A twin study design was used to examine the the genetic validity of attention deficit hypoeractivity disorder (ADHD)-related phenotypes. Parent-rated symptoms were reported by questionnaires received for 2082 twin pairs, and teacher-rated symptoms were available for 1470 twin pairs. Parent-rated, teacher-rated, and both parent and teacher-rated ADHD categories were highly heritable. Shared environmental effects were found for teacher-rated ADHD. ADHD reported by both parent and teacher is as heritable as ADHD symptoms defined only by maternal reports. A common genetic factor influences maternal and teacher-rated ADHD but not all teacher-rated ADHD. Teacher reports alone may be distinct from parent-rated ADHD, since they are influenced by shared environmental factors and by additional genetic and nonshared environmental factors. 
COMMENT. Broadly defined pervasive (both parent and teacher-rated) ADHD symptoms are as heritable as ADHD behaviors defined by maternal reports alone. A common genetic factor influences maternally rated and teacher rated ADHD. ADHD symptoms are highly heritable.
Dopamine D4 receptor gene and ADHD. Linkage of the dopamine D4 receptor gene and ADHD is supported by further studies at Toronto Western Hospital. 
Is ADHD a noradrenergic disorder? This topic is reviewed by Biederman J and Spencer TJ . Data implicate norepinephrine dysfunction and frontosubcortical pathways that control attention and motor behavior. Drugs with anti-ADHD activity share a common noradrenergic/dopaminergic activity.