = 0762 2=23

17 = 0 665 = 760 = 0728 1=30 7 = 1= = 625 = 800 = 0747 2,34 2o = 808 = 800 =0608 0=13

3 = = 680 = 840 = 0720 2=22 910 = 830 o0442 1 = 81 29

Table 2 ‘

Differences between Expected and Observed Concordance

Frequencies Expressed in Terms of the Standard jEffrorsikm::.

of Expected Concordance Frequencies in relation to Monozygotic

Twin Pairs

V A ' ' E ' ' C ■ ■ • : F

Trait Expected Observed Standard \ Ar -' E ]' concordance concordance ( ' E r r o r s _ frequencies frequencies u

I 0 ^ ,237 ,969 ,0656 11,15

H o I, *678 ,969 ■ ,0 7 2 1 4,03

2 * ,565 o9 0 6 o 0765 4,46

4= ,507 -0.906 * ,0 7 7 1 5 ,1 7

5= .565 1=000 ,.0765 : 5,83

H l = l = 0625 *973 = 0747 4,66

3.= 0691 o940 o0713 4*08

4= =580 =940 =0762 4*72 iv, =665 1 * 0 0 0 , *0 7 2 8 4 * 6 0

V. 1 , =625 *940 , 0747 4*22

2= =,808 1=000 =0.608 3 = 11

3 = =680 1= 000 =0720 4 , 4 4

4 = =910 1= 000 =0442 2 * 0 4 Dizygotic Twin Pair Plato IV, Monozygotic Twin Pair POSSIBLE ETHNIC VARIATION IN ASPECT FREQUENCY

The collection of dental casts at the Laboratory of

Physical Anthropology included samples of several ethnic groups0 These were observed to determine possible aspect variations within each group0 ■

Ethnic groups are defined by Ashley-Montagu^ as being s

o o a arbitrarily recognised groups which in virtue of the possession of a more or less distinctive assemblage of physical traits, through a common heredity^ are statistical­ ly distinguishable, from other groups within the species®

Ethnic groups are clusters within a division* He defines a division as being r

o ® * comprised of a number of ethnic groups classified together on the basis of their possession of- certain common characters whichS'Serve to distinguish that division from others* ; • . v ' ■ . ' yC

He recognizes four divisions, Caucasoid, Mongoloid,. Negroid, and Australoid*; In summing up the basls for classification into ethnic groups and divisions, Montagu states:

, » * ethnic group and divisional differences simply represent more or less, temporary expressions of variations in the relative frequencies of genes in different parts of the whole species.

Three of the four divisions were represented in the casts observed, Caucasoids, Mongoloids and Negroids, A fourth group of Caucaso-Mongoloid mixture was also considered as a division

1, Ashley-Montagu, 195isPP«'29l|--295» 31 for simplicity in classification of tlie material0 Under the

Mongoloid division^ three ethnic groups were manifest:*

Papagos * Yaquis* and Chinesee The Caucaso-Mongoloid division

consisted of two ethnic groups* Mexicans and Yaqui-Mexicans«,

The sample of Caucasoids was represented by llj.0 teeth*

the random population and the dizygotic twin group being

combinedo Since both groups have been shown to be a function

of chance statistically* such a combination is permissiblec

The Negroid sample consisted of l|j) teeth* and the Mongoloid

of 120 teeth* 37 of which were Papago* 5>7 Y&qui, and 26

Chineseo The Caucaso-Mongoloid sample consisted of 6£ teeth*

I4I .being Yapui-Mexican and 2i}. being Mexican, These samples were too small to ascertain accurate variations in ethnic groups or divisions* but indications of any probable differ*-' ences will be shqwh,. : ,

The same traits whicli'were described •for the Caucasoid sanple appear on the crowns :of all the other groups in the same, manner® The frequency with which the aspects of the traits are manifest varies within each group. Tables 1|_* 3 and 6 show the expected frequencies of occurrence of the aspects of each group for the five defined traits. Aspect frequencies for Trait I indicate that the Chinese have a high frequency of aspect f and a very low frequency of e. The reverse is true for the other groups. Trait II in­ dicates in general that the Eegroids have the highest fre­ quency of paracone ridging of all of the groups* the Caucasoid . am# Gaucaso“Mongoloid grottisg stiowiiig relatively the lowest

' ’ i . - ■ . ■ . . - " frequencies* The same situation is largely true .for the prdtoeone ridging (Trait III)e Ah in Trait IIs the legroids e^iblt relatively the highest degree of ridging9 the-

Caucasoids the lowest» Trait IV Indicates that Mongoloids do =not have as high a frequency of presence of a mesio-buccal

' - ■ \ ^ ■ , paracone parastyle as do Negroids and Caucasoids,, Trait V indicates that the Negroids have a higher frequency of mar­ ginal ridging than do the : other groups® Studies of .larger samples coUid well prove -flie above trends in variation® 33

$able Ij.

Aspect Frequencies in Ethnic Groups trait le Group Aspect a b e d a f Caucasoid , 029 ®1# .108 *209 *309 *201 Eegroid »oli5 * Oii.3 ,27k *318 *227 ,0 9 1 Mongoloid 9.01? ■ *023 >133 *133 *417 *273 Pap ago . .,..►000 .ooo *031)-. *081 *487 *378 Yaqui ♦ 000- *033 *11)0 ol73 *344 *106 Chinese *077 *038 .231 oil 6; *038 *300 Caucaso-Mongoloid .*032 *016 0231). *190 *238 2270 Mexican 0 000 *000 *373 *123 . *230 *230 Yaqui-Mexiean o09i *026 ol79 =231 *231 *282

trait IIe Group trait and Aspects 1® 2o li-e ; 3* a b a b a b a b Caucasoid s250 *730 061I1. *386 *437 *343 *36.4 *636 Eegroid *81)3 *267 *733 *089 *911 *067 *933 Mongoloid >746 *383 *413 *203 *797 ol32 * 848 Pap ago ,31k @686 ®3l4 *486 *029 *971 *086 *914 Yaqui a 123 9877 *649 *331 *228 *772 *246 *734 Chinese c, l|-6l *339 *339 *461 *384 G 6l6 *038 *962 C aueas. o. -Mong^ oloid 1 tl ,,301 *699 *371 *429 *317 o683 *301 *699

Mexican *363 *637 *409 *391 sl8 l *819 *318 *682 Yaqui-Mexic an »268 *732 *639 *346 >390 0610 *292 *708 lable $ '

Aspect Frequencies in Ethnic Groups

Trait: III o

Group Trait and Aspects ' 1* 3» : 4° a b a b a b

Caucasoid .2 3 0 «i770 #770 #230 .280 o7 2 0

Negroid o089 .911 o600 olj.00 .200 .800

Mongoloid .2 9 1 .709 #892 *108 .500 .500

.Pap ago .351 o6i}9 .919 ,081 0676 f32k

Yaqui .2 6 3 o737 o912 , »088 .526 oli74-

Chinese .269 .731 .8 0 8 ol9 2 .192 .808

Caucaso"Mongoloid 01+23 o577 .766 .234 .5 0 0 .500

Mexican o30ij- 0 696 .826 <>17k- .565

Yaqui-Mexican olt.87 .513 .732 .268 o il-63 . .537

- ■ * Trait IV.

Group Aspect - a b

Caucasoid ol90 - #810

- - Negroid .81£

Mongoloid o W .517

Pap ago 0676 > 3 2 ^

Yaqui o35l *649

C auc aso-Mongoloid 0I4.78 .5 2 2

Mexican #333 .66?

Yaqui-Mexic an 0 ^ 6 okMb ' V.

35

.Table 6

Aspect Frequencies in Ethnic Groups

5?rait Vo

. Group Trait and Aspects : 1 . 2= 3o 4= a b a b a b a b

Caucasoid 0.795' o205 =887 =113 =845 =155 =936 =o64

Negroid >318 =682 =467 =533 =444 =556 o 6oo =400

Mongoloid = 716 o 281j. .825 =175 =875 =125 =917 =083

Pap ago ■■>■892 =108 =946 =o54 = 946 =054 1= 000 =000

Yaqui =632 =368 = 7 72 > 228 =877 =123 =930 = 070

Chinese o3k& =654 = 769 o 231 =769 =231 =769 = 231 ' V' Caucaso-Mongoloid o 6 46 =354 =788 =212 =701 =299 =865 =135

Mexican o84o =260 =913 =087 =840 =260 1= 000 = 000

Yaqui-Mexican • =596 = 404 =721 =279 =682 =318 =795 =205- SUMMARY AZD C ONCLUSIONS

The problem was to define and Isolate morphological

traits on the crown surface of the maxillary first premolar^ „

and then to determine through a comparison of monozygotic - '

and dizygotic twin pairs with a population taken In random

pairs whether these traits were heritable,

lo In the examination of casts of 1+38 teeth discrete morpho­

logical structures were manifest on the crown surface of the

maxillary first premolar«, Those structures which appeared

frequently on the teeth were considered as Individual 'units ^

or traits„

2o In an examination of the casts of 17 monozygotic and 13

dizygotic Caucasoid twin pairs, monozygotics were found, to

possess a higher degree, of concordance frequencies for the

defined traits than did the dizygotics*

• 3© The expected frequencies of concordance were obtained

for a Caucasoid population taken in random pairs,, To test

for significance between expected and observed frequencies,.

the standard errors of the expected concordance frequencies; were computed and the differences expressed in terms of these

standard errors0

q© Dizygotic twin pairs generally fell within the non- significance range statistically, showing that chance . .V- h ■' - v ■ ■ ■■■ :: ' : largely operated in determining the frequencies of the 37

structureso

5d Monozygotic twin pairs fell well within the rai^p of

significance statistically^ showing that, their high degree

of concordance frequencies was.subject to other factors than

chance«, External environmental factors.-, cannot-play a role

in determining the frequency of concordances since the

structures on the crown surface are completely formed before

eruption of the tooth. The very similar environment in

which the twins are generally reared (both internal and

external) cannot account for the high degree of; concordance

frequencies since dizygotic twins.s who are. subject to: like

similarity of environments fell within the statistical range

of chance0 Since monozygotic twins are genotypically identi­

cal genetic factors must be. the cause of this relationship«,

6 S The structures of the crown surface were the same for

all ethnic groups examined0 The frequencies with which aspects of-these structures occurred varied with individual ethnic groupss Due to the limited si&e of the sampless these varia­ tions can be considered only as trends® The possibility exists that differential frequencies of phenotypic .expressions in ethnic groups may be the result of different gene si acting to produce practically the same end effects® It is, therefore* important to recognize the genetic mechanisms controlling the formation of the structures in such an ethnic classification® BIBLIOGRAPHY

Arkin$ H ss and CoIton$ R» Re . ' ,1937 Table' of Squaress Square Roots# Cubes# and Cube Rootse Barnes and Noble# Inceg Hew York# 1937*

.. Askley“Montagu# M, Pa . • • I933 The Dentition of Identical Twins with Particular Reference to an Idential Pathological. Condition, Human Biology# v0 J, Hoe 4# Deca 1933# pp* 6 2 9 -64^0

195>1 An Introduction to Physical Anthropology = Charles G. : Thomasg = Springfield# Illinois9 19^1® "

Bachrach# F. H 0 # and Young# Mo I9 27 A Comparison of the Degree-of Resemblance in the ' Dental Characters Shown in Paris of Twins of- ■ Identical and Rratemal Types 0 British Dental Journal y, X M I I I # Ho, .21# Ho Vo 1# 1927# PP® 1293.

, Birdsell# ' : • . 1949 The Problems of the Early Peopling of the Americas as Viewed from Asia0 Papers in the Physical Anthro-~ pology of the American Indian# the Viking Fund, Incc„ 1 9 4 9 # pp. i “6d. ■ ■" - ...

Boydy W* C.-- , - . 1950 Genetics and the Races of Man® D0 C, Heath and Coo# Boston# 195)0° Dahlberg# A» A. : . 1 9 4 9 The Dentition of the American Indians,, Papers on ... .. the Physical Anthropology of the American Indian# the Viking Pund» y Inc . 1949T^P° 138“176°

Goldberg# S, I929 Biometries of Identical Twins from the Dental View­ point a Journal of Dental Research# v0 IX# Ho®. 3# June# 1929# ppo 3 6 3 - 4 0 9 © ‘ “ .

Gregory# H,- H, ■ ; : . , 1 9 2 2 The Origin and Evolution of the Human Dentition, Williams and Wilkins Co® # Baltimore# 1922.

Honseholder# James E 0 Pb- : .. . Personal Conference., '.• . ' ' .

Hrdlleka, Ales, ' ' — — ' - 19 2 1 Further Studies of Tooth Morphology, . Arneri0 an Journal 39

of Physical Anthropology^, v 0 IVs N qo 2 s April - June, 1921, pp. lifl-17'6* .

Korkhaus, G-o . " ■ • ; 1930 The Inheritance of Crown Form and Size of Human Teeth , Zt s Anata Entwgs Gescho, V» 91g 1930, po

Kraus, B; So, and Purr, M. I».- . -• Lower First Premolars, Part Is A Definition and Classification of Discrete Morphological Traits, Journal of Dental Besearch. In Press,

Kraus, B, So, Householder, James E 0, and Jones, B, - A Study of the.Heritability in Lower First Premolars, . , In Manuscripts : ' ; '

Hewman, H, H« ■ ■ v:/ ; ; : 19i}.0 , Multiple Human- Births 0 Doubleday, Doran and Co0, , Inc ,, Hew Mork,. 19W:, / . : '

Herman, H,. H», Frpeman, F, . H6.s and. Hoizinger, K. J, ; 1937 Twins: i A' Study, of Heredity and Environment , Univer­ sity of Chicago Press, Chicago, Illinois, 1937* Pedersen, p. 0*., ’ - . ' 19l}-9 The Bast .Greenland Eskimo Dentition, Numerical • Varia­ tions' and Anatomy,. Branco Lunos Bogtrykkeri, K^benhavn, I9I19* •

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