I have just read more evidence about the complexity of ADHD genetics. Twin and family studies make it very clear that ADHD is an inherited disorder, but no specific causative genes have been identified. Current thinking is that many genes are responsible for ADHD and that outside factors like toxins and disciplinary styles can change how it appears. As part of the human genome project, scientists looked at over 3000 genome samples including 896 ADHD individuals and could not find a significant ADHD gene. They did find a few sequences which were promising for future research especially for gene-gene interactions.
The authors also theorized that there may be interactions between genes and the environment. Another study looked at one of the genetic sequences which has been associated with some cases of childhood ADHD. 548 families with an ADHD child contributed DNA to a study and answered questions about discipline, ADHD symptoms and attitudes in the home. It was found that if the child had the sequence being studied and received "inconsistent parenting" that child exhibited significantly more inattentive and oppositional symptoms. If the child took responsibility for problems between the parents, he or she was also more inattentive if the studied sequence was in his or her genes.
A lot more information needs to be gathered to make conclusions about this aspect of ADHD, but it is certainly a promising beginning! Thinking about some of the other associations with ADHD symptoms brings up possibilities for future research. For instance, it was shown that high levels of a pesticide in the urine of children were associated with ADHD diagnoses. Wouldn't it be interesting to look at their status with respect to some of the associated gene sequences?