QTL uses maps of generations to trace and locate various genes. Using the genome of the mouse, researchers have made maps of genes that are common for certain traits. QTL discovery is the first step in identifying genes that influence complex traits. The approach is tedious and is limited by the low precision of the DNA region. Behavioral traits pose special and difficult problems.

To date there have been few QTL association studies in ADHD. Several family studies have investigated the effect of comorbid disorders in ADHD. These studies suggest that relatives of probands with ADHD and comorbid conduct disorder are at greater risk for ADHD than relatives of probands with ADHD alone. The proband is the family member though whom a family's medical history comes to light. For example, a proband might be a baby with Down syndrome; the proband may also be called the index case. Given the difficulty of defining the diagnostic phenotype, more objective measures of behavior are essential. However, advances in gene mapping technology may identify new genes and new neurological targets.

Research to pinpoint the abnormal genes is honing in one two genes: a dopamine-receptor (DRD) gene on chromosome 11 and the dopaminetransported gene (DAT1) on chromosome 5. Several studies have found evidence that children with ADHD have genetic variations in one of the dopamine-receptor genes (DRD4). Several other studies have found abnormalities of the dopamine-transporter gene (DAT1) in children with severe forms of ADHD.

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