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Complex segregation analysis of dental morphological variants.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 78:37-59 (1989)
Complex Segregation Analysis of Dental Morphological Variants
CHRISTIAN R. NICHOL
Department of Anthropology, Arizona State University,
Tempe, Arizona 85287
KEY WORDS
Dental anthropology, Genetics, Environmental
effects, Mode of inheritance, Crown traits, Dental crown
ABSTRACT
A set of 20 morphological variants of the dental crowns and
four characteristics of the jaws are tested for probable mode of inheritance
using the complex segregration analysis method of Morton et al. (Am. J. Hum.
Genet. 23:602-611, 1971). Models tested include three two-allele single-locus
models (dominant, codominant, and recessive) and a model employing the
polychotomized normal distribution of liability (an additive polygenic model),
with transmissibility estimated via maximum likelihood. Most of the traits
studied are observed using ordinal scales with several grades, and many are
tested using more than one dichotomy of their scale. These multiple analyses
allow for a n examination of such factors as trait incidence on the results of the
statistical analysis. The results of the analysis yield propositions of major
genes for 13 of the 24 traits examined. Two traits give good evidence of being
polygenic in origin. The remaining nine characters present methodological
problems that do not allow for a definite conclusion on their mode of inheritance at this time. The ability to test varying levels of transmissibility in the
polygenic model allows for a n estimation of the percentage of trait variance
determined by familial factors. Estimates of transmissibility for all characters
examined range from 0 to 1,with a mean of 0.36. These findings may suggest
a large environmental role in the development of dental crown morphology.
However, the possibility exists that difficulties in the ability to classify the
expression of certain traits consistently result in overestimates of the environmental influences on the development of those characters.
Over the years, several investigations into
the mode of inheritance for various characteristics of the human dentition have been
performed. Of the attributes examined, those
that have received the most attention are the
tooth crown diameters and Carabelli’s trait.
There is general agreement that the inheritance of crown dimensions is multifactorial
(Goose, 1971; Townsend and Brown, 1978a).
Divergent opinions do exist on the role of sexlinked genes in the formation of tooth size.
Some studies have indicated that X-linked
genes play a role in size development (Garn
et al., 1965; Lewis and Grainger, 1967; Alvesalo, 1971); others have demonstrated that a
sex-linkage model is not necessary to explain
the observed sexual dimorphism (Goose,
1971; Townsend and Brown, 1978a; Potter et
al., 1983). Most of the more recent studies of
0 1989 ALAN R. LISS, INC
the genetics of crown dimensions have accepted polygenic inheritance and have concentrated on the analysis of the relative
contributions of genetic and environmental
factors to the development of tooth crown size
(Potter et al., 1968, 1976, 1983; Goose, 1971;
Alvesalo and Tigerstedt, 1974; Townsend and
Brown, 197813; Corruccini and Potter, 1981).
Of the morphological characteristics of the
tooth crowns, the one that has provoked the
most argument over its mode of inheritance
is Carabelli’s trait. Several researchers
(Kraus, 1951; Tsuji, 1958; Turner, 1967b; Kolakowski et al., 1980)have suggested the possibility of single or major gene involvement
Received November 9, 1987; accepted August 31,1988.
Address reprint requests to C.R. Nichol, 66 Grandview DriveLeft Side, N. Tonawanda, NY 14120.
38
C.R. NICHOL
in the expression of this trait. Cadien (1970)
has proposed a two-locus model to explain
the inheritance of Carabelli’s trait. Also,
analyses by Goose and Lee (1971) and Lee
and Goose (1972) have indicated that the development of the trait is under the control of
many genes.
Various other dental variants have also
been suggested to have single or major gene
determination. Evidence of major gene influence on the development of winging of the
upper central incisors has been found via
segregation analysis (Escobar et al., 1976).
Turner (1967b)suggests the possibility of major gene involvement in the development of
both the hypocone and incisor shoveling
based on correspondence of categories of
expression to expected Hardy-Weinberg proportions in a population sample. However,
Turner himself (1967a, 1969) has indicated
the potential weakness of such a method for
determining the mode of inheritance of a
character, and Sofaer (1970) has pointed out
that even obviously nongenetic events can
sometimes fit Hardy-Weinberg expectations.
Studies that have analyzed various sets of
characters (Scott, 1973; Harris, 1977; Berry,
1978) indicate that morphological traits of
the dentition tend to be inherited in a complex, probably polygenic, fashion.
The methods employed in the above-mentioned studies include a variety of population
and familial analyses of trait expression.
Much of the methodology used in the past
does not allow for discrimination between a
complex, multifactorial model and a singlegene model. Usually, testing was done to find
goodness-of-fitto a single-gene model. If single-gene determination was rejected, then
polygenic inheritance was accepted. Recent
developments in complex segregation analysis allow for simultaneous testing of polygenic (at least using a simple additive
hypothesis) and single-locus models. These
developments permit rejection of the additive
polygenic model even when the data do not
yield good fit to a single-gene model.
Segregation analysis is generally the
method of choice for a familial analysis of
trait inheritance. Early methods of segregation analysis (Snyder, 1932; Smith, 1956)
were designed to test statistically whether
the distribution of expression of a character
among the offspring produced by matings of
individuals with certain combinations of trait
expression corresponds to expected Mendelian proportions. Beginning in the late 1950s,
work by Falconer (1960,1965) and others laid
the groundwork for testing the goodness-offit of data to polygenic models compared to
single-gene models. In the 1970s, methods
developed by Morton and his colleagues
(Morton et al., 1971; Morton and MacLean,
1974) out of this work began to allow for
simultaneous testing of single-gene and polygenic models.
In the present study, a series of morphological traits of the human dentition undergo a
complex segregation analysis in a n attempt
to find information concerning their mode of
inheritance. Parameters of concern to the
study are the degree of dominance of the
allele for affection when the data yield best
fit to a single-locus model and the transmissibility when the data correspond to a polygenic model. The results of the study are
intended to give insight into the possibility
of major gene influences on dental trait
expression and the relative proportion of genetic and environmental influences on the
development of multifactorially determined
characters.
MATERIALS AND METHODS
Sample
Between 1946 and 1972, A.A. Dahlberg directed the collection of casts of dental impressions taken from persons residing in the Gila
River Indian and Salt River Pima-Maricopa
Communities near Phoenix, Arizona. During
this period, approximately 8,800 sets of dental casts were collected. Because many individuals had impressions taken on more than
one occasion, the 8,800 casts actually represent 3,632 individuals. The number of people
with casts in the collection is approximately
one-third of the total population of these two
reservations a t the termination of the collection period. Accompanying the collection are
detailed records giving a range of information concerning each individual represented.
Most important to the present study are records concerning known relatives. From this
information, extensive pedigrees have been
compiled. Because the familial information
comes from several sources, including medical, educational, and tribal personnel, as well
as the individuals themselves, it is reasonable to expect that the familial data are accurate. This magnificent research collection is
now housed in the Department of Anthropology a t Arizona State University.
A sample of casts representing 600 individuals from 83 nuclear families is selected from
the collection for use in the present study.
SEGREGATION ANALYSIS OF DENTAL TRAITS
39
TABLE 1. Distribution ofsibship sizes i n the sample’
Number of
sibshios
2
3
4
5
13
11
14
11
Number of sibs
6
7
8
9
10
11
12
7
7
3
0
1
9
7
‘Total number of sibships, 83; total number of sibs, 434; total number of males, 215; total number of females, 219;
average sibship size, 5.22.
These 83 nuclear families are all sibships
identified in the collection for which casts of
both parents and at least two sibs are available for observation. Information concerning
the sample is given in Table 1. The sizes of
the sibships range from 2 to 12, and the number of sibships with each number of sibs is
presented. Because both parents from each
sibship are included in the sample, the 600
individuals includes 166 parents and 434
sibs.
The sex distribution among the 434 sibs in
the sample is nearly equal, with a slight
excess of females (50.5%).The sex distribution in the sample is fairly representative of
that of the collection, in which 50.9% of the
individuals are female. Although sex dimorphism does occur in tooth crown dimensions, most of the morphologic attributes of
the crowns have not shown strong or consistent sex differences in occurrence or expression (Nichol, n.d.). One notable exception is
the distal accessory ridge of the upper and
lower canines. Scott (1973, 1977) first demonstrated strong sexual dimorphism in the
expression of this trait, and his findings have
since been borne out in several studies (Harris, 1977; Kieser and Preston, 1981; Kaul and
Prakash, 1981). Sex differences have also
been found for shoveling of the upper central
incisor (Harris, 1980). However, these differences are so slight that it took a n analysis of
19, 137 individuak to achieve significance.
Some significant differences in trait frequencies have been found in this sample. In that
a sex-linkage model is not examined in this
study, any sex dimorphism is not directly
dealt with. However, significant findings of
sex differences will be noted as possibly complicating factors to the stated goals of this
study.
The segregation analysis method selected
requires neither of the aforementioned conditions, that both parents and at least two
sibs be available, for inclusion in the sample.
Therefore, there is no concern for missing
data, except in those cases when none of the
sibs in a family can be observed. When none
of the sibs in a family can be observed for a
trait, that sibship provides no information to
the segregation analysis and is therefore
eliminated from the analysis of that character. The column in Table 2 labeled ‘‘ > 0 Sibs
observed” gives the number of sibships used
for each trait, on each tooth the trait is observed on, and a t each of the dichotomies of
the observation scale submitted to a run of
the segregation analysis. The numbers of sibships used in any run range from 83 €or several traits to 72 for UI2 interruption grooves.
The condition that casts of both parents be
available is implemented because a n assumption concerning equal fertility among
phenotypes need not be made when the status of both parents is known. However, for
each of the traits examined, one or both parents of some sibships are not observable;
therefore, the assumption of equal fertility
must be retained for these families. The column in Table 2 labeled “Both parents observed’’ gives the number of sibships in which
both parents could be observed for each data
run. Because of attrition and caries some
traits are difficult to observe in adults (parents). For six characters; distal accessory
ridge UC and LC, groove pattern LM1 and
LM2, cusp 5 LMl, and the maxillary torus
(the latter because of casting methodology
early in the collection period rather than
wear and caries); more than half of the sibships used in the analysis have one or both
parents unobserved. The worst case is distal
accessory ridge LC, for which only 14 of the
80 (17.5%) sibships used have both parents
observed. Also contained in Table 2 is a
breakdown of the number of sibships with
both parents observed by the number of parents affected.
Two traits have been tested to determine
whether using the sibships in which the status of both parents is not known had any
effect on the results. The traits selected are
40
C.R. NICHOL
maxillary incisors: winging, procumbency,
and insetting; four characteristics of the jaws:
the maxillary and mandibular tori and the
forms of each of the two arches; and 17 morphological attributes of the tooth crowns.
Several of the 17 crown variants are observed on more than one tooth, bringing the
total number of attributes examined in the
study to 38. All traits are observed using
either the Arizona State University standard
reference plaques or standards developed by
the author (Nichol, n.d.1. The names of the
traits and the teeth for which they are observed are given in the first columns of
Tables 2-4.
Because the segregation analysis method
selected is designed for use with dichotomous
characteristics, and the traits are generally
observed using ranked scales with several
classes, decisions must be made on where to
dichotomize the distribution of expression for
each trait into the normal and affected categories. For all but four characters, a dichotomy is made at what is considered to be the
presence/absence level. In the four cases
when a trait is not analyzed at the presence/
absence breakpoint (shoveling of both the
central and lateral maxillary incisors and
hypocones of the maxillary first molar and
the hypoconulid of the mandibular first molar), a dichotomy a t this level places 100% of
the observed sib (or, in the case of the hypoconulid, all but one observed sib) in the affected category, making analysis a t this level
of expression pointless.
Furthermore, all traits that could be analyzed a t more than one scale breakpoint
underwent more than one analysis. The determination of whether the analysis is to be
done at a certain breakpoint is made on the
basis of the frequency of individuals demonstrating expression at or beyond the breakpoint. Frequencies below 5% or above 95%
are considered to be too extreme to be informative. In two cases, this guideline is violated. For labial convexity of the lateral
incisor, a n analysis is done a t the presence/
absence level even though the frequency of
individuals in the affected class (trait present) is 95.4%. For the protostylid of the lower
second molar a t the 112 breakpoint, a run is
made even though the frequency is only 3.3%
for reasons to be discussed. When the trait
frequency changes less than 5% when the
breakpoint is moved up one grade, the inforCharacters examined
mation is considered to be redundant (test
Twenty-four traits are analyzed for proba- runs indicated this to be true), and a run is
ble mode of inheritance in this study. In- not made a t the higher of the two breakcluded are three positional variants of the points. The reasons for performing multiple
those showing the greatest raw (transverse
ridge LP2 at the 011 breakpoint) and ratio
(protostylid LM2 at the 112 breakpoint) differences in frequency among sibs between
those sibships with both parents observed
and those with one or both parents not observed. The results of these tests show some
slight changes in the calculated likelihood
ratios, but no changes in what model is selected as the best fit, for both traits. These
findings suggest that a concern over the assumption of equal fertility among phenotypes is unwarranted.
The condition that casts of at least two sibs
be observable is employed because the
amount of information obtained per sibship
increases with the size of the sibship. Furthermore, the same sample is used in a path
analysis of the inherited and random factors
involved in the development of dental crown
diameters (Nichol, n.d.). The path analysis
requires sib correlations, making the availability of at least two siblings per family a n
important consideration. The column in Table 3 labeled “Sibs observed under total sample” gives the number of sibs who could be
observed for each trait in each run and is the
number used as N in calculating the trait
incidence. The number of sibs observed
ranges from 423 for Carabelli’s trait to 205
for UI2 interruption grooves. Careful examination of this column reveals that there are
two different numbers of sibs observed for
cusp 5 on the lower second molar. The difference is due to there being 13 individuals for
whom it is possible to see that cusp 5 is
present but for whom it is not possible to
estimate the size of the cusp because of the
presence of cusp 6.
Table 3 also gives the trait incidence used
in each run of the segregation analysis. The
incidence in any run is the total number of
offspring considered affected, a t the scale
breakpoint being analyzed in that run, divided by the total number of offspring observed. The incidence given under the
heading “Total sample” is that used to estimate certain parameters of the models tested
in the segregation analysis. These parameters are mentioned in the discussion of the
statistical method. Also presented in this table are the number of sibs in families, with
each number of affected parents and the incidence among the sibs in each category.
SEGREGATION ANALYSIS OF DENTAL TRAITS
analyses of certain traits are 1)to determine
if the analysis produces consistent results if
the trait incidence is changed and 2) to get
a n idea whether the genetics of trait presence and trait expression are the same. In
all, 76 separate runs of the complex segregation analysis are done. The scale breakpoints
for each of the runs are presented in the
second columns of Tables 2-4. A description
of the scales and breakpoints thereof follows.
Winging. Rotation of the upper central incisors is classified according to the categories
of Enoki and Dahlberg (1958). The classifications have been assigned grade numbers as
follows: grade 1 is bilateral winging; grade 2
is unilateral winging; grade 3 is the “normal” position; and grade 4 is counterwinging
(either unilateral or bilateral). Because of the
understanding that counterwinging and unilateral winging are the result of tooth crowding and that bilateral winging is not, it was
decided to analyze only bilateral winging in
this study. The dichotomy of the trait is made
with bilateral winging placed in the affected
class and all other conditions in the normal
category.
Central incisor procumbency. The degree
of procumbency of the upper central incisor
is scored using a standard developed by the
author (Nichol, n.d.1. This standard presents
a range from nonprocumbent incisors, those
oriented perpendicularly to the occlusal
plane, in grade 0 to pronounced labial inchnation of the teeth in grade 3. It is possible
to analyze procumbency at two breakpoints
of the scale under the conditions set out.
These include absence/presence (0/1) and
slightlmoderate expression (1/2) dichotomies.
The higher degree of expression in both cases
is placed in the affected category.
Lateral incisor insetting. The degree of insetting of the upper lateral incisor is rated
according to a standard developed by the author (Nichol, n.d.1. This scale assesses the
alignment in the arch of the lateral incisor
in relation to the central. The “normal” relationship between the two incisors, when
the alignment is good, is scored as grade 1.
Grade 0 is the opposite of insetting, when the
lateral incisor is out of alignment in a labial
direction, and grades 2-6 demonstrate varying degrees of lingual placement of the lateral incisor from slight insetting (grade 2) to
where the lateral incisor is so far out of alignment with the central that the teeth are no
longer in contact (grade 6). Three dichotomies of the insetting scale are tested. These
are the 1/2 (absencelpresence), 2/3, and 3/4
breakpoints. For all three breakpoints, the
41
higher grades are considered as affected in
the analysis.
Incisor labial convexity. The standard of
Nichol et al. (1984) is used to score the degree
of convexity of the labial surface of the upper
incisors. For both the central and lateral incisors, dichotomies are made between grades
0 and 1(the absencehrace presence level) and
between grades 1 and 2 (the tracelmoderate
expression level). In both cases, the greater
degree of convexity is treated as affected.
Interruption grooves. The presence of interruption grooves of the upper lateral incisors is assessed using the procedure of Turner
(1967a). The trait is dichotomized with the
absence of any grooves considered a s normal
and the presence of a groove crossing the
cingulum in any location classified as
affected.
Shoveling. Scott’s (1973) standards are used
to classify the degree of expression of lingual
marginal ridges (shoveling) on the upper central and lateral incisors. The high frequency
of absence and the weaker degrees of expression of shoveling on the central incisor in this
sample mean that testing cannot be done at
any breakpoint lower than that between
grades 2 and 3. This division is equivalent to
a tracehemishovel dichotomy using the
Hrdlicka (1920) scale. Additionally, it is possible to test the 314 breakpoint on this tooth.
For the lateral incisor, a somewhat greater
spread of individuals across the classes allows for analysis a t the 1/2, 2/3, 3/4, and 415
breakpoints. The latter is the equivalent of a
semishovellshovel dichotomy on the Hrdlicka
scale. In all cases, the higher degrees of trait
expression at the breakpoint are treated as
affected.
Double-shoveling. The degree of expression
of lingual marginal ridges (double-shoveling)
of the upper incisors, canine, and first premolar is scored using the standard of Turner
and Dowda (1979). For all four teeth, a dichotomy is made at the absencelpresence (01
1)breakpoint. Low frequency of expression
beyond the trace level on the canine and
premolar indicates that analysis be done only
a t this point for these two teeth. However,
further analyses are possible for the two incisors. For the central incisor, three other
breakpoints-1/2, 213, and 314-are examined. On the lateral incisor, the 1/2 breakpoint is also analyzed. The higher set of
grades is placed in the affected class in each
data run.
Canine distal accessory ridge. The size of
the distal accessory ridge on both the upper
and lower canines is rated according to
42
C.R. NICHOL
Scott’s (1977) standards. On the upper canine, all possible breakpoints are analyzed,
from the absenceltrace presence (011) level to
the point at which only the largest form of
the ridge (grade 5) is considered affected. On
the lower canine, the low frequency of grade
5 ridges means that analysis cannot be done
at the 415 breakpoint. All other breakpoints
can be and are studied. The affected category
contains the higher set of grades at each
dichotomy.
Premolar mesial and distal accessory cusps.
The presence of mesial and distal accessory
cusps on the maxillary first and second premolars is assessed using the procedures of
Turner (1967a). The trait is dichotomized,
with the absence of any accessory cusp
treated as normal and the appearance of an
accessory cusp on either the mesial or distal
margin classified as affected.
Hypocone (Cusp 4). The standard of Larson
et al. (1975)is employed to classify size variation for the hypocone of the upper molars.
The complete lack of absent and the rarity of
greatly reduced (grades 1 and 2) hypocones
on the first molar prohibits analysis at any
level lower than the 314 breakpoint. The 415
breakpoint, which is the equivalent of a 4-1
4 division of the Dahlberg (1956) scale is also
examined. For the second molar, all possible
breakpoints up to and including the 314 dichotomy are studied. The set of grades at a
particular breakpoint that exhibit the greatest degree of reduction (lower-numbered
grades) are placed in the normal category,
and the higher grades (larger hypocones) are
consigned to the affected class.
Metaconule (Cusp 5). The size of the metaconule (cusp 5 ) of the upper first molar is
rated using the standard of Turner and Warner (1977a). The standard presents a range
from a small to a large cusp. Because of the
low frequency of any degree of expression of
the trait in this sample, the only practical
dichotomy to be made is at the absence (grade
Ofipresence(grades 1-5) level. Absence is considered as normal, and presence is classed as
affected.
Carabelli’s trait. Dahlberg’s (1956) standard is used to score variation in the expression of Carabelli’s trait on the upper first
molar. Two breakpoints are examined in the
study. One is at the absenceipresence (011)
level, and the second (the 112 breakpoint) includes the pit form of the trait (grade 1)with
absence (grade 0). It would be highly desirable to analyze this trait at the noncusp
(grades 0-3)lcusp (grades 4 and higher) level
to obtain an idea whether the cusp and seemingly incipient forms (pits and grooves) have
the same genetics. However, the low freconquency of the cusp in this sample ( - 1%)
traindicates analysis of this question. In both
cases studied, the higher grades are treated
as affected.
Premolar lingual cusp number. Variations
in the number and relative sizes of the lingual cusps of the mandibular premolars are
categorized according to slight variations of
Scott’s (1973) standards. For both plaques,
absence of any lingual cusp is scored as grade
0, a single cusp is scored as grade 1, two
lingual cusps are assigned grades 2-7 depending on the relative sizes of the mesial
and distal cusps, and three or more cusps are
placed in grade 8. The rarity of absence of
any cusp dictates that the no cusp/cusp(s)(01
1)breakpoint cannot be examined. For both
teeth, the onelmore than one cusp (112)
breakpoint is tested. Sufficient frequencies of
two lingual cusps occur on the second premolar for the 213 and 314 breakpoints (considering increases in size of the distolingual
cusp) to be studied as well. In all cases, the
higher grades are relegated to the affected
category.
Transverse ridge. The transverse ridge of
the lower premolar is scored using a standard developed by the author (Nichol, n.d.1,
following a description given by Kraus et al.
(1969). This standard is nearly identical to
Scott’s (1973) standard for the uninterrupted
sagittal sulcus. Grade 0 exhibits no expression of the transverse ridge (the uninterrupted sagittal sulcus), grade 1shows a ridge
with some interruption along its course by
the sagittal sulcus, and grade 2 is a transverse ridge that completely interrupts the
sulcus. Both possible dichotomies are analyzed for both teeth, with the affected class
containing the higher grade(s).
Molar groove pattern. Jorgensen’s (1955)
classifications are used to categorize the
groove pattern of the lower molars. The dichotomy has the Y-pattern treated as normal
and any other cusp contact pattern (i.e., + or
X) considered as affected for both teeth.
Hypoconulid (cusp 5). The size of the hypoconulid (cusp 5) of the lower molars is scored
using the standard of Turner and Warner
(1977b). Because absence of the cusp and occurrence of the smaller forms of the cusp
(grades 1 and 2) are relatively rare on the
first molar, analysis for this character is not
SEGREGATION ANALYSIS OF DENTAL TRAITS
possible a t levels lower than the 314 breakpoint on this tooth. On the second molar,
absence is much more frequent ( 36%).
Therefore, the absencelpresence (011) dichotomy can be analyzed for this tooth. The low
frequencies of tiny (grade 1)and large (grade
5) cusps meant that the 112 and 415 breakpoints cannot be examined. However, the 213
and 314 levels are studied. The presence of
the cusp or the larger forms of the cusp are
placed in the affected category.
Entoconulid (cusp 6). Turner’s (1970) standard is employed to classify the size variation
for the entoconulid (cusp 6) of the lower molars. The first three possible breakpoints-01
1, 112, and 213-are examined for the first
molar, and only the absencelpresence level is
considered on the second because of the low
frequency of the sixth cusp on this tooth. The
presence of the cusp (or the larger cusp in the
cases of the 112 and 213 dichotomies for M1)
is considered as affected.
Metaconulid (cusp 7). The size of the metaconulid (cusp 7) of the lower first molar is
rated according to Turner’s (1970) standard.
Because of the relative rarity of this trait,
analysis was performed only a t the presence1
absence level. Absence of cusp 7 (grade 0) is
treated as normal, and the presence of any
degree of expression of the cusp (grades 1-4)
is considered as affected.
Deflecting wrinkle. The degree of expression of the deflecting wrinkle of the lower
first molar is scored using the standard of
Seybert and Turner (1975). All possible
breakpoints of this scale can be tested. The
011 breakpoint has complete absence of the
deflecting wrinkle (grade 0) in one category
and any form of the trait in the other. The 11
2 breakpoint adds a ridge without a deflection to the absent class. The 213 breakpoint
combines a deflecting wrinkle that does not
extend into the molar grooves with grades 0
and 1. In each case, the higher grade(s) is
placed in the affected class.
Protostylid. Dahlberg’s (1956) standard is
used to rate the degree of expression of the
protostylid of the lower molars. The class
frequencies allow the scale to be divided at
the 011 (absencelpresence) and 112 (combining
the buccal pit with absence) breakpoints for
both the first and second molars. (Actually,
the trait frequency of grade 2 and higher on
the second molar is, as mentioned, lower than
the stated guideline, but analysis at this
breakpoint was retained in the study because of the results obtained, as will be
-
43
shown.) Furthermore, for the first molar, it
was also possible to perform analysis a t the
213 (adding the distal deflection of the buccal
groove to absence) and 314 (further adding
the minimal expression of a secondary groove
to absence) breakpoints. The affected category contains the higher set of grades a t each
breakpoint used.
Maxillary and mandibular tori. The standards of N.T. Morris (1970) are used to score
the size of the maxillary and mandibular
tori. Low frequency of occurrence of the torus
in either jaw dictates that only a n absence
(grade 0) as normal and presence (grades 13) as affected scheme be used in both cases.
Maxillary and mandibular arch form. Variation in the form of the maxillary and mandibular arches is categorized according to
classifications developed by the author (Nichol, n.d.). Each of these standards has two
narrow (parabolic) and two wide (U-shaped)
categories. For both jaws, the dichotomy is
made between grades 1 and 2, placing the
wide forms into the normal class and the
narrow forms in to the affected category. Low
frequencies in both the extreme grades (0
and 3) contraindicate analysis a t any other
breakpoint for either arch.
Scoring procedure
Each tooth in the normal human dentition
exists in two copies. However, only one observation per individual for each character is
desired in order not to inflate the sample size
falsely. A procedure often used to surmount
this problem is to score only one side of the
dentition in each individual. This method
would seem to restrict the sample size unnecessarily, although some authors (i.e., Zubov
and Khaldeyeva, 1979)have surmounted that
difficulty by observing the antimere if the
tooth on the selected side is missing. Turner
and Scott (1977) recommend a procedure in
which the teeth on both sides of the mouth
are observed, and then the maximum of the
two scores for a trait is used as the observation for that individual. This method assumes that the member of a pair of
asymmetric antimeres that exhibits the
greatest degree of trait expression best represents the genetic potential for that character in that individual. Turner and Scott’s
procedure is followed in the present study for
all traits except winging. As mentioned, unilateral and bilateral winging are thought to
be the result of different factors. Therefore,
44
C.R. NICHOL
the expression of the trait on both teeth is of
interest for this trait.
Statistical methods
The statistical method selected to accomplish a complex segregation analysis is the
1971 method of Morton, Yee, and Lew. Although Morton himself (Morton and Rao,
1978) has stated that this particular method
has less power to discriminate between complex hypotheses than does the 1974 method
of Morton and MacLean, the method was selected for two reasons. The overriding concern is that, with the number of traits
analyzed, a considerable amount of computer
CPU time and memory is required for parameter estimation. Because the 1971
method involves the estimation of fewer parameters than does the 1974 method, the
1971 method is more realistic when a large
number of characteristics are examined, as
in the present study.
Second, the 1974 method is designed for
use with characters that are continuous, or
a t least may have their range of expression
trichotomized. Whereas the majority of traits
in the present study are observed using ordinal scales, making trichotomization possible,
two traits, interruption grooves and maxillary premolar accessory cusps, are observed
on a dichotomous basis. To date, no acceptable ranked scoring scheme has been found
for these two traits, which makes trichotomization into normal, intermediate, and affected categories impossible at this time.
Furthermore, some of the traits studied occur
in this sample at such high or low frequencies that trichotomization would strain even
further the limited available information.
Three different general models for the mode
of inheritance can be tested using this
method. These are the generalized two-allele
single-locus model, the beta distribution of
risk (which is not considered in this study),
and the polychotomized normal distribution
of liability. Under the two-allele single-locus
model, three main parameters may be estimated. The first of these is the degree of
dominance (d) of the allele for affection (GI.
Following Morton’s suggestion of the most
important special cases of d, three levels of d
have been tested. These are: when G is recessive (d = 0), G is dominant (d = l), and a
model that Morton calls additive in which d
= 112. The single-gene additive model, which
is not to be confused with the additive poly-
genic model described below, is the equivalent of a codominant two-allele system in
which the heterozygote has a 50% chance of
being affected. The two other parameters to
estimate are the penetrance (t)of G and the
frequency (z) of nonheritable (or sporadic)
cases. In the present study, G is estimated to
be completely penetrant, and the frequency
of sporadic cases is estimated to be 0. The
frequency G is estimated from trait incidence
and Hardy-Weinberg assumptions given the
chosen values of d, t, and z.
The model for the polychotomized normal
distribution of liability is a n application of
Falconer’s (1965) proposition of a n additive
polygenic liability scale that has a sharp
threshold for affection. Falconer’s model is a
mathematical consideration of Gruneberg’s
(1952) model of quasicontinuous inheritance.
Under this model, two parameters need to be
estimated. These are the standardized normal deviate of the threshold for affection (Z),
which is estimated from trait incidence, and
the transmissibility (TI, which is estimated
via maximum likelihood.
For each model, the probability of observing r affected out of s sibs, given the probability of parental matings of each combination of genotypes and the risk of affection in
children resulting from such matings, can be
estimated for each sibship. The expected
number of affected sibs can then be estimated for each sibship and a likelihood ratio
criterion, which is a x2 with a n undefined
number of degrees of freedom, is calculated
for the model. Subtracting the smaller likelihood ratio criterion from the larger between a pair of models gives the likelihood
ratio that is equivalent to a x2 with one degree of freedom. In their discussion of the
method, Morton et al. (1971) have stated that
a goodness-of-fit comparison between the two
models requires the estimation of three parameters (i.e., d, t, and z) in the two-allele
single-locus model and two parameters (i.e. T
and Z) in the polychotomized normal distribution of liability model. This guideline is
followed in the present study.
In their discussion of the method, Morton
et al. state that
Limited experience suggests that
discrimination between model 1
[two-allele single locus] and quasicontinuous model 3 will often be difficult. Dominance and a high ratio
SEGREGATION ANALYSIS OF DENTAL TRAITS
of recurrence risk to incidence favor
model 1. Lack of dominance and a
low or moderate ratio of recurrence
risk to incidence suggest quasi-continuity but do not rule out a discontinuous model, which, with more
parameters, is necessarily more
flexible.
With this in mind, the recurrence risks and
the recurrence risk to incidence ratios are
calculated and are relied on in the interpretation of the results of the complex segregation analysis. A high recurrence risk to
incidence ratio and dominance would tend to
improve the chances that a n underlying major gene would be detected should one exist.
However, in those cases when the recurrence
risk to incidence ratio is low and the quasicontinuous model is accepted, but the recessive model gives the best fit of the single
gene models, it is not possible to completely
rule out the existence of a major gene. In
those cases, the recommendation for further
study of that character must be made.
Because high or low frequencies are a potential source of difficulty for the statistical
analysis, the trait frequency, when approaching 0 or 1, also has a role in the interpretation of the results of the complex segregation
analysis. Under Gruneberg’s (1952) model for
quasicontinuous inheritance, a character is
under the control of many genes, with alleles
that have either positive or negative effects
on its development. The number of positive
alleles must add up to a certain threshold
value for the trait to be expressed. When a
quasicontinuous trait has a high frequency
in a population, it would be expected that
many of the individuals affected with the
trait have values well above the threshold,
whereas the values of most nonaffected individuals would be just below it. Therefore,
matings of the most informative type, affected x nonaffected, would tend to produce
many affected offspring, mimicking the dominant pattern. When such a trait is a t low
frequency, most affected individuals would
have values just above the trait threshold,
and the values among the nonaffected individuals would tend to be well below the
threshold. In this case, the affected x nonaffected matings would tend to produce few
affected offspring, mimicking the recessive
pattern.
45
RESULTS
Table 4 presents the results of the complex
segregation analysis. Information contained
therein for each run includes the recurrence
risk, the recurrence risk to incidence ratio;
the model with the best fit to the data (which
is that yielding the lowest likelihood ratio
criterion), the best estimate of transmissibility (T) when the polychotomized normal distribution of liability is selected, the likelihood
ratio between the model with the best fit and
the model with the second best fit, the level
of significance of that likelihood ratio, and
any other models producing likelihood ratio
criteria that are not significantly different
from that produced by the model with the
best fit at the P < 0.05 level. The following
is a discussion of the results for each of the
characters studied.
Winging
Winging produces a highly significant likelihood ratio ( P < 0.001), favoring the polygenic model, with a transmissibility estimate
of 0.19. Having used a n earlier segregation
analysis method that was also developed by
Morton (1959), Escobar et al. (1976) proposed
that a major gene with a dominant allele for
affection is involved in the development of
winging. In the present study, the singlegene model with the best fit is recessive
rather than dominant. The recurrence risk
to incidence ratio is somewhat low, which, as
was mentioned, may inhibit the detection of
discontinuity, particularly when there is a
lack of dominance. Given these findings,
judgement on the mode of inheritance of
winging must await further study.
Central incisor procumbency
Another positional variant of the upper
central incisors, procumbency, shows best fit
to the quasicontinuous model with a transmissibility of 0.47 when examined at the 011
breakpoint and to a discontinuous model with
a dominant allele for affection when analyzed a t the 112 breakpoint. The significance
level a t the 0/1 breakpoint is quite high ( P <
0.001); at the 112 breakpoint, P is just barely
< 0.05. These findings may suggest that the
detection of dominance at the 112 dichotomy
is a statistical error. On the other hand, the
frequency a t the 0/1 breakpoint is fairly high,
which can interfere with the detection of a
dominant major gene. A definite conclusion
Winging UIl
Procumbency UI1
Procumbency UI1
Insetting U12
Insetting U12
Insetting UI2
Labial convexity UI1
Labial convexity UI1
Labial convexity U12
Labial convexity UI2
Interruption grooves UI1
Shoveling UI1
Shoveling UI1
Shoveling U12
Shoveling U12
Shoveling U12
Shoveling UI2
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI2
Double-shoveling U12
Double-shoveling UC
Double-shoveling UP1
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge LC
Distal accessory ridge LC
Distal accessory ridge LC
Accessory cusps UP1
Accessory cusps UP2
Hypocone (cusp 4) UM1
Traititooth
>O
Sibs
observed
83
83
83
82
82
82
83
83
82
82
72
83
83
82
82
82
82
83
83
83
83
82
82
77
76
77
77
77
77
80
80
80
77
75
83
Categorization
of grades
normal/
affected
2-411
011-3
0-112-3
0-112-6
0-2/3-6
0-314-6
011-4
0-112-4
011-4
0-112-4
Abs/Pres
0-213-6
0-3/4-6
0-112-7
0-2/3-7
0-3/4-7
0-415-7
0/1-6
0-112-6
0-213-6
0-314-6
0/1-6
0-112-6
0/1-6
011-6
011-5
0-1/2-5
0-213-5
0-3/4-5
011-5
0-112-5
0-2/3-5
AbsiPres
AbsiPres
0-314-6
62
62
60
60
60
62
62
68
68
51
49
49
57
57
57
57
54
54
54
54
65
65
62
54
28
28
28
28
14
14
14
50
48
56
~~
63
Both
parents
observed
1
4
0
3
12
24
0
1
10
33
37
0
17
14
6
25
10
10
18
7
7
10
4
7
9
3
14
10
5
11
31
13
9
18
28
21
16
11
11
1
55
19
13
31
5
16
41
47
1
31
39
48
4
55
1
25
14
17
20
23
12
26
17
0
27
5
42
5
24
48
6
45
3
1
51
1
0
36
0
51
32
21
18
42
6
1
43
3
35
13
0
30
0
56
2
4
15
2
34
10
3
11
2
Number of parents affected
TABLE 2. Number o f sibships used in the analysis and break down by number ofparents affected
?
20
21
21
22
22
22
21
21
14
14
21
34
34
25
25
25
25
29
29
29
29
17
17
15
22
49
49
49
49
66
66
66
27
27
27
Hypocone (cusp 4) UM1
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Metaconule (cusp 5) UMl
Carabelli’s trait UM1
Carabelli’s trait UM1
Lingual cusp number LP1
Lingual cusp number LP2
Lingual cusp number LP2
Lingual cusp number LP2
Transverse ridge LP1
Transverse ridge L P l
Transverse ridge LP2
Transverse ridge LP2
Groove pattern LM1
Groove pattern LM2
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM2
Metaconulid (cusp 7) LM1
Deflecting wrinkle LMl
Deflecting wrinkle LM1
Deflecting wrinkle LM1
Protostylid LM1
Protostylid LM1
Protostylid LM1
Protostylid LMI
Protostylid LM2
Protostylid LM2
Maxillary torus
Mandibular torus
Maxillary arch form
Mandibular arch form
X
0-1/24
0-213-8
0-314-8
0/1-8
0-1/2-8
0/1-3
011-3
0-1/2-3
0-112-3
0/1-8
0-314-5
0-415
011-5
0-213-5
0-3/4-5
0/1-5
0-112-5
0-2/3-5
011-5
011-4
0/1-3
0-112-3
0-2/3
Y/+,
0-415-6
011-6
0-1/2-6
0-213-6
0-314-6
011-5
011-7
0-112-7
0-112-8
0-112-8
0-213-8
0-314-8
011-2
0-112
on-2
0-112
Yli, x
83
75
75
75
75
83
83
83
80
76
76
76
80
80
73
73
82
73
80
80
74
74
74
83
83
83
74
83
82
82
82
83
83
83
83
76
76
82
83
83
83
~~
56
61
61
61
61
56
57
57
58
51
51
51
57
57
50
50
37
28
33
33
43
37
37
54
54
54
44
59
45
45
45
55
55
55
55
47
47
29
45
48
51
21
11
43
27
24
~~
35
34
24
34
35
46
11
15
0
0
4
29
49
22
43
50
25
37
46
0
6
11
40
24
0
4
20
7
17
26
28
34
44
37
51
0
10
10
23
17
9
23
17
9
7
8
7
21
10
25
19
20
9
12
4
2
20
21
23
5
27
11
19
32
28
6
22
14
7
20
12
5
1
27
23
9
12
7
0
56
24
16
1
1
23
19
3
13
3
2
3
3
1
0
0
38
13
1
6
2
0
0
0
0
0
1
16
z
13
0
1
6
1
14
50
42
25
4
27
14
14
14
14
27
26
26
22
25
25
25
23
23
26
26
45
45
47
47
31
37
37
29
29
29
30
24
37
37
37
28
28
28
28
29
29
53
38
35
32
__
Categorization
of grades
normal/
affected
2-4/1
011-3
0-112-3
0-112-6
0-213-6
0-314-6
011-4
0-112-4
011-4
0-112-4
Abs/Pres
0-213-6
0-314-6
0-112-7
0-2/3-7
0-314-7
0-415-7
Oil-6
0-112-6
0-213-6
0-314-6
011-6
0-112-6
011-6
011-6
011-5
0-112-5
0-213-5
0-314-5
011-5
0-1/2-5
0-213-5
AbsPres
AbsiPres
0-314-6
Raitltooth
Winging UI1
Procurnbency UI1
Procurnbency UI1
Insetting U12
Insetting U12
Insetting U12
Labia1 convexity UI1
Labial convexity UI1
Labial convexity U12
Labial convexity U12
Interruption grooves UI1
Shoveling UI1
Shoveling UI1
Shoveling U12
Shoveling U12
Shoveling U12
Shoveling U12
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling U12
Double-shoveling U12
Double-shoveling UC
Double-shoveling UP1
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge LC
Distal accessory ridge LC
Distal accessory ridge LC
Accessory cusps UP1
Accessory cusps UP2
Hypocone (cusp 4) UM1
396
394
394
369
369
369
393
393
371
371
205
389
389
368
368
368
368
393
393
393
393
370
370
274
303
264
264
264
264
283
283
283
310
281
422
Sibs
observed
60.0
18.8
15.0
9.9
5
32
113
131
-
-
-
100.0
44.7
11.6
22.3
69.6
14.5
73.1
52.0
24.0
50.0
4.2
100.0
8.3
47.8
37.7
16.4
48.0
21.6
14.3
6.0
44.4
36.0
20.7
6.3
75.0
8.7
71.4
48.1
(%)
Incidence
7
38
69
-
266
69
69
152
25
88
175
215
9
161
198
237
20
253
7
27
1
112
23
200
26
127
221
26
212
24.5
88.8
16.5
90.0
60.4
25.5
71.8
6.1
95.4
7.8
51.2
55.0
20.6
70.4
34.8
18.2
7.3
93.9
43.5
25.4
6.1
91.4
9.2
92.3
74.9
83.3
73.9
42.4
11.7
71.0
45.6
14.8
23.9
8.5
92.9
(%)
Sibs
observed
Incidence
0
40
79
99
67
78
128
72
41
57
80
69
34
128
49
42
58
17
23
25
8
20
27
11
64
44
20
51
131
62
78
97
104
50
126
86
22.9
82.3
33.3
90.7
73.1
48.0
66.7
10.5
90.2
5.0
57.0
58.6
38.8
57.7
42.2
27.8
17.1
87.7
57.5
42.0
8.8
88.3
14.3
95.2
70.7
88.2
78.3
36.0
0.0
75.0
59.3
27.3
35.9
6.8
90.0
~
-
25
13
2
17
1
265
-
72.0
46.2
50.0
29.4
0.0
95.1
-
-
92.5
75.8
85.0
74.5
42.9
-
173
124
60
47
14
-
95.5
-
154
264
10
9
61
10
156
43
12
3
205
30
4
96.2
10.0
44.4
73.8
50.0
80.1
44.2
33.3
33.3
98.0
70.0
75.0
-
-
79.5
146
-
32.2
91.9
18.8
95.3
77.5
59
209
16
148
40
~
Number of parents affected
1
2
Sibs
Incidence
Sibs
Incidence
observed
observedI
(%)
(%I
TABLE 3. Number of sibs observed and trait incidence in the sample and i n each type of sibship based on number ofparents affected
55
55
48
160
160
109
109
109
109
122
122
122
122
68
68
52
94
187
187
187
187
238
238
238
116
105
137
98
98
98
95
95
100
94
100
Sibs
observed
?
24.5
91.0
7.0
85.7
51.0
17.3
72.6
6.3
96.4
7.3
47.9
53.1
15.0
70.6
33.0
16.5
5.5
93.4
37.7
22.1
4.9
92.6
7.4
92.3
84.0
82.4
72.2
42.8
12.3
70.6
43.7
13.4
25.0
7.6
89.1
(%I
Incidence
Hypocone (cusp 4) UM1
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Metaconule (cusp 5) UM1
Carabelli’s trait UM1
Carabelli’s trait UM1
Lingual cusp number LP1
Lingual cusp number LP2
Lingual cusp number LP2
Lingual cusp number LP2
Transverse ridge LP1
Transverse ridge L P l
Transverse ridge LP2
Transverse ridge LP2
Groove pattern LM1
Groove pattern LM2
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM2
Metaconulid (cusp 7) LM1
Deflecting wrinkle LM1
Deflecting wrinkle LM1
Deflecting wrinkle LM1
Protostylid LM1
Protostylid LM1
Protostylid LM1
Protostylid LM1
Protostylid LM2
Protostylid LM2
Maxillarv torus
Mandibdar torus
Maxillary arch form
Mandibular arch form
0-314-5
0-415
011-5
0-213-5
0-314-5
011-5
0-112-5
0-213-5
011-5
011-4
011-3
0-112-3
0-213
011-8
0-112-8
0-213-8
0-314-8
011-8
0-1/2-8
011-3
011-3
0-112-3
0-112-3
Yl+,X
YI+,X
0-415-6
011-6
0-112-6
0-213-6
0-314-6
011-5
011-7
0-112-7
0-112-8
0-112-8
0-2/3-8
0-314-8
011-2
0-112
011-2
0-112
422
227
227
227
227
420
423
423
313
292
292
292
316
316
291
291
347
223
307
307
235
222
222
415
415
415
237
420
378
378
378
413
413
413
413
270
270
370
398
401
404
21.9
91.5
81.8
33.2
63.8
39.2
13.1
36.1
20.0
8.2
8.4
8.6
93.7
63.2
12.4
28.8
23.0
18.2
7.7
8.5
3.3
13.8
11.3
57.6
32.7
5.5
47.4
93.4
87.2
63.0
15.9
7.6
61.9
21.5
9.3
38.7
21.9
9.6
93.4
53.5
40.2
-
58.1
18.1
26.7
23.6
20.1
10.4
10.7
5.2
19.6
13.0
43.4
25.5
-
43
144
120
165
169
230
121
153
138
123
53
__
106
-
72.2
34.1
58.6
21.1
6.4
20.1
10.6
6.1
7.1
7.1
51.9
19.4
4.2
21.6
18
88
29
57
78
139
170
212
113
239
-
27
36
144
102
-
66.7
11.9
5.4
45.4
18.2
9.3
26.6
11.0
8.8
12
84
259
108
214
182
94
136
181
-
-
-
31.5
73
-
55
38
21
107
76
44
18
59
35
102
47
106
92
101
40
40
8
9
104
104
131
47
12
2
75
90
39
42
16
40
34
78
._
22
43
68
08
01
23
11
81
30
59.6
32.8
9.6
..
45.1
93.0
80.9
54.6
15.8
0.0
60.4
27.2
16.7
44.9
42.6
8.3
50.0
48.0
28.9
7.7
38.1
87.5
85.0
20.6
54.5
47.4
4.8
44.9
26.3
15.9
5.6
23.7
85.7
65.7
14.9
33.0
29.3
23.8
7.5
5.0
0.0
22.2
-
21
10
5
95
32
-
-
158
48
2
44
13
0.0
64.2
40.6
-
-
-
94.3
60.4
0.0
36.4
30.8
-
-
-
94.7
65.1
49.2
0.0
40.0
93.4
89.6
53.8
79.5
72.7
28.6
70.6
58.8
42.9
209
109
61
4
5
61
77
13
39
11
7
17
17
7
-
0.0
61.9
80.0
-
1
65.6
92.7
89.6
74.0
62.5
50.0
89.5
90
150
125
73
8
2
76
137
34
34
34
34
136
128
128
100
99
99
99
105
105
104
104
198
146
172
172
112
116
116
152
152
152
106
122
185
185
185
143
143
143
143
109
109
223
166
149
135
3fi.3
46.0
97.1
91.2
64.7
14.7
12.5
60.9
23.4
7.0
40.4
21.2
11.1
91.4
45.7
51.9
6.7
18.2
91.1
78.5
33.7
64.3
42.2
18.1
40.8
23.0
7.2
10.4
4.1
94.6
63.8
7.6
25.2
17.5
11.9
3.5
7.3
0.9
9.9
11.4
57.0
Categorization
of grades
normaliaffected
2-411
011-3
0-112-3
0-112-6
0-213-6
0-314-6
011-4
0-112-4
011-4
0-112-4
AbsPres
0-2/3-6
0-314-6
0-112-7
0-213-7
0-314-7
0-415-7
011-6
0-112-6
0-213-6
0-314-6
0/1-6
0-112-6
Oil-6
Oil-6
011-5
0-112-5
0-2/34
0-3/4-5
011-5
0-1/24
0-2/3-5
AbsPres
AbsiPres
0-3/4-6
Traititooth
Winging UI1
Procumbency UI1
Procumbency UI1
Insetting U12
Insetting UI2
Insetting U12
Labial convexity UI1
Labial convexity UI1
Labial convexity UI2
Labial convexity U12
Interruption grooves UI1
Shoveling UI1
Shoveling UI1
Shoveling U12
Shoveling U12
Shoveling U12
Shoveling U12
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling UI1
Double-shoveling U12
Double-shoveling U12
Double-shoveling UC
Double-shoveling UP1
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge UC
Distal accessory ridge LC
Distal accessory ridge LC
Distal accessory ridge LC
Accessory cusps UP1
Accessory cusps UP2
Hypocone (cusp 4) UM1
16.9
85.9
13.7
87.1
54.7
20.8
66.7
8.3
94.1
7.8
45.4
50.7
27.1
66.0
36.2
18.2
4.0
92.3
39.9
30.0
9.7
88.9
11.4
97.8
70.1
79.0
67.2
29.6
9.6
67.7
38.6
12.1
25.9
5.6
91.7
Recurrence
risk (%)
68.9
96.6
82.8
96.8
90.6
81.5
92.9
136.5
98.6
100.3
88.7
92.2
131.8
93.7
103.9
99.9
55.1
98.3
91.6
117.9
159.2
97.3
124.4
99.5
93.6
94.8
91.0
69.7
82.1
95.4
84.8
81.7
108.3
65.0
98.7
Recurrence
risk to
incidence
ratio (%)
Likelihood
ratio
22.708
14.560
3.852
4.355
24.351
46.322
6.874
22.003
19.324
21.283
16.008
20.072
33.473
30.803
17.562
5.140
1.585
9.327
29.779
19.716
5.140
33.768
5.543
10.081
26.185
20.727
25.231
19.841
16.105
4.444
8.645
12.158
2.601
27.145
8.224
Model with
best fit
Polygenic T = 0.19
Polygenic T = 0.47
Dominant
Polygenic T = 0.54
Dominant
Dom in a nt
Recessive
Polygenic T = 0.44
Polygenic T = 0.35
Polygenic T = 0.16
Dominant
Dominant
Dominant
Recessive
Recessive
Dominant
Dominant
Polygenic T = 0.51
Dominant
Dominant
Dominant
Polygenic T = 0.38
Dominant
Polygenic T = 0.19
Recessive
Polygenic T = 0.04
Polygenic T = 0.00
Polygenic T = 0.07
Polygenic T = 0.04
Polygenic T = 0.46
Polygenic T = 0.13
Polygenic T = 0.20
Dominant
Polygenic T = 0.08
Recessive
0.001
0.001
0.05
0.05
0.001
0.001
0.01
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.025
n.s.
0.005
0.001
0.001
0.025
0.001
0.025
0.005
0.001
0.001
0.001
0.001
0.001
0.05
0.005
0.001
ns.
0.001
0.005
Significance
(P<)
Recessive
Polygenic T
=
0.29
Other models
not reiected
TABLE 4. Results of the complex segregation analysis: recurrence risk to incidence ratio, model with best fit, likelihood ratio, and significance
Hypocone (cusp 4) UM1
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Hypocone (cusp 4) UM2
Metaconule (cusp 5) UM1
Carabelli’s trait UM1
Carabelli’s trait UM1
Lingual cusp number LP1
Lingual cusp number LP2
Lingual cusp number LP2
Lingual cusp number LP2
Transverse ridge LP1
Transverse ridge LP1
Transverse ridge LP2
Transverse ridge LP2
Groove pattern LM1
Groove pattern LM2
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM1
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Hypoconulid (cusp 5) LM2
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM1
Entoconulid (cusp 6) LM2
Metaconulid (cusp 7) LM1
Deflecting wrinkle LM1
Deflecting wrinkle LM1
Deflecting wrinkle LM1
Protostylid LM1
Protostylid LM1
Protostylid LM1
Protostylid LM1
Protostylid LM2
Protostylid LM2
Maxillary torus
Mandibular torus
Maxillary arch form
Mandibular arch form
0-415-6
011-6
0-112-6
0-213-6
0-314-6
011-5
011-7
0-112-7
0-112-8
0-112-8
0-213-8
0-314-8
011-2
0-112
011-2
0-112
Yl+,x
Yl+, x
0-314-5
0-415
011-5
0-213-5
0-314-5
011-5
0-112-5
0-213-5
011-5
011-4
011-3
0-112-3
0-213
011-8
0-112-8
0-213-8
0-314-8
011-8
0-112-8
011-3
011-3
0-112-3
0-112-3
43.6
90.8
83.4
65.9
17.2
9.3
55.9
17.9
10.6
31.7
28.1
12.5
91.5
44.0
34.5
2.0
21.2
88.7
77.3
31.5
58.7
32.8
9.1
32.2
15.5
11.9
7.4
10.9
91.9
57.7
11.0
19.9
18.4
12.4
3.6
5.7
3.3
11.6
6.9
50.3
30.2
91.9
97.2
95.7
104.6
108.4
122.4
90.3
83.1
114.8
82.0
128.1
130.4
98.0
82.3
85.7
35.7
96.7
96.9
94.6
95.0
91.9
83.6
69.6
89.1
77.4
145.0
87.8
127.5
98.1
91.3
88.7
69.2
79.9
68.2
47.1
67.1
100.0
84.5
61.2
87.4
92.6
Recessive
Polygenic T = 0.00
Recessive
Recessive
Polygenic T = 1.00
Polygenic T = 0.51
Dominant
Dominant
Polygenic T = 0.29
Dominant
Dominant
Dominant
Polygenic T = 0.36
Recessive
Recessive
Polygenic T = 0.11
Polygenic T = 0.37
Polygenic T = 0.18
Polygenic T = 0.30
Dominant
Dominant
Dominant
Polygenic T = 0.20
Dominant
Polygenic T = 0.42
Polygenic T = 0.59
Polygenic T = 0.18
Dominant
Polygenic T = 0.23
Polygenic T = 0.19
Polygenic T = 0.11
Polygenic T = 0.06
Dominant
Polygenic T = 0.10
Polygenic T = 0.00
Polygenic T = 0.04
Dominant
Dominant
Polygenic T = 0.00
Recessive
Polygenic T = 0 25
_ _
7 678
17.299
8.635
0.938
22.403
11.750
2.024
30.791
2.882
12.342
13.882
30.661
17.306
15.773
27.043
16.114
20.933
4.619
11.650
17.426
15.315
16.534
4.400
16.865
28.981
20.628
4.025
4.318
10.040
19.352
16.094
12.536
31.551
11.729
17.327
8.624
23.435
15.491
11.302
53.109
28.585
n- ni
__
0.001
0.005
ns.
0.001
0.001
ns.
0.001
n.s.
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.001
0.05
0.001
0.001
0.001
0.001
0.05
0.001
0.001
0.001
0.05
0.05
0.005
0.001
0.001
0.001
0.001
0.001
0.001
0.005
0.001
0.001
0.001
0.001
0.001
Polygenic T
Dominant
Polygenic T
=
=
0.22
0.38
52
C.R. NICHOL
likelihood ratio comparing the best quasicontinuous and discontinuous models and the
strength of a possible major gene is a one-toLateral incisor insetting
one correspondence. However, the percentInsetting of the lateral incisor yields best age contribution of a single locus to the disfit to the single-gene model with a dominant continuity of the genetic background for a
allele for affection for two of the three break- trait should correspond in some way to the
points (213 and 314) a t which it was tested. At ability to detect this discontinuity.
the third breakpoint (1/2), the polygenic
Shoveling
model with a transmissibility estimate of
Six separate runs of shoveling are per0.54 is accepted a t the P < 0.05 level. Given
that the significance level in the two cases formed in this study. Two different breakwhen the dominant model is selected is high points (314 and 415) are tested on the central
( P < 0.001), whereas at the other breakpoint incisor, and four breakpoints (213, 314, 415,
the significance level is lower and the fre- and 5-6) are examined on the lateral. In each
quency is quite high (90.0%), it is logical to case, the model showing the best fit was a
reject the latter case and propose that inset- discontinuous one, although on the lateral
incisor at the 5-6 breakpoint the quasicontinting has dominant major gene inf’Iuence.
uous model is not rejected. The reason for the
Incisor labial convexity
inability to discriminate between models at
Labial convexity is examined at two break- this breakpoint is probably the low trait frepoints on both the central and lateral inci- quency (7.3%) and a low recurrence risk to
sors. On the central, at the 011 breakpoint, incidence ratio (55.1%). For both cases on the
the single gene model with a recessive allele central incisor the allele for affection in the
for affection yields the best fit to the data. best model is dominant, with a P of < 0.001.
At the 112 breakpoint, the recessive mod- However, for the lateral incisor the degree of
el, which gives the lowest likelihood ratio cri- dominance of the allele for affection is not
terion of the three single gene models, is the same in all cases. At the two lower breakrejected by a polygenic model with a trans- points, the allele is suggested to be recessive;
missibility estimate of 0.44. For the lateral a t the two higher dichotomies, the best fit is
incisor, the quasicontinuous model is ac- to a dominant model. Given that the best fit
cepted a t both breakpoints. The transmissi- in all six runs is to the single-gene model, a
bility estimate is 0.35 a t the 011 breakpoint proposal of a major gene for shoveling can be
and 0.16 at the 112 dichotomy. One can note made. Dominant gene involvement in the
that, in three cases when the quasicontin- development of shoveling seems clear for the
uous model is accepted, the trait frequency is central incisor. The situation for the lateral
either very high (95.4% for U12 a t the 011 incisor is more confusing, but it seems undichotomy) or quite low (6.1 and 7.8% for UI1 likely that shoveling has different genetic
and UI2, respectively, at 112 division). Also, etiologies for each incisor. Therefore, the
a significant sex difference in incidence is most logical conclusion would be to dismiss
seen on 12 a t the 112 breakpoint. Again, no the findings of recessiveness a t the two lower
final answer on the genetics of this trait can breakpoints as statistical error and propose
dominant major gene influence on shoveling
be put forward at this time.
for both teeth. A significant sex difference in
Interruption grooves
trait frequency is noted for shoveling of the
Quasicontinuity is overwhelmingly re- lateral incisor a t the 213 breakpoint.
jected for interruption grooves on the maxillary lateral incisors. The single locus model
Double-shoueling
with a dominant allele for affection produces
Double-shoveling
is tested on four different
better fit to the data than does any other
model a t the P < 0.001 level. If the level of teeth and for a total of eight dichotomies of
significance of the likelihood ratio can be the observational scale. On the central inciused to estimate the strength of the major sor, the quasicontinuous model with a transgene for a trait, then such a major locus for missibility estimate of 0.51 yields best fit a t
interruption grooves plays a very large role the 011 breakpoint. For the other three dichoin the development of the character. It is tomies tested on this tooth, the discontinuous
doubtful that the relationship between the model with a dominant allele for affection is
on the mode of inheritance of this trait also
depends on further analysis.
SEGREGATION ANALYSIS OF DENTAL TRAITS
accepted. For the lateral incisor, the polygenic model (T = 0.38 in this case) is again
accepted a t the 011 breakpoint, and the dominant model is chosen for the 112 dichotomy.
The polygenic model with transmissibility
estimate of 0.19 yields best fit for the canine,
which is tested only at the 011 division of the
scale. For the premolar, the single-gene
model with a recessive allele for affection is
selected. From the observation that the polygenic model is selected for presencelabsence
on all three anterior teeth, one might be led
to the conclusion that trait presence is under
the influence of many genes, whereas expression is influenced by a major gene with a
dominant allele for stronger degrees of affection. However, in all cases when the polygenic model is the best fit, the trait frequency
is > 90%. It is more likely that this high
frequency is causing methodological problems, and the quasicontinuous findings can
be dismissed. Also, significant sex differences in the trait frequencies occur a t the 01
1 breakpoint for both the incisors. The findings of recessive major gene influence for
labial (or, more correctly in this case, buccal)
marginal ridges on the premolar, as opposed
to the dominant allele influence noted for the
anterior teeth, could indicate that there are
different genetic backgrounds for these two
morphologically similar traits. On the other
hand, this might be some sort of statistical
aberration, as is suspected for lateral incisor
shoveling.
Canine distal accessory ridge
The canine distal accessory ridge is tested
using four dichotomies of the upper canine
scale and three divisions of the lower canine
standard. In all seven cases, the model using
the polychotomized normal distribution of liability is selected. For the upper canine, all
transmissibility estimates are quite low
(ranging from 0 to 0.07), suggesting a n overwhelming environmental influence on the
character on that tooth. The transmissibility
estimates are higher for the mandibular canine, ranging from 0.13 to 0.46. Two problems with the distal accessory ridge are
worthy of note. First, as mentioned, the canine distal accessory ridge is the most sexually dimorphic morphological feature of the
human dentition. In this sample, very significant ( P < 0.001) sex differences in trait frequencies are found in all runs, except for
those at the 011 and 112 breakpoints on the
upper canine. Second, this trait has one of
53
the worst records for percentages of sibships
with one or both parents not being observed.
The reason for this is that canine distal accessory ridge is affected by relatively minor
degrees of attrition and is frequently totally
obliterated by the age of 30 years. Given this,
the analysis of this trait in this sample is
highly dependent on making the assumption
of equal fertility among parents, which if not
true could yield unreliable results.
Premolar mesial and distal accessory cusps
The presence of mesial and distal accessory
cusps on the upper premolars yields best fit
to a discontinuous model on the first premolar. However, there is not significant discrimination between the dominant and recessive
estimates of the degree of dominance for the
allele for affection. Both the recessive and
dominant models reject the polygenic model
a t the P < 0.005 level. For the second premolar, the greatest correspondence is to a
quasicontinuous model with a low transmissibility estimate (T = 0.08). The results for
the first molar suggest discontinuity in trait
inheritance, implicating a major gene, but it
is impossible to estimate the degree of dominance at this time. On the second premolar,
there is a very low transmissibility estimate,
suggesting large environmental influences.
Also, the trait frequency (8.5%)and recurrence risk to incidence ratio (65%) are both
low. Each of these factors could make the
major gene, if it exists, impossible for this
method to detect. The results point to the
existence of a major gene on the first premolar, although, at this time, the degree of dominance is in question, and it is unlikely that
the genetics, of this feature are different on
the second premolar. Also, note that there
are significant sex differences in trait frequencies for both teeth, and possible sex-linkage could be causing some of the problems
noted.
Hypocone (cusp 4)
Expression of the hypocone is tested a total
of six times on two teeth. On the first molar,
the recessive single locus model was accepted
at both breakpoints (314 and 415) tested. For
the second molar, the recessive model is a
significantly better fit ( P < < 0.001) at the 21
3 breakpoint. At the 112 breakpoint the recessive model is the best fit, but the quasicontinuous model with a transmissibility
estimate of 0.38 is not rejected. At the other
two scale divisions tested on this tooth, the
54
C.R. NICHOL
polygenic model is a significantly better
choice. At the presence/absence level (Oh) the
transmissibility estimate is 0; a t the 314
breakpoint the estimate is 1. It is apparent
that, if a major gene exists for this trait, the
allele for greater trait expression is recessive. In all six cases, including the two when
the single gene model was rejected, the best
estimate of the degree of dominance for the
allele for affection out of those tested was 0.
In that both tests of the first molar hypocone
give best fit to the recessive model, a proposition of major gene influence for first molar
seems reasonable. It is unlikely that the genetics of the hypocone are different on the
second molar. Some of the difficulties in the
results for the second molar could be due to
high frequency and overwhelming environmental influence at the 0/1 breakpoint, high
frequency at the 112 breakpoint, and low frequency a t the 314 breakpoint. Significant frequency differences between the sexes occur
at the 213 breakpoint on M2 and at the 314
breakpoint on both molars.
Metaconule (cusp 5)
The metaconule of the upper first molar
does not provide a significant level of discrimination between a discontinuous model
with a dominant allele for affection and a
quasicontinuous model with a moderate level
of transmissibility (T = 0.51). A factor that
may be the cause of the inability of the
method to distinguish between models is the
relatively low incidence of the trait in the
sample (7.6%). Possibly, a major gene with a
dominant allele for affection exists for this
trait, but it does not contribute to discontinuity in trait inheritance with such strength
that it results in rejection of additive multifactorial inheritance in-a sample of this size
with the low level of incidence observed.
Carabelli’s trait
Carabelli’s trait of the maxillary first molar undergoes two analyses of its probable
inheritance. At the 0/1breakpoint the dominant model is accepted overwhelmingly accepted (P < < 0.001). At the 1/2 breakpoint
there is not significant discrimination between this model and the polygenic one with
a transmissibility estimate of 0.22. Using
Morton’s and MacLean’s (1974) complex segregation analysis method, and trichotomizing the trait with cusp forms as affected,
other forms of the trait as intermediate, and
absence as normal, Kolakowski et al. (1980)
have found evidence of a weak major gene
for Carabelli’s trait. The findings of this
study are in agreement with the suggestion
of major gene influence on this trait. Significant frequency differences between sexes are
seen at both dichotomies.
Premolar lingual cusp number
The presence of multiple lingual cusps on
the lower premolars yields best fit to a quasicontinuous model with a low to moderate
estimate of transmissibility (T = 0.29) for the
first premolar a t the presencelabsence (1/2)
level. On the second premolar, the greatest
correspondence is to a discontinuous model
with a dominant allele for affection for all
three breakpoints tested. Probably, a major
gene exists for the trait, but its influence is
weakened away from the center of the trait
field. Other evidence, such as the trait frequency (9.3%) and the transmissibility levels
selected for the polygenic model (estimates of
T for P2 range from 0.38 to 0.941, suggests
that the center of the field for this character
is on the second premolar rather than the
first. Different genetics for the trait on each
tooth are unlikely.
Transverse ridge
The transverse ridge of the lower premolars is analyzed a t two different dichotomies
of the observation scale for each of the two
teeth. In both cases, the recessive model is
accepted at one breakpoint, and the polygenic model is accepted at the other. For the
first premolar the recessive model is chosen
a t the 112 breakpoint; on the second the recessive model is selected at the 011 breakpoint. In both cases when the polygenic model
produces best fit, the frequency approaches
fixation. For the first premolar at the 011
breakpoint the frequency is 93.4%, and for
the second premolar at the 112 breakpoint
the frequency is 5.5%. There is also a very
low recurrence risk to incidence ratio and a
significant sex difference in frequencies on
P2 at the 1/2 division. Because of these findings, the inclination is to reject the results
indicating quasicontinuity and to go forward
with a proposal of recessive major gene involvement in the development of the transverse ridge.
Molar groove pattern
The groove pattern of both lower molars
demonstrates best fit to a model employing
the polychotomized normal distribution of li-
SEGREGATION ANALYSIS OF DENTAL TRAITS
ability. For the first molar, the transmissibility estimate is moderate (T = 0.37), and, for
the second, the estimate is somewhat low (T
= 0.18). The groove pattern has been shown
to be statistically associated with the size of
molar cusps 5 and 6 (Nichol, 1985). Also, the
presence of the Y-pattern has been noted to
be related to the presence of the deflecting
wrinkle (D.H. Morris, 1970). Because of the
previous findings that the groove pattern is
associated with several other crown features
(and therefore may be the result of the presence andlor size of these characters), it is not
surprising that the trait would correspond to
a model in which the character is determined
by many additive factors.
Hypoconulid (cusp 5)
The hypoconulid is examined a t five dichotomies, three on the first molar and two on
the second. In three of the five cases, the
discontinuous model with a dominant allele
for affection is accepted. In the other two
cases, the 3/4breakpoint on both M1 and M2,
the data best fit the quasicontinuous model.
The two findings of fit to the polygenic model
occur in the analyses with the highest (81.8%
on M1) and lowest (13.1% on M2) frequencies
of the five runs for this trait. The findings of
discontinuity for the other three analyses
lead to a proposition of dominant major gene
influence on the development of lower molar
cusp 5.
55
incidence ratio is quite high (127.5%)in this
case. The logical conclusion is that there is a
major locus with a dominant allele for affection for this cusp. However, the caution must
be put forward that a significant sex difference in trait frequency is found.
Deflecting wrinkle
The deflecting wrinkle of the lower first
molar demonstrates best fit to the quasicontinuous model at all three of the possible
breakpoints. In each case the estimate of
transmissibility is low, ranging from 0.23 a t
the 011 breakpoint to 0.11 at the 213 breakpoint. The presence of this character would
appear to be under the influence of many
additive genes modified by relatively strong
environmental effects.
Protostylid
The protostylid of the lower molars is tested
for a total of six dichotomies on two teeth.
Four of the data runs yield best fit to the
quasicontinuous model with low transmissibility estimates (ranging from 0 to 0.10). The
other two analyses, both of which are a t the
112 breakpoint on each of the two teeth, give
the greatest correspondence to the dominant
model. Because this phenomenon occurred
on both teeth, the findings for the second
molar are retained even though the frequency was below the guideline stated earlier. From the transmissibility estimates
found, it is evident that there is a tremenEntoconulid (cusp 6)
dous amount of environmental influence on
Cusp six is tested a t the presencelabsence the development of the protostylid trait. The
level on both the first and second molars and findings of possible dominant major gene efis further examined a t the 112 and 213 break- fects at one breakpoint are potentially interpoints on the first molar. In all cases, except esting, but determination of whether this has
for the 011 dichotomy on the first molar, some meaning awaits further analysis.
which corresponds to the dominant model,
the polygenic model gives best fit. In two of
Maxillary and mandibular tori
the cases when the polygenic model is acTwo different situations are found for the
cepted, the trait frequency (8.2% for the 213
dichotomy on M1 and 8.4% for the 011 divi- maxillary and mandibular tori. The maxilsion on M2) and the significance levels for lary torus demonstrates best fit to a singlerejection of the dominant model are low. locus model with a dominant allele for affecThese findings cast suspicion on the results tion. On the other hand, the mandibular tofor these two runs. The results for this trait rus shows best fit to a polygenic model with
do not make the proposition of a major gene a transmissibility estimate of 0. Proposition
a straightforward one, but they at least raise of a major gene with a dominant allele for
affection is indicated for the maxillary torus.
the possibility that such a gene exists.
For the mandible, problems of low incidence
Metaconulid (cusp 7)
and low recurrence risk to incidence ratio
Cusp seven of the lower first molar shows exist, and overwhelming environmental inbest fit to the single-gene model with a dom- fluences on the development of the mandibinant allele for affection. The recurrence to ular torus make it impossible to say just what
56
C.R. NICHOL
the inherited portion of the trait’s develop- three tests show significant correspondence
to the polygenic model. Two of these three
mental background might be.
give best fit to the recessive submodel when
Maxillary and mandibular arch form
the single locus model is examined. These
Wide forms of the two arches also demon- results bear out the contention that, with
strate correspondence to different models. For dominance and a high incidence to recurthe maxilla, the wide forms yield best fit to a rence risk ratio, detection of discontinuity is
discontinuous model with a recessive allele enhanced. On the other hand, low recurrence
for affection. The likelihood-ratio in this case risk to incidence ratio and lack of dominance
is very significant ( P < < 0.001). For the are suggested to inhibit detection of disconmandible, the best fit is to the polygenic tinuity. The results seem to bear this out, in
model with a transmissibility estimate of that, for the 11 runs where the recurrence
0.25. A relatively strong major gene with a risk to incidence ratio is < 70%, ten give
recessive allele for affection by wide arches significantly better fit to the polygenic model,
is suggested for the maxilla, whereas multi- and the other does not produce significant
factorial inheritance, including fairly strong discrimination between the two models. In
environmental effects, is apparent for the five of the ten runs where quasicontinuity is
mandible. It should also be noted that there favored, the best fit under tests of the single
is sexual dimorphism for this trait on the gene model is to the recessive submodel.
mandible in this sample, with males more
An overall view of the results suggests that
frequently having a wide arch.
the polygenic model is favored a t high or low
levels of trait incidence. In 30 of the 39 cases
when the polygenic model is favored, the trait
DISCUSSION
frequency is below 20% or above 80%,
The results of the 76 individual runs of the whereas this is true in only 11 of the 37 cases
complex segregation analysis produce 39 when the single-locus model is the best fit.
cases showing best fit to the polychotomized The potential effects of high and low frequennormal distribution of liability model. In 38 cies on the results of the segregation analysis
of the 39 instances, the likelihood ratio is were mentioned above. In all, 14 of the 76
significant, and in the other a dominant sin- runs have trait incidences > 80%. An inorgle gene model is not rejected. The other 37 dinate number of these (12) show signifiruns show that the two-allele single-locus cantly greater correspondence to the quamodel is the best fit to the data. For 34 of sicontinuous model than to the single-locus
these the likelihood ratio is significant, and model. For 8 of the 12, the dominant submofor the other three the quasicontinuous model del is the best fit in the analysis of the single
is not rejected. Dominance of the allele for gene model. Fears that confusion of domiaffection is indicated in 26 of the 37 instances nance and quasicontinuous inheritance could
when the single-gene model is the best fit occur when a trait is a t high frequency seem
and in 24 of the 34 runs when the polygenic to be realized. In 27 runs the incidence is <
model is rejected.
20%.For 17 of these 27, the goodness-of-fitto
Discussed above was the possible relation- the polygenic model is significant. In the
ship between the recurrence risk to incidence other ten runs, eight when the single gene
ratio and the goodness-of-fittest between the model is a significantly better fit and two
discontinuous and quasicontinuous models. when discrimination between the models is
Analysis of the results of the segregation not significant, the dominant submodel is seanalysis reveals that some of the concerns of lected as the best of the single-gene models
the developers of the method are realized. In in all cases. The 17 instances when the qua11 runs the recurrence risk to incidence ratio sicontinuous model is selected show a n equal
is > 110%.In seven of these cases, the single- division (seven dominant, three additive,
locus model yields significantly better fit to seven recessive) between dominance and rethe data than does the quasicontinuous cessiveness under tests of the single-gene
model. In all seven instances the allele for model. The suggestion of these findings is
affection is estimated to be dominant. Of the that there is a n inability of the model to
four remaining tests, one favors the poly- discriminate between single-locus recessivegenic model, with a dominant single gene ness and quasicontinuity when the trait is a t
model not being rejected. The remaining low frequency.
SEGREGATION ANALYSIS OF DENI'AL TRAITS
For some of the traits examined on more
than one tooth, a proposition about the genetics of the character on one tooth is extended to another in the face of conflicting
results from the complex segregation analysis. Under the two major models for dental
development, the field model (Butler, 1939;
Dahlberg, 1945) and the clone model (Osborn, 19781, a trait that occurs on more than
one tooth has a genetic background shared
between teeth rather than a separate genetic
background for each tooth. It is therefore logical to extend a proposition about the genetics of a trait from one tooth to another,
especially when complications of recurrence
risk to incidence ratios or frequencies call the
results from one tooth into question.
Given these reasons for rejecting the results of certain data runs, the results of the
complex segregation analysis suggest major
locus involvement in the development of 13
of the 24 traits examined. Nine of theseinsetting of UI2, interruption grooves of UI2,
incisor shoveling, incisor and canine doubleshoveling (as was mentioned above; premolar buccal marginal ridges may not be genetically linked to double-shoveling of the
anterior teeth), Carabelli's trait of UM1,
multiple lingual cusp of the lower premolars,
the hypoconulid of LM1 and LM2, cusp 7,
and the maxillary torus-show indication
that the allele for affection by the trait is
dominant. Three traits-the hypocone of
UM1 and UM2, the transverse ridge of the
lower premolars, and the maxillary torusgive evidence of a recessive allele for affection. Premolar double-shoveling also suggests recessiveness, but further study is
necessary to determine whether this finding
is a statistical error or if in fact the trait is
separate from double-shoveling on the incisors and canine. One character, accessory
cusps of the upper premolars, does not give a
clear picture on the degree of dominance of
the major gene indicated in the results. Five
other traits-procumbency of UI1, labial convexity of UI1 and UI2, metaconule UM1, and
cusp 6 of LM1 and LM2, and protostylid of
LM1 and LM2-give hints of major gene influence, but definite conclusions on their true
mode of inheritance must await further
study. The results for the remaining six traits
show that their development is under the
control of many genes. However, the results
in two of these cases-winging of UI1 and the
mandibular torus-are suspect because of
57
findings of low recurrence risk to incidence
ratios. Two others of these six-the distal
accessory ridge and the mandibular arch
form-show significant sex differences in frequencies, which may call the results into
question. Therefore, only two characters-the
groove pattern of LM1 and LM2 and the deflecting wrinkle of LM1-give clear results
indicating their polygenic origin.
Overall, the transmissibility estimates
found for the polygenic model tend to be low.
Being magnanimous and taking the highest
estimate of transmissibility from any of the
runs for a trait on each tooth, the average for
the 38 traits is only 0.36. These findings suggest that there is a large amount of environmental influence on the development of
dental morphological variants. However, this
conclusion must be qualified by recognizing
that there are observational difficulties for
many of these characters (Nichol and Turner,
1986). Although the traits analyzed in the
present study generally have observational
discrepancies that have been found to be nondirectional, and would therefore cancel out
in population studies of large samples, it
must be recognized that the difficulties in
accurately replicating observations may well
result in overestimation of the environmental influences on these characters. Valid
transmissibility estimates must await the
development of more refined observational
technique.
Certainly this analysis has left open many
questions about the mode of inheritance of
the dental morphological characters examined. In 20 of 76 data runs, there are significant differences between the sexes for the
trait frequency, meaning 11 of 24 traits examined show some evidence of sexual dimorphism. If any of these characters are
under the influence of sex-linked genes, the
conclusions drawn about their mode of inheritance could be in error. The possibility of sex
effects on dental trait development certainly
warrants the future testing of sex-linkage
models, to determine whether these are due
to sex-linked genes, or other circumstances
such as sex differences in growth factors.
Also, more intensive analysis of these characters using more advanced methods of segregation analysis is needed to verify the
presence of the major genes proposed here; to
estimate the relative contributions of major
genes and the multifactorial backgrounds;
and to investigate the relative contributions
58
C.R. NICHOL
of genetic, familial environmental, and ran- Falconer DS (1965)The inheritance of liability to certain
diseases, estimated from the incidence among reladom environmental effects on trait developtives. Ann. Hum. Genet. 295-76.
ment and expression.
Garn SM, Lewis AB, and Kerewsky RS (1965) X-linked
inheritance of tooth size. J. Dent. Res. 44t439-441.
Goose DH (1971)The inheritance of tooth size in British
families. In AA Dahlberg (ed.): Dental Morphology and
The author would like to recognize the conEvolution. Chicago: University of Chicago Press.
tributions to this study of Dr. and Mrs. A.A.
DH, and Lee GTR (1971) The mode of inheritance
Dahlberg. Without their tremendous and ex- Goose,
of Carabelli’s trait. Hum. Biol. 43t64-69.
tended effort in putting together the collec- Gruneberg H (1952) Genetical studies on the skeleton of
tion from which the sample used in this study
the mouse. IV. Quasi-continuous variation. J. Genet.
51t95-114.
came, this analysis would not have been possible. The information that they gave me Harris EF (1977) Anthropologic and genetic aspects of
dental morphology of Solomon Islanders, Melaabout the collection and Dr. Dahlberg’s in- the
nesia. PhD Dissertation, Arizona State University,
sightful comments about the dentition were
Tempe.
also invaluable. I would also like to thank Harris EF (1980) Sex differences in lingual marginal
ridges on the human maxillary central incisor. Am. J.
Dr. C.G. Turner I1 under whose guidance
Phys. Anthropol. 52541448.
with study was performed and with whose
Hrdlicka
A (1920) Shovel-shaped teeth. Am. J. Phys.
encouragement it was submitted for publicaAnthropol. 3.429-465.
tion. The three anonymous reviewers also Jorgensen KD (1955)The dryopithecus pattern in recent
deserve thanks for their comments, which
Danes and Dutchmen. J. Dent. Res. 34t195-208.
have resulted in the final form of this paper. Kaul V, and Prakash S (1981) Morphological features of
Finally, I wish to acknowledge Dr. M.Y. Is- the J a t dentition. Am. J. Phys. Anthropol. 54:123-127.
can, who organized the session at the annual Kieser JA, and Preston CB (1981) The dentition of the
Lengua Indians of Paraguay. Am. J. Phys. Anthropol.
meetings of the American Association of
55,485-490.
Physical Anthropologists where this paper Kolakowski D, Harris EF, and Bailit HL (1980)Complex
was originally presented and who oversaw
segregation analysis of Carabelli’s trait in a Melanethe organization of the papers from that ses- sian population. Am. J. Phys. Anthropol. 53r301-308.
Kraus BS (1951) Carabelli’s anomaly of the maxillary
sion into this issue of the Journal.
molar teeth. Am. J. Hum. Genet. 3t348-355.
Kraus BS, Jordan RE, and Abrams L 11969) Dental
LITERATURE CITED
Anatomy and Occlusion. Baltimore: The Williams and
Wilkins Company.
Alvesalo LM (1971) The influence of sex chromosome
genes on tooth size in man. A genetic and quantitative Larson MA, Turner CG 11, and Scott GR (1975)Standard
plaque for the observation of the hypocone. Dental
study. Suomen Hammaslaak. Toim. 67:3-54.
Anthropology Laboratory, Arizona State University,
Alvesalo LM, and Tigerstedt PMA (1974) Heritabilities
Tempe.
of human tooth dimensions. Hereditas 77:311-318.
Berry AC (1978) Anthropological and family studies on Lee GTR, and Goose DH (1972)The inheritance of dental
traits in a Chinese population in the UK. J. Med.
minor variants of the dental crown. In PM Butler and
Genet. 9t336-339.
KA Joysey (eds.): Development, Function, and EvoluLewis DW, and Grainger RM (1967) Sex-linked inherition of the Teeth. London: Academic Press.
tance of tooth size. Arch. Oral Biol. 12t539-544.
Butler PM (1939) Studies of the mammalian dentition.
Differentiation of the post-canine dentition. Proc. Zool. Morris DH (1970)On deflecting wrinkles and the dryopithecus pattern in human mandibular molars. Am. J.
Soc. London 109:l-36.
Phys. Anthropol. 32r97-104.
Cadien J D (1970) Models for the inheritance of dental
characteristics. PhD Dissertation, University of Cali- Morris NT (1970) The occurrence of the mandibular torus at Gran Quivira. MA Thesis, Arizona State Unifornia, BerkeIey.
versity, Tempe.
Corruccini RS, and Potter RHY (1981) Developmental
correlates of crown component asymmetry and occlu- Morton NE (1959) Genetic tests under incomplete ascertainment. Am. J. Hum. Genet. 11tl-16.
sal discrepancy. Am. J. Phys. Anthropol. 5521-31.
Dahlberg AA (1945) The changing dentition of man. J. Morton NE, and MacLean C J (1974) Analysis of family
resemblance. 111. Complex segregation analysis of
Am. Dent. Assoc. 32,676-690.
quantitative traits. Am. J. Hum. Genet. 26~489-503.
Dahlberg AA (1956) Materials for the establishment of
standards for classification of tooth characteristics, at- Morton NE, and Rao DC (1978)Quantitative inheritance
in Man. Yrbk. Phys. Anthropol. 21:12-41.
tributes, and techniques in morphological, studies of
the dentition. Zoller Laboratory of Dental Anthropol- Morton NE, Yee S, and Lew R (1971) Complex segregaogy, University of Chicago.
tion analysis. Am. J. Hum. Genet. 23t602-611.
Enoki K, and Dahlberg AA (1958) Rotated maxillary Nicol CR (1985) Correlations within and between size
central incisors. Orthodont. J. Jpn. 17:157-153.
measurements and morphological variants of the human dentition. Paper presented at 29th Annual MeetEscobar V, Melnick M, and Conneally PM (1976) The
ings ArizonaiNevada Academy of Science, Las Vegas.
inheritance of bilateral rotation of maxillary central
incisors. Am. J. Phys. Anthropol. 45t109-115.
Nichol CR (n.d.) Dental genetics and biological relationships of the Pima Indians of Arizona. PhD DissertaFalconer DS (1960) Introduction to Quantitative Getion, Arizona State University, Tempe (in progress).
netics. New York Ronald Press.
ACKNOWLEDGMENTS
SEGREGATION ANALYSIS OF DENTAL TRAITS
Nichol CR, and Turner CG I1 (1986) Intra- and interobserver concordance in observing dental morphology.
Am. J. Phys. Anthropol. 69t229-315.
Nichol CR, Turner CG 11, and Dahlberg AA (1984)Variation in the convexity of the human maxillary incisor
labial surface. Am. J. Phys. Anthropol. 63:361-370.
Osborn JW (1978) Morphogenetic gradients: Fields versus clones. In PM Butler and KA Joysey (eds.): Development, Function and Evolution of the Teeth. London:
Academic Press.
Potter RH, Nance WE, Yu PL, and Davis WB (1976) A
twin study of dental dimension. 11. Independent genetic determinants. Am. J. Hum. Genet. 20239-100.
Potter RHY, Rice JP, Dahlberg AA, and Dahlberg T
(1983) Dental size traits within families: path analysis
for first molar and lateral incisor. Am. J. Phys. Anthropol. 61t283:-289.
Potter RH, Yu PL, Dahlberg AA, Merritt AD, and Conneally PM (1968) Genetic studies of tooth size factors
in Pima Indian families. Am. J. Phys. Anthropol.
44t397-412.
Scott GR (1973) Dental morphology: A genetic study of
American white families and variation in living Southwest Indians. PhD Dissertation, Arizona State University, Tempe.
Scott GR (1977) Classification, sex dimorphism, association, and population variation of the canine distal accessory ridge. Hum. Biol. 49t453-469.
Seybert L, and Turner CG I1 (1975) Standard plaque for
the observation of the deflecting wrinkle. Dental Anthropology Laboratory, Arizona State University,
Tempe.
Smith CAB (19561 A test for segregation ratios in family
data. Ann. Hum. Genet. 2Ot257-265.
Snyder LH (1932)Studies in human inheritance. IX. The
inheritance of taste deficiency in man. Ohio J. Sci.
32t436-444.
Sofaer JA (1970)Dental morphological variation and the
Hardy-Weinberg law. J. Dent. Res. 49:1505-1508.
59
Townsend GC, and Brown T 11978a)Inheritance of tooth
size in Australian Aboriginals. Am. J. Phys. Anthropol. 48t305-314.
Townsend GC, and Brown T (1978b) Heritability of permanent tooth size. Am. J. Phys. Anthropol. 49:497504.
Tsuji T (1958) Incidence and inheritance of Carabelli’s
cusp in a Japanese population. Jpn. J. Genet. 3t21-31.
Turner CG I1 (1967a) The dentition of Arctic peoples.
PhD Dissertation, University of Wisconsin, Madison.
Turner CG I1 (1967b) Dental genetics and microevolution in prehistoric and living Koniag Eskimos. J. Dent.
Res. 46t911-917.
Turner CG I1 (1969) Microevolutionary interpretations
from the dentition. Am. J. Phys. Anthropol. 30t421426.
Turner CG I1 (1970) New classifications of non-metrical
dental variation: Cusps 6 and 7. Paper presented at
the 39th annual meetings American Association of
Physical Anthropologists, Washington, DC.
Turner CG 11, and Dowda L (1979) Standard plaque for
the observation of double-shoveling.Dental Anthropology Laboratory, Arizona State University.
Turner CG I1 and Scott GR (1977) Dentition of Easter
Islanders. In AA Dahlberg and TM Graber (eds.): Orofacial Growth and Development. The Hague: Mouton
Publishers.
Turner CG 11, and Warner R (1977a) Standard reference
plaque for the observation of cusp 5 on the upper molars. Dental Anthropology Laboratory, Arizona State
University, Tempe.
Turner CG 11, and Warner R (1977b)Standard plaque for
the observation of the hypoconulid (cusp 5). Dental
Anthropology Laboratory, Arizona State University,
Tempe.
Zubov AA, and Kahldeyeva NI (1979)Etniceskaja Odontologija SSSR. Moscow: Hauka (English translation by
A.M. Haeussler).
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