IntroductionMalocclusion is the manifestation of complex genetic and environmental interactions on the development of the oral-facial region.Historically, orthodontists have been interested in genetics as a means to better understand why a patient has a particular occlusion, and to determine the best course of treatment for the malocclusion.The application of genetic information in treatment, however, has been hampered by several factors including: 1) the presumption that heritability studies have some clinical relevance to the individual patient, which they do not (Harris, 2008); 2) the presumption that whatever genetic factors may have contributed to the occlusion will also affect how the patient responds to treatment, which they may not; and 3) a lack of understanding to the extent at which genetic factors may interact with environmental factors (such as those created during orthodontic and dentofacial orthopedic treatments) to influence single gene (Mendelian) traits versus "Complex" traits which are more frequently observed in the clinic.(Hartsfield, 2011) While it is essential to consider genetic factors when diagnosing the underlying cause for virtually all oral-facial anomalies and developmental variations, the importance of how genetic factors will affect the outcome of treatment is often not appreciated.Understanding the etiology of a malocclusion is important, e.g., if the patient is a thumb sucker, then that habit must stop.But in terms of etiology, the factors that influenced a malocclusion to develop may not be the same ones that will influence how the patient responds to treatment of that malocclusion.In addition, the patient's developmental stage during treatment is typically a later stage then when the basis of the malocclusion first formed.Although an environmental modification may alter the development of the phenotype at a particular moment, gross structural morphology, already present, may not change readily unless the environmental modification is sufficient to alter preexisting structure.(Buschang & Hinton, 2005) As every orthodontist knows, the ability of the practitioner to affect a change is dependent both on the time of intervention (treatment) and the patient's stage of development.Knowing whether the cause of the problem is genetic has been cited as a factor in eventual outcome; that is, if the problem is genetic, then orthodontists may be limited in what they can do (or change), because of an intrinsic "predestination."This is a misapplication of genetics to clinical practice since most malocclusions we treat appear to not be the result of a single dominant (Mendelian) gene.(Mossey, 1999b) There are inappropriate uses of www.intechopen.com