agenesis of permanent maxillary lateral incisors in south australian twins
TRANSCRIPT
Australian Dental Journal 1995;40(3):186-92
Agenesis of permanent maxillary lateral incisors in South Australian twins
Grant Townsend* James Rogers* Lindsay Richards* Tasman Brown*
Abstract The frequemy of agenesis of maxillary lateral incisors (Iz) in a sample of South Australian twins was determined, and associations with gender, zygosity, birthweight and dental crown size were investigated. Ten of the 446 twins examined (2.24 per cent) displayed agenesis of one or both Iz, a similar frequency to that reported for Australian singletons. Seven of the ten affected individuals were mono- zygous female twins, including two pairs, while the three affected dizygous twins were each from different male twin pairs. Five pairs of monozygous twins were identified who displayed varying expres- sions of normal, small, peg-shaped or missing I*. These twin pairs displayed disparate birthweights suggesting they may have been mono-chorionic. These findings are consistent with a multifactorial threshold model linking size and number of I' with developmental influences modifying phenotypic expression in those monozygous twins whose genetic predisposition places them near to the threshold for agenesis.
Key words: Agenesis, genetics, lateral incisors, twins.
(Received for publication January 1994. Accepted March 1994.)
Introduction Reported frequencies of individuals with congenitally
missing teeth (dental agenesis or hypodontia) vary between approximately 2 and 12 per cent in different human populations, with females tending to be affected more often than males. 1-4 Third molars are most commonly missing, followed by second premolars and maxillary lateral incisors, and individuals identified as having one
*Department of Dentistry, The University of Adelaide, South Australia.
186
missing tooth are more likely to display agenesis of other teeth.5.6 There also appears to be a tendency for reduction in size and morphological complexity of remaining teeth in individuals with dental agene~is,'-~O as well as a higher frequency of disturbances in dental spacing, tooth erup- tion and exfoliation.' Significant associations of dental agenesis with ankylosis of primary molars, taurodontism, enamel hypoplasia, and peg-shaped incisors have also been reported.12 Although there is a familial basis to dental agenesis, the precise mode of inheritance is still unclear, with both single gene and polygenic models proposed.l3
In relation to the permanent maxillary lateral incisor, reported frequencies of agenesis vary considerably, from approximately 0-22 per cent, with most estimates being around 1-3 per cent.14 Meskin and Gorlin15 reported that 1.78 per cent of over 8000 American students demon- strated either peg-shaped or missing maxillary lateral incisors, with a higher frequency in females and a predominance of left-sided occurrence.
Results of family studies point to a genetic relationship between peg-shaped and missing lateral inci~ors.'~.'' Witkop18.19 proposed that small, peg-shaped or missing maxillary lateral incisors were inherited in an autosomal dominant fashion, modified by a polygenic component. Alvesalo and P ~ r t i n ' ~ also felt that agenesis and peg- shaping were different expressions of one dominant autosomal gene, but that small maxillary lateral incisors of normal morphology resulted from a different mechanism.
Comparisons of concordance between identical (mono- zygous) and non-identical (dizygous) twins provide an opportunity to assess the relative importance of genetic and environmental influences on dental agenesis. Boruchov and GreenY2O in a study of 367 pairs of twins, found no significant difference in concordance for agenesis between the two twin types, indicating that environmental factors were likely to be important. Although no associa- tion was found between maternal health during pregnancy or early childhood diseases and the incidence of agenesis,
Australian Dental Journal 1995;40:3.
there was a tendency for children with very low birth- weight to display a higher frequency of dental agenesis. Markovic,z’ in contrast, found a significantly higher concordance rate for dental agenesis among monozygous than dizygous twins in his investigation of 165 twin pairs, leading him to suggest a strong genetic influence.
While the frequencies of twins with missing teeth reported in the studies of Boruchov and Greenz0 and Markoviczl fell within the range of those reported for the general population, KeeneZZ noted that agenesis of teeth other than third molars was two to three times more frequent in his sample of 262 twins than in the general population. Keene also observed that the frequency of dental agenesis was higher in the lower birthweight twins.
Another approach involving twins, aimed at learning more about the role of genetic and environmental influences on dental agenesis, is to note variation in the expression of dental agenesis within pairs of identical twins. For example, Gravely and Johnsonz3 reported on three pairs of monozygotic twins who displayed different expressions of dental agenesis, including the maxillary lateral incisor, and commented that the cases supported the view that agenesis is genetically determined but that its expression could be altered by non-genetic factors. Hartneyz4 and Zvolanekz5 have also provided examples of variation in expression of maxillary lateral incisors in identical twins, supporting the notion that developmental factors may modify expression of the gene($ for dental agenesis.
The general aim of this study was to investigate whether associations existed between agenesis of permanent maxil- lary lateral incisors and gender, zygosity, birthweight and dental crown size patterns in South Australian twins. Specifically, this study sought to determine the frequency of agenesis of lateral incisors in a sample of South Australian twins and to compare these findings with other published results. By comparing the frequencies of concor- dance between monozygous and dizygous twin pairs, the study aimed to assess the importance of genetic differ- ences on agenesis of lateral incisors. By identifying those monozygous twin pairs who displayed different expres- sions of small, peg-shaped or missing lateral incisors, clarification of the nature of the interaction between genetic and environmental influences on phenotypic variation was sought. Monozygous twin pairs are gener- ally thought to share the same genes, so phenotypic differences between pairs are likely to reflect environ- mental differences during development.
Study population and methods All subjects included in this study are participating in
an on-going investigation of dentofacial variation in South Australian twins and their families being undertaken in the Department of Dentistry at the University of Adelaide.z6 A range of observations and records is obtained from subjects, including direct examinations and photo- graphs of the teeth and oral structures, dental impressions from which stone models are prepared, colour and stereo- photographs of the face, and blood samples for the deter- mination of zygosities. Information relating to handedness, birthweight and medical histories is also obtained. The ethical guidelines issued by the National Health and
Australian Dental Journal 1995;40:3
Medical Research Council (of Australia) are followed and the informed consent of all participants is obtained, including permission to publish facial photographs.
Subjects in this study ranged in age from 10 to 46 years, the majority being teenagers. All were Caucasians, with no history of major medical disorders likely to be associated with missing teeth. Zygosities were confirmed by compar- ison of a number of genetic markers in the blood (ABO, Rh, Fy, Jk, MNS), together with several serum enzyme polymorphisms (GLO, ESD, PGM1, PGD, ACP, GPT, PGP, AK1) and protein polymorphisms (HP, C3, PI, GC). The probability of dizygosity, given concordance for all systems, was less than 1 per cent. The study sample included 223 pairs of twins: 50 monozygous (MZ) male pairs, 72 M Z female pairs, 24 dizygous (DZ) male pairs, 31 DZ female pairs and 46 opposite-sexed DZ pairs.
Agenesis of maxillary lateral incisors (Iz) was recorded initially during dental examinations performed on all subjects, then subsequently confirmed by reference to intra-oral radiographs previously obtained by the subjects’ own dentists. Previous dental histories of those twins with missing I’ were checked to determine whether any I’ had been extracted. Frequencies of I’ agenesis were compared between MZ and DZ twins using chi-squared analysis, and comparisons were also made with previously published data for twins and singletons. Concordance for Iz agenesis was compared between MZ and DZ twin pairs as a means of clarifying the roles of genetic and environ- mental influences in determining trait expression. In those MZ twin pairs identified as displaying different expres- sions of small, peg-shaped or missing lateral incisors, possible associations with gender, birthweight, and size of other permanent teeth were investigated. The term ‘small’ was used to refer to Iz of normal crown morphology but with a mesiodistal diameter more than one standard deviation below the mean. The term ‘peg-shaped’ was applied where the mesiodistal crown diameter was reduced and the proximal surfaces tapered toward the incisal edge. Measurements of mesiodistal and buccolingual crown diameters were obtained from dental models using specially-modified calipers following the definitions of Moorrees et al.”
Table 1. Relationships between gender and birthweight in five MZ twin pairs who displayed different expressions of missing (M), peg-shaped (P), small (S) and normal (N) maxillary lateral incisors (I2)
I’
R L Ident No Gender Birthweight (9)
T34 A M S F 2264 B S S F 2745
T73 A M M F 1472 B P M F 1726
F 3113 T76 A S M B S N F 1967
T155 A S P F 2406 B M S F 1910
T188 A P* M F Unavailable B M M F Unavailable
*Peg-shaped tooth extracted and canine orthodontically moved mesially.
107
Fig. 1. -Monozygous female twins T34A and B. T34A displays agenesis of the maxillary right lateral incisor and a small maxillary left lateral incisor, whereas T34B has two small maxillary lateral incisors. The mandibular premolars had been extracted for orthodontic reasons.
Fig. 2.-Monozygous female twins T73A and B. T73A displays bilateral agenesis of the maxillary lateral incisors, whereas T73B has a peg-shaped maxillary right lateral incisor and agenesis of the maxillary left lateral incisor. The maxillary right canine of T73A is more advanced in its eruption
than its antimere. The maxillary right canine of T73B is not fully erupted.
188 Australian Dental Journal 1995;40:3.
Fig. 3. -Monozygous female twins T76A and B. T76A displays a small maxillary right lateral incisor and agenesis of the maxillary left lateral incisor, whereas T76B shows a small maxillary right lateral incisor and a normal-sized maxillary left lateral incisor.
Fig. 4.-Monozygous female twins T155A and B. T155A displays a small maxillary right lateral incisor and a peg-shaped maxillary left incisor (in crossbite relationship), whereas T155B displays a missing maxillary right lateral incisor and a small maxillary left lateral incisor.
Australian Dental Journal 1995;40:3. 189
Fig. 5 . -Monozygous female twins Tl88A and B. T188A displays missing maxillary lateral incisors. A peg-shaped maxillary right lateral incisor had been extracted previously and the canine moved mesially. The maxillary left lateral incisor was congenitally missing. T188B displays bilateral
agenesis of maxillary lateral incisors.
Results Of the 446 individuals included in this study, 10
displayed agenesis of one or both 12, representing a frequency of 2.24 per cent. In five cases, agenesis was bilateral, whereas in the remainder the feature occurred unilaterally, being right-sided in three instances and left- sided twice. Seven affected individuals were MZ twins, including two pairs, and all were female. In the two cases where both members of an MZ pair displayed agenesis of 12, one twin showed bilateral agenesis and the other unilateral agenesis. In contrast, the three affected DZ twins were each from different DZ male pairs. Frequencies of I2 agenesis were therefore 2.87 per cent (7 in 244) for MZ twins and 1.49 per cent (3 in 202) for DZ twins. Although the frequency of I2 agenesis in MZ twins approached twice that in DZ twins, chi-squared analysis failed to indicate a statistically significant association between agenesis and zygosity or,’ = 0.97, p = 0.33). As only two pairs of MZ twins showed concordance for agenesis of at least one 12, and no DZ twin pairs were concordant, a statistical test of association between concordance and twin type was not attempted. Only one of the ten affected individuals was confirmed to display agenesis of any other tooth, a mandibular right second premolar. There was also no marked evidence of reduction in morphological complexity of other teeth or alterations in the timing or sequence of tooth exfoliation or emergence. In two of the ten instances there was a familial history for missing teeth. The mother and two siblings of one of the affected DZ individuals had agenesis of second premolars and the cousin of one
190
pair of affected MZ girls also displayed bilateral peg- shaped 12.
Interestingly, five pairs of MZ twins, including the seven females identified with agenesis of I2 displayed varying expressions of normal, small, peg-shaped or missing 12. The findings for these twin pairs relating to gender and birthweight are summarized in Table 1. In addition, facial photographs of the seated twins, together with intra-oral photographs are provided in Fig. 1-5.
In both cases where one member of an MZ pair displayed bilateral agenesis of 12, the co-twins showed agen- esis of I2 on one side and a peg-shaped lateral on the other. The other three MZ pairs showed different expressions of missing, peg-shaped, small and normal 12. For the four MZ pairs where records of birthweight were available, discrepancies in birthweight between co-twins were noted that ranged from 254 to 1146 g. In three of these four cases, the co-twin with the lower birthweight appeared to show the more extreme expression of I’ agenesis.
Table 2 provides dental crown size data for the five pairs of MZ twins with different expressions of I2 morphology. Average crown dimensions for all female M Z twins in the study are provided for comparison. No clear trend was evident in relation to the size of the other permanent teeth in the five twin pairs. In one pair (T34) there was a general reduction in tooth size compared with the average, in another there was evidence of increased size (T155), and in the other cases the pattern differed between dimensions. The general patterns of permanent tooth size within each twin pair were very similar.
Australian Dental Journal 1995;40:3.
Table 2. Dental crown size (mm) of five MZ female twin pairs with different expressions of lateral incisor morphology*
All MZ female twinst T34 T73 T76 T155 T188 Tooth dimension
n X SD A B A B A B A B A B
Mesiodistal Maxilla
I1 I2 C P1 P2 M1 M2
I1 I2 C P1 P2 M1 M2
Buccolingual Maxilla
I1 I2 C P1 P2 M1 M2
I1 I2 C P1 P2 M1 M2
Man d i b 1 e
Mandible
161 8.49 0.55 147 6.65 0.53 143 7.56 0.36 124 6.89 0.45 140 6.66 0.44 153 10.26 0.58 89 9.82 0.70
160 5.25 0.37 163 5.83 0.43 156 6.53 0.35 136 6.98 0.43 137 7.08 0.41 152 10.82 0.65 89 10.35 0.67
158 6.99 0.42 140 6.26 0.50 131 7.99 0.48 127 9.12 0.56 144 9.22 0.59 158 11.22 0.61 108 10.92 0.69
160 5.90 0.40 159 6.29 0.42 145 7.25 0.52 135 7.89 0.53 142 8.51 0.52 156 10.58 0.52 122 10.27 0.58
8.6
7.3 6.6 6.2 9.8 9.0
5.2 6.2 6.7
6.9 10.5 9.6
-
-
6.4
7.3 8.2 8.4
10.2 10.1
-
- 5.6 6.9
8.4 9.7 9.0
-
8.5 5.8 7.4 6.5 6.3 9.8 9.2
5.3 6.2 6.6
7.0 10.8 10.0
-
6.6 5.1 7.6 8.0 8.1
10.6 10.0
5.6 6.0 7.1
8.2 9.9 9.2
-
9.6
7.4 7.4
11.3
-
-
-
5.3 5.8 6.6 7.1 7.6
11.8 -
6.6
7.4 9.5
11.7
-
-
-
5.3 6.2 6.6 7.7 8.6
11.0 -
9.2 - - 7.2
10.9 -
-
5.6 5.5 6.4 6.9 7.5
11.2 -
7.0 - -
9.4
10.7 -
-
5.4 5.9 6.6 7.9 8.6
11.0 -
8.7 5.2 7.2 6.5 6.1
10.4 9.9
5.3 5.4 6.8 7.0 6.9
10.8 10.5
7.5
8.4 8.6
19.2 11.7 11.5
6.5 6.6 7.9 8.5 8.9
10.4 10.8
-
9.0 9.4 5.2 4.8 7.4 - 6.5 - 6.9 7.0 - 10.6 - -
5.5 5.4 5.6 6.0 6.6 6.8 7.0 7.3 7.0 7.4
10.9 11.4 10.0 -
7.3 6.9 6.1 4.5
8.6 9.6 9.1 9.9
11.6 11.7
8.5 -
- -
6.1 6.2 6.4 6.6 7.9 7.0 8.4 7.9 9.1 9.1
10.5 10.8 10.6 -
9.2
7.8 7.1 6.8
10.4
-
-
5.5 5.9 6.7 7.3 7.4
11.1 -
6.7 -
- 9.0 9.8
11.3 -
6.2 6.5 7.1 8.3 9.1
10.7 -
8.2
7.8 6.3 5.9 9.1 9.5
5.1 5.4 6.1 6.7 6.5
10.2
-
-
7.4
8.2 8.7 8.3
10.3 10.0
6.4 6.8 7.5 7.7 8.0
9.8
-
-
8.4
7.6 6.2
9.0 9.2
4.9 5.4 6.0 6.6
9.9 10.0
-
-
-
7.4
8.0 8.5
10.2 10.5
6.1 6.7 7.6 7.7
9.7 9.7
-
-
-
*Values for right side only. ?Average tooth size dimensions for all MZ female twins provided for comparison.
Discussion The overall frequency of I2 agenesis in South Australian
twins was of the same magnitude (that is, 2.2 per cent) as that recently reported in a large sample of Australian ~ h i l d r e n . ~ There was, therefore, no evidence of an increased frequency of I2 agenesis in Australian twins compared with singletons. The frequency of agenesis of I2 was greater in female twins (7/252 = 2.78 per cent) than male twins (3/194 = 1.55 per cent), a result consistent again with the finding of a 2 to 1 ratio of affected females to males in the study of Stamatiou and S y m o n ~ . ~ There was, however, no definite trend for I2 agenesis to show a definite side predilection in the present study, nor was a signifi- cant association between I2 agenesis and zygosity observed although the statistical tests lacked power due to the small sample sizes available. That the only examples of concor- dance for agenesis were both noted in MZ twins, who presumably share the same genes, provides support for a genetic basis to the condition, but larger samples will be needed before a formal statistical comparison can be made.
No definite trend for a generalized reduction in tooth size or alteration in crown form of other teeth in those individuals with I2 agenesis was noted in the present study, but the sample size of affected individuals was very small. However, Le Bot, Gueguen and Salmon2' found that the
Australian Dental Journal 1995;40:3.
morphology of other teeth was affected in their study of 192 males with missing or reduced 12. The modifications in morphology of other teeth included reduction in molar cusp number, altered groove patterns, decreased frequency of Carabelli trait and altered shape of mandibular first premolars. These researchers suggested that changes in developmental timing within the dentition may have contributed to the simplification of dental morphology. Interestingly, Le Bot and colleagues pointed out that reduction in size of I2 was associated with more pronounced modification of size and morphology in other teeth than agenesis of 12. They suggested that this effect may be explained by the hypothesis of Sofaer et that agenesis of a tooth may lead to an increased growth poten- tial of other neighbouring teeth, reflecting a compensatory effect in growth of adjacent developing tooth germs. Stalker and Mahany30 also noted a reduction in tooth size and cusp number in accessional teeth of individuals with small, peg-shaped or missing I2 who belonged to a North American genetic isolate displaying a high prevalence of dental agenesis.
Large-scale studies therefore confirm a link between agenesis of I* and morphological variation in other teeth. Such a generalized effect on the dentition seems more likely to reflect polygenic influences rather than a simple monogenic mode of inheritance. Brook3' has provided a
191
Table 1. Intra- and inter-examiner reliability of dental indices
Table 2. Distribution of subjects by age group and by sex
Index Examiner
DMF-T (caries) Intra-examiner Examiner 1 Examiner 2 Inter-examiner
Plaque Index (oral hygiene) Intra-examiner Examiner 1 Examiner 2 Inter-examiner
Gingival Index (gingival condition) Intra-examiner Examiner 1 Examiner 2 Inter-examiner
~
kappa value Variable n 7 0
0.93 1 .oo 0.75
0.07 0.22 0.07
Age group (years) 6-8
11-13 14-16
Total Sex
Male Female
Total
122 30.3 177 43.9 104 25.8 403 100.0
203 50.4 200 49.6 403 100.0
0.09 0.36 0.32
A questionnaire eliciting standard demographic infor- mation was administered to each subject and an intra-oral examination for dental caries, oral hygiene, and gingival condition was performed by either of two calibrated epidemiological examiners. The teeth and soft tissues were not dried prior to examination. A sickle-shaped probe and a standard fibre-optic light source with mouth mirror attached were used with every subject.
Data for the dental caries examination were recorded in terms of DMF-T (decayed, missing, filled teeth) using the standard World Health Organization criteria.’*
Oral hygiene was measured using the Silness and Loe Plaque Index” by assessing the level of dental plaque along the gingival margins of 6 selected teeth. These teeth were: upper right first molar (16); upper right lateral incisor (12); upper left second premolar (25); lower left first premolar (34); lower left lateral incisor (32); and lower right first premolar (44). When any of these teeth was absent, the tooth adjacent or preceding it was used instead.
The gingival status was assessed by using the Loe and Silness Gingival Index.*O The marginal gingivae of the same 6 teeth selected in estimating the Plaque Index score were examined.
The DMF-T, Plaque Index and Gingival Index were the same indices used in the 1981 study.”
Ten per cent of the sample was re-examined to measure the intra- and inter-examiner reliability for each index. Intra-examiner reliability was calculated by comparing individual tooth scores for each index from the first with the second examination for each examiner. The kappa value was then calculated using the following equation:21
where Po is the observed proportion of agreement and P, is the expected proportion of agreement. Inter-examiner reliability was calculated by comparing individual tooth scores for each index tooth reported by the first examiner with those reported by the second examiner. The kappa value was calculated using the same equation as above.
Intra- and inter-examiner reliability of the caries scores were high. For both the plaque and gingival scores, intra- and inter-examiner reliability were low (Table 1).
194
Results A total of 403 subjects, or 50 per cent of those who were
approached to participate, from 5 primary schools and 3 secondary schools in Heidelberg, Victoria, participated in this study. The subjects were classified into three age groups. Table 2 shows that the largest proportion of subjects belonged to the age group 11-13 years old (43.9 per cent). There was an almost equal representation of males (50.4 per cent) and females (49.6 per cent).
Occupation of father or guardian was used to determine socioeconomic status, and ethnic background was deter- mined by using the information on the language spoken at home. Non-manual occupations, indicating higher SES, formed the greater proportion of subjects in this study (46.2 per cent). The majority of the subjects were from Anglo-Saxon ethnic backgrounds (84.4 per cent).
Caries experience The overall mean DMF-T score was 1.19k2.04
(Standard Deviation), with 58.9 per cent of the sample caries free. Table 3 summarizes the mean DMF-T and component scores for each age group. The mean DMF- T score increased as age group increased. A Pearson corre- lation (r = 0.34; pC0.01) between age group and DMF-T score was significant. Analysis of variance showed that there was a significant difference in the scores of the different age groups (p < 0.01).
It was also observed that the proportion of caries free individuals decreased as the age group increased. Freedom from caries was observed in 9 1.8 per cent of the 6-8 year old subjects; 49.2 per cent of the 11-13 year olds; and 36.5 per cent of the 14-16 year olds. Analysis of variance of DMF-T for each age group showed that caries experience was not associated with any other socio- demographic variable.
Table 3. Mean DMF-T and component scores ( f SD) Tooth 6-8 year olds 11-13 year olds 14-16 year olds condition
1991 1981 1991 1981 1991 1981
Decayed 0.06f0.30 0.27t0.66 0.38f0.98 0.70f1.47 0.17t0.55 0.79f1.44 Missing 0.0 0.1OtO.09 0.03f0.28 0.10t0.48 0.03t0.17 0.21t0.59 Filled 0.06f0.27 0.42t0.91 1.09t1.64 2.46t2.49 1.71t2.30 5.25f4.42 DMF-T 0.12t0.41 0 . 6 9 t 1 . 1 3 1 .50t2 .16 3.22f2.94 1.91 t 2 . 4 7 6.26f4.32 FIDMF-T 0.92t0.20 0.57t0.46 0.87t0.21 0.71 f0 .40 0.96f0.11 0.79t0.31 Caries free (To) 91.8 25.0 49.2 18.2 36.5 9.1
Australian Dental Journal 1995:40:3.
unifying explanation for the relationship between tooth number and size. He proposed a multifactorial model, combining polygenic and environmental influences, that affected the entire dentition. An underlying continuously variable distribution related to tooth size was postulated, upon which thresholds were superimposed to account for agenesis and supernumerary teeth. The model is consis- tent with observations of greater frequencies of dental agenesis in females, whose teeth are generally smaller than those in males where supernumeraries are more common. Brook suggested that the different prevalence of anoma- lies in different regions of the dental arches could be associated with developmental timing, later-developing teeth displaying more variability than earlier-forming teeth in the same class.
Detailed investigation of the five pairs of MZ twins who displayed different expressions of normal, small, peg- shaped and missing I2 provided further insights into the possible role of the genetic and environmental contribu- tions to observed variability. As indicated by their disparate birthweights, the affected MZ twin pairs were likely to be mono-chorionic, that is, they shared a common placenta and chorionic membrane during prenatal develop- ment. In about 30 per cent or more of mono-chorionic twin pairs, arterio-venous anastamoses can lead to relatively large differences in birthweight, with one member of the twin pair receiving better nourishment than the Differences in blood flow to developing tooth germs at critical stages of their formation, resulting in nutritional discrepancies, could presumably also influence dental phenotypes. Indeed, the I2 normally shows consider- able morphological instability associated with a relatively long period of time spent in the soft tissue phase prior to crown ca l c i f i~a t ion .~~-~~ Subtle alterations in local blood supply to sensitive, developing I2 in genetically identical individuals may then explain the range of variations observed in final I2 crown form, from agenesis at one extreme to normal morphology at the other. Another possible explanation for I2 instability may relate to its positioning at the site of embryonic fusion between the premaxilla and palatal plates of the maxillary processes, labelled as a ‘fragile site’ by Stamatiou and S y m o n ~ . ~
This study of twins does not allow a definite statement as to whether the mode of inheritance of I2 agenesis is monogenic with reduced penetrance or multifactorial. However, the results are consistent with a multifactorial threshold model linking size and number of 12, with developmental influences modifying phenotypic expres- sion in those MZ twins whose genetic predisposition places them near to the threshold for agenesis.
Acknowledgements This study was supported by the National Health and
Medical Research Council of Australia. Special thanks are accorded to Ms Sandy Pinkerton for her technical assistance.
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Address for correspondenceheprints: Department of Dentistry,
The University of Adelaide, Adelaide, South Australia 5005.
Australian Dental Journal 1995;40:3.