When the issues of risk for problem behaviors arise, it seems reasonable to turn to genetics to aid in the search for important risk factors. In both the popular and scientific lexicon , genes are implicated in the language we use to discuss the origins of behavioral disorders. We speak of the genesis of problems; we believe that diatheses interact with stressors and view genetic factors as key elements of the diatheses; we speak of genetic liability predisposing some individuals to develop a disorder. We prefer the term ontogenesis to imply a dynamic view of development and the term epigenesis to escalate the needed complexity of that term to allow for simultaneous and/or alternating reciprocal changes in both effective genotypes and behavioral phenotypes . Genes have only a probabilistic effect on the development of behavior, both normal and disordered . However, the degree of probability ranges widely according to the genotype and phenotype under analysis. The unfortunate individual who inherits a particular pair of recessive genes on chromosome pair 12 will surely exhibit the symptoms of phenylketonuria early in life. Only one copy of a stretch of a particular, soon-tobe-specified dominant mutant gene on chromosome 4 will also surely lead to Huntington's Disease although the affected person will live for 40 or 50 years before frank signs of the illness appear. Thus, the probability of a particular gene leading to a disorder can be 100%.