Objective: The aim of this study was to quantify the contributions of genetic and environmental factors to variation in the timing of emergence of primary incisors and timing of colonisation of MS in a sample of monozygotic (MZ) and dizygotic (DZ) twins, using a model-fitting approach.
Methods: The sample comprised 83 pairs of MZ twins and 82 pairs of DZ twins, all of European ancestry, aged from 0-2.5 years. Zygosities were confirmed by DNA analysis. Emergence timing was based on parental reports. Colonisation timing was based on microscopy of microbiological samples obtained quarterly. The software program, Mx, was used to fit genetic models to raw data.
Results: There were no significant differences between zygosity groups or sexes for either emergence times or colonisation time. The mandibular central incisors emerged first, with a mean of 267±68 days, followed by the maxillary central incisors (330±61 days), the maxillary lateral incisors (358±76 days), and the mandibular lateral incisors (404±75 days). Mean colonisation time was 334±152 days.
The model that best explained observed variation included an additive genetic component (A) and a unique environmental component (E). Estimates of heritability averaged 95% for tooth emergence timing and 82% for colonisation timing.
Conclusions: These findings indicate that genetic factors are of considerable importance in determining variation in human primary incisor emergence and oral colonisation of MS. We now plan to examine whether the two processes share common genetic liabilities.
Supported by the NHMRC of Australia and the ATR.