Heredity, genetics and orthodontics: How much has this research really helped?

Uncovering the genetic factors that correlate with a clinical deviation of previously unknown etiology helps to diminish the unknown variation influencing the phenotype. Clinical studies, particularly those that consider the effects of an appliance or treatment regimen on growth, need to be a part of these types of genetic investigations in the future. While the day-to-day utilization of “testing” for genetic factors is not ready for practice yet, genetic testing for monogenic traits such as Primary Failure of Eruption (PFE) and Class III malocclusion is showing more promise as knowledge and technology advances. Although the heterogeneous complexity of such things as facial and dental development, the physiology of tooth movement, and the occurrence of External Apical Root Resorption (EARR) make their precise prediction untenable, investigations into the genetic factors that influence different phenotypes, and how these factors may relate to or impact environmental factors (including orthodontic treatment) are becoming better understood. The most important “genetic test” the practitioner can do today is to gather the patient’s individual and family history. This would greatly benefit the patient, and augment the usefulness of these families in future clinical research in which clinical findings, environmental, and genetic factors can be studied.

Introduction

Many orthodontic clinicians think of genetics in terms of controlling and therefore predicting facial growth, and/or in terms of a patients’ genetics defining the limits of what can be changed with treatment. Knowing whether “genetics” can cause a phenotype has been cited as a factor in a patients’ eventual outcome; that is, if the phenotype is genetically programmed, then orthodontists may be limited in what they can do to change it.¹ However, this concept has often been misapplied. Inappropriate use of heritability estimates in the orthodontic literature have occurred when authors misinterpret these estimates to be a proxy for determining whether a phenotype is of “genetic origin;” particularly when a malocclusion or other anatomic morphology (e.g., arch width) exhibits a complex inheritance pattern, or etiology, as most phenotypes do.²

The most practical and significant way that genetics will help in clinical practice is through the identification of specific genetic factors and factor variations that can influence the craniofacial traits that are identified within an individual; not by using heritability estimates. However, since there is practically no aspect of orthodontic practice that can be precisely predicted or explained by only one mutation in one gene, the expectation that genetics will be a crystal ball that tells all is unfounded. To paraphrase the writer HL Mencken, “For every complex problem there is an answer that is clear, simple, and wrong.”³ Hence, it most likely will be a combination of genetic factor effects and variations that will be central to enhancing our understanding of the genetic influence(s) that act in on numerous complex oral–facial phenotypes and responses to treatment. An overview of several studies pertinent to the orthodontist are presented below which have examined different genetic factors in the attempt to explain a portion of the individual variation observed in facial growth and response to treatment.