The aims of this study were to determine the frequency of taurodontism of the first and second permanent molars in twins discordant or concordant for clefts, the frequency for each type of cleft in twins, and the concordance-discordance ratio in mono- and dizygotic twins. Thirty-nine pairs of twins (13 monozygotic, 26 dizygotic) between the ages of 7 and 23 yrs were investigated. Taurodontism and hypodontia were determined from orthopantomograms of the dentition. Of the 39 pairs of twins investigated, 16 (41%) had taurodontism, equally in mono- and dizygotic pairs. The highest frequency of the trait was noted in subjects with isolated cleft palate and the lowest in cleft lip subjects. Four of 13 monozygotic pairs with taurodontism were concordant for the trait, as were 9 of 12 dizygotic pairs. Taurodontism was symmetric in 91% of the affected molar pairs. Concomitant taurodontism and hypodontia was observed in 8 of 16 taurodontic twin pairs. We conclude that the etiology of clefting in our group of twins seems to have an effect on the frequency of taurodontism. A high concordance for taurodontism and a high symmetry of the trait may indicate a strong genetic etiology of taurodontism.
The frequency and expression of the Carabelli trait among 36 twin pairs discordant or concordant for cleft lip and/or palate were investigated. Four of 10 monozygotic (MZ) and 24 of 26 dizygotic (DZ) pairs of twins were discordant for clefts. The frequency of the trait on the first upper molars of the noncleft subjects was 79%. It varied from 54% to 61% in the subjects with clefts, and the frequency of the trait decreased with increasing extension of the cleft. The occurrence of the trait was bilateral in more than 90% of the MZ and the DZ subjects. MZ twins had a higher concordance rate for the corresponding molar comparisons and for cross-twin comparisons than did the DZ pairs. The corresponding polychoric correlations for MZ twin pairs were estimated to be 1.0, and for the DZ twin pairs estimates ranged from 0.4 to 0.5. Thus, genetic factors appear to control the Carabelli trait, and the heritability seems to be high even in our cleft lip and/or palate twins.
Lctatikctitien T, Rcmta R; Hypodontia in twins discordant or concordctnt for cleft lip atid/or palette. Scatui J Detit Res 1994; 102; 88-91. © Munksgaard, 1994.The aims of this study were to determine the frequency of hypodontia of permanent teeth in twins discordant or concordant for clefts in each type of cleft, to determine the concordance of hypodontia in mono-and dizygotic twins, and to compare the findings with some earlier data. Thirty-nine pairs of twins between 7 and 23 yr of age were investigated. Six of 13 monozygotic and 24/ 26 dizygotic pairs were discordant for clefts. Orthopantomograms of the dentition, treatment records, and anamnestic data were studied. Twenty-three pairs of twins (59' Mi) had at least one twin with hypodontia: 9/13 (69%) monozygotic and 14/26 (54'^) dizygotic. The prevalence of hypodontia was 37'y;. for monozygotic and 32.7% for dizygotic twins, 16.7% in the noncleft and cleft lip groups, 41.1% in the cleft palate group, and 64.3% in the cleft lip and palate group. All these were above the values observed earlier in the noncleft twins and in the Einnish normal and cleft group populations, Eour of 13 pairs of monozygotic twins had no hypodontia, 8/9 pairs were discordant, and only 1/9 was concordant for hypodontia. The corresponding figures for the 26 dizygotic pairs were: 12/26, 11/14, and 3/14 pairs. Maxillary second premolars were the most frequently absent teeth, followed in order of frequency by the maxillary lateral incisors and the mandibular second premolars. Hence, for this sample of twins, the genetic component seems to be weak.
The craniofacial morphology of 11 pairs of monozygotic (MZ) and 28 pairs of dizygotic (DZ) Finnish twins, discordant or concordant for cleft of the lip (CL), unilateral cleft lip and palate (UCLP), or cleft palate only (CP) were investigated by means of lateral cephalometric radiographs. The results were compared to those of age-, sex-, and cleft-type matched single-birth cleft subjects, and also with normative data from the Nordic population. The co-twins with no cleft lip or palate (NONC) showed only slightly more obtuse gonial and steeper mandibular angles compared to normative data. Twins with CL, UCLP or CP had a more retrusive mandible, a wider cranial base and mandibular angle, and a wider angle between the maxilla and mandible than did the single-birth cleft subjects. Comparison of the noncleft twin group with the CL, UCLP, and CP twin groups for the CL twins showed no significant differences. For the UCLP twins, a more retrusive and down- and backward rotation of both jaws, a wider gonial angle, and a wider cranial base angle was seen. The CP twins had their maxillae slightly retrusive, the down- and backward rotation of both jaws was apparent, and the gonial angle was more obtuse. A comparison between the noncleft MZ and noncleft CDZ twins showed no significant differences. The MZ CP twins had a more retrusive mandible and more down- and backward rotation of both jaws than did DZ CP twins. It thus can be suggested that twinning itself does not seem to have an effect on maxillofacial morphology, but the features of the mandibular structure, the cranial base angulation, and the inclination of the jaws are at least partly genetically induced.
Extensive review of the literature since 1884 on cleft-twin sets yielded 364 cleft-twin sets. Of these, 118 were monozygotic (MZ) and 246 dizygotic (DZ) sets. In addition, Danish material on cleft twins, like our Finnish material, reflects the total number of clefts and cleft twins on a well-defined population during a well-defined time interval. Both sets of material also contain slightly over 100 pairs of twins. The Danish material and the literature review were compared to the Finnish material. The hospital records of all Finnish patients with operated clefts who were born between 1948 and 1987 were reviewed. Information was gathered regarding each patient, his parents, the pregnancy, and his twin or triplet siblings and other siblings. This search produced 105 sets of twins and three sets of triplets with clefts, 15 sets of twins being concordant regarding clefting. This resulted in a total of 120 cleft siblings, and the corrected cleft incidence of 1.72 promille, close to the overall cleft incidence rate in Finland between 1948 and 1975. Twinning was found to be associated neither with an increased nor with a decreased risk of clefting, and clefting could not be seen to increase twinning. Zygosity could be verified in 88 sets of twins; the total number of MZ sets was 17, and of DZ was 71, a 19% MZ rate. Although a higher incidence of clefting in MZ-twin sets has been proposed, no such higher or lower incidence could be found in our material. Recognized syndromes were found in 15 sets (14%), slightly higher than found in a large Finnish study on cleft probands (8.4%). Of these, three sets were monozygotic (MZ), all of them cleft palate (CP) and male sets, whereas eight sets were dizygotic (DZ). All 15 sets were CP only, with no one set with cleft lip and palate [CL(P)]. In our total Finnish-twin material of 105 sets, we found the CL(P)/CP ratio to be 39/66 (37%/63%). In all of the 120 affected siblings, the ratio was 35%/65%. The very high rate of 63% of CP twins is about two to three times higher than that reported in the literature of 364 sets where the CP ratio is 23%; compared to the Danish material with a CP ratio of 17%, it is almost four-fold. The overall CP rate in all clefts (not restricted to twins)in Finland compared to the rate In our neighboring Scandinavian countries was very much in line with this very big difference seen in the CP rate In our twin material. The CL(P) Incidence in our Finnish material is 0.61 promille and the CP incidence 1.11 promille. For MZCL(P), the incidence was 0.38 promilie; for MZCP, 0.91 promille; for DZCL(P), 0.52 promille, and for DZCP, 0.99 promille. Compared to the Danish figures, both the MZCP and DZCP incidence figures are nearly four-fold, with the MZCL(P) somewhat lower, and DZCL(P) less than half that number. The sex distribution of all cleft patients in our material was 44% male/56% female. Both in the Danish material and in the literature, it was the reverse. This difference is probably due mostly to the higher ratio of CP in the Finnish material. The CP group has a higher proportion of females in all these materials. The concordance (C) of the whole Finnish-twin material is 14%, compared to 16.5% reported in the literature and 8% for the Danish twins. The concordance for CL(P) In our material is many times lower (2.6%) than in the literature (17.1%) and In the Danish material (8%). In the Finnish twins, the C for CP is higher (17%) than that for the Danish (6%) and for the literature cleft-twin populations (14.3%). This is also true for the MZ and DZ subgroups. The heritability index (H) in CL(P) is lower for the Finns (17%) than for the Danish (45%) and for the literature materials (43%), and higher for CP (Finns 49%, Danish 33%, literature 36%). All of these data strongly suggest the quite different genetic behavior of both CL(P) and CP in Finland, with a much lower genetic component in the CL(P) and a higher In the CP.
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