Twin Half-sibs: A Research Design for Genetic Epidemiology of Common Dental Disorders

Abstract

Dental caries and the chronic type of periodontal disease are classic examples of common familial diseases that are complex and multifactorial in etiology. Due to previous methodological limitations, there is no information at present on the genetic and shared environmental risks within families that account for familial aggregation of these diseases. Such information, however, is needed in the long run to help specify modifiable family environments that affect the frequency and/or severity of the disease, and thereby to aid in the planning of prevention strategies to be targeted at the family level. Recently, an innovative genetic model has been described to test the genetic and environmental basis of chronic familial diseases in identical (monozygotic) twins, their spouses, and their offspring, who are genetically half-sibs. In this paper, the method is detailed and shown to be appropriate not only for partitioning shared genetic and environmental risks within families, but also for delineating maternal effects and assortative mating as two non-genetic mechanisms that may ultimately be found to affect the incidence of disease.

Genetic Influences in Caries and Periodontal Diseases

“…Population studies, family studies, the twin model, and variations of the twin paradigm (e.g., the twin half-sib method described by Potter, 1990) have all been used in dentistry to evaluate the inheritance potential of numerous orofacial traits (see Table 3). Significant and major genetic effects have been reported for the mechanisms that influence intra-alveolar dental development (Green and Aszkler, 1970), mesiodistal tooth dimension (Osborneeta!., 1958;Horowitz et al, 1958b), buccolingual tooth dimension (Potter et al, 1976), morphologic variation in permanent molar teeth (Biggerstaff, 1970), the Carabelli cusp trait (Biggerstaff, 1973;Townsend and Martin, 1992), cleft lip/palate (Fogh-Anderson, 1942;Cronin and Hunter, 1980), as well as various other aspects of the craniofacial complex (reviewed by Nakata, 1985).…”

“…Yet today, the precise reasons for such familial clustering remain largely unclear, although the genetic mechanism for commonly occurring chronic diseases is generally acknowledged to have a complex etiology (Potter, 1990;Nishimura et al, 1990). In its traditional and most restrictive form, the family method for ascertainment of information about the heritability of certain characteristics consists of pedigree analysis.…”

“…This can be particularly troublesome and misleading in studies of dental disorders such as caries or chronic adult periodontitis, which are known to be influenced more or less dramatically by within-family factors such as diet, oral hygiene, etc. (Potter, 1990). The family method is generally effective in linkage studies as well as in studies designed to identify the carrier state (Osborne, 1963).…”

The value of twins in dental research

“…Potter 23 emphasized the potential value of the twin halfsib approach and indicated that it could be applied not only for partitioning shared genetic and environmental risks within families, but also for determining the involvement of maternal effects or assortative mating. The advantage of this model is that the children of MZ pairs who are born to different mothers are themselves genetically half-siblings.…”

Genetic aspects of dental disorders

“…6 Monozygous twins are assumed to have identical genotypes, so their offspring are genetically related as half-sibs but are socially first cousins. A nested analysis of variance similar to that used in analysing data from half and full-sibling litters in animal studies can be applied to provide estimates of genetic and environmental effects.…”