Complex segregation analysis of dental morphological variants

Few dental anthropological studies have investigated the associations between tooth crown size and crown traits in humans using quantitative methods. We tested several hypotheses about overall crown size, individual cusp areas, and expression of Carabelli cusps in human permanent first molars by obtaining data from standardized occlusal photographs of 308 Australians of European descent (171 males and 137 females). Specifically, we aimed to calculate the areas of the four main molar cusps, and also Carabelli cusp, and to compare the relative variability of cusp areas in relation to timing of development. We also aimed to compare cusp areas between males and females and to describe how Carabelli cusp interacted with other molar cusps. Measurements included maximum crown diameters (mesiodistal and buccolingual crown diameters), the areas of the four main cusps, and the area of Carabelli cusp. The pattern of relative variability in absolute areas of molar cusps corresponded with their order of formation, the first-forming paracone displaying the least variation, and the last-forming Carabelli cusp showing the greatest. Overall crown size and areas of individual cusps all showed sexual dimorphism, with values in males exceeding those in females. Sexual dimorphism was smallest for paracone area and greatest for Carabelli cusp area. Overall crown size and cusp areas were larger in individuals displaying a Carabelli cusp, especially the hypocone area. Although the combined area of the protocone and a Carabelli cusp was greater in cuspal forms than noncuspal forms, protocone area alone was significantly smaller in the former. Our findings lead us to propose that, in individuals with the genotype for Carabelli trait expression, larger molar crowns are more likely to display Carabelli cusps, whereas molars with smaller crowns are more likely to display reduced forms of expression of the trait. We suggest that the pattern of folding of the internal enamel epithelium in developing molar crowns, particularly in the protocone region, can be modified by a developing Carabelli cusp.

Section: Discussionmentioning

confidence: 90%

Associations between Carabelli trait and cusp areas in human permanent maxillary first molars

Kondo

Townsend

2005

“…Kolakowski et al (1980) noted significant major gene and common sibling environmental effects for Carabelli trait by ap lying a tained from a large number of families living on the Solomon Islands. More recently, Nichol (1989) has also applied segregation variance. rs evertheless, in general it seems for dm2 is a complicating P actor in our analcomplex segregation analysis to I f ata ob-analysis to family data in an attempt to clarify the mode of inheritance of a number of dental crown traits.…”

Section: Discussionmentioning

confidence: 98%

Expression of the entoconulid (sixth cusp) on mandibular molar teeth of an Australian aboriginal population

Townsend

Yamada

Smith

1990

The expression and genetic basis of the entoconulid (sixth cusp) on mandibular molars were examined in a geographically isolated group of aboriginals from Yuendumu in the Northern Territory of Australia. Four grades of trait expression, ranging from trace to small, medium, and large cusps, were defined on dental casts of 399 subjects. Frequencies of occurrence were among the highest reported in human populations. Approximately 80% of dm2s showed the trait, whereas frequencies in the permanent dentition ranged from around 50% on M2 to 70% on M1 and 80% on M3. The degree of expression increased distally along the molar series, with only 3% of dm2s showing large cusps compared with 25% of M3s. Fluctuating asymmetry was highest for M2 and lowest for dm2. No strong evidence for sexual dimorphism in occurrence or degree of expression was found. Based on a quasi-continuous threshold model, a genetic contribution to entoconulid variability was observed that was strongest for M1. Significant associations were noted between entoconulid expression on mandibular molars and metaconule expression on maxillary molars, indicating that similar developmental mechanisms may influence these traits. The entoconulid and the metaconule both provide additional bulk on the distal occlusal surface of molar teeth, an area subjected to early wear during mastication in aboriginals.

“…Several studies of heritability of dental traits support this assumption (Mizoguchi, 1977;Townsend and Brown, 1978;Nichol, 1989). As such, differences in trait frequencies between samples are presumed to represent genetic variation between the populations.…”

mentioning

confidence: 89%

“…The results of the present study are interpreted under the assumption that dental traits are phenotypic characteristics that reflect genetic differences (Scott and Turner, 1985;Sofaer et al, 1986;Nichol, 1989;Rightmire, 1999). Several studies of heritability of dental traits support this assumption (Mizoguchi, 1977;Townsend and Brown, 1978;Nichol, 1989).…”

mentioning

confidence: 97%

Microevolution of African American dental morphology

Edgar

2007

The African American (AA) gene pool is primarily the result of gene flow between two biologically disparate groups: West Africans (WA) and Americans of western European descent (EA). This research utilizes characteristics of dental morphology to trace genetic relationships among WA, western Europeans (EU), AA, and European Americans. Dental morphological traits are useful for this purpose because they are heritable, do not remodel during life (although they can be lost to wear or pathology), and can be compared equally among samples from past and present populations. The results of this research provide new information about human microevolution through time and space in a biocultural setting. The mean measure of divergence is used to analyze dental morphological data from 1,265 individuals in 25 samples grouped by ancestry and time. Three hypotheses associated with admixture in AA are tested. When compared with known history, results from dental morphological data are equivocal in documenting admixture in AA. Dental morphological traits do appear to reflect admixture in AA. However, changes in trait frequencies do not closely correspond with important cultural events and trends such as the institutionalized racism of the Civil War and Jim Crow era. Results are mixed concerning whether AA with greater admixture were more likely to take part in the Great Migration to southern urban centers and to the North.