Human sex chromosomes in oral and craniofacial growth.

Studies on tooth crown size and structure in families and in individuals with various sex chromosome anomalies have demonstrated differential direct effects of the human X and Y chromosome genes on growth. The Y chromosome promotes both tooth crown enamel and dentin growth, whereas the effect of the X chromosome on crown growth seems to be restricted to enamel formation. Enamel growth is decisively influenced by cell secretory function and dentin growth by cell proliferation. It is suggested that these differential effects of the X and Y chromosomes on growth explain the expression of sexual dimorphism in various somatic features. These include tooth crown and root size, crown shape and the number of the teeth, and under the assumption of genetic pleiotropy, torus mandibularis, statural growth, and sex ratio. It is of interest that molecular studies have shown that the gene loci for human amelogenin, the major protein component of the organic matrix in enamel are on both the X and Y chromosomes. Future questions include the role of the Y chromosome in the mineralization process, the concentric control of enamel and dentin growth, and gene expression.