In another post, I described that there were at least 7 well-known genes that are associated with ADHD. In fact, since the publication of this paper, additional ones have been identified. Within the past couple of months, another key study on ADHD genes has been discussed.
ADHD Gene #2: Dopamine D5 receptor gene (DRD5): CA repeat, 148 bp
In a previous posting, we discussed an ADHD gene found on the 11th chromosome in humans, called the DRD4. One of the forms (also called alleles) of this gene was associated with the disorder of ADHD, in particular the inattentive component of ADHD.
A second ADHD gene called the "Dopamine D5 receptor gene", or DRD5, is also thought to have strong familial ties to the disorder. Like the DRD4 gene listed above, DRD5 has multiple forms (or alleles). This is located on chromosome number 4 for humans. The "ADHD allele" which is referred to as "CA repeat, 148 bp" (this notation is commonly used by geneticists and refers to the length and DNA makeup of the "ADHD form" of the gene, the exact details aren't entirely important) is slightly greater in length than the non-ADHD form(s). While different studies on this allele have produced different results, it appears that this form of the gene, like the form of the DRD4 ADHD gene listed above, is tied more towards the inattentive than hyperactive component of ADHD. Statistically, however, there appears to be a weaker association between the DRD5 gene and ADHD than the DRD4 gene.
From what I know about GxE and genetics, the power must be very high to narrow down what genes cause what. There are also lots and lots of interactions that can blur the gene effect. This makes these genetic studies less reliable (esp if it n=>1,000).
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