Attention deficit hyperactivity disorder (ADHD) is the most frequently diagnosed 
behavioral disorder in children with a high frequency of co-morbid conditions 
like conduct disorder (CD) and oppositional defiant disorder (ODD). These traits 
are controlled by neurotransmitters like dopamine, serotonin and norepinephrine. 
Monoamine oxidase A (MAOA), a mitochondrial enzyme involved in the degradation 
of amines, has been reported to be associated with aggression, impulsivity, 
depression, and mood changes. We hypothesized that MAOA can have a potential 
role in ADHD associated CD/ODD and analyzed 24 markers in a group of 
Indo-Caucasoid subjects. ADHD probands and controls (N = 150 each) matched for 
ethnicity and gender were recruited following the Diagnostic and Statistical 
Manual for Mental Disorders-IV. Appropriate scales were used for measuring CD 
and ODD traits. Markers were genotyped by PCR-based methods and data obtained 
analyzed using the Cocaphase program under UNPHASED. Only eight markers were 
found to be polymorphic. rs6323 "G" allele showed higher frequencies in ADHD (P 
= 0.0023), ADHD + CD (P = 0.03) and ADHD + ODD (P = 0.01) as compared to 
controls. Haplotype analysis revealed statistically significant difference for 
three haplotypes in ADHD cases (P < 0.02). Statistically significant differences 
were also noticed for haplotypes in ADHD + CD and ADHD + ODD cases (P < 0.01). 
LD analysis showed significant variation in different groups. 
Multidimensionality reduction analysis showed independent as well as interactive 
effects of markers. Genotypes showed correlation with behavioral problems in 
ADHD and ADHD + CD. We interpret that MAOA gene variants may contribute to the 
etiology of ADHD as well as associated co-morbid CD and ODD in this ethnic 
group.