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Dr. Kacir's ADHD Blog

ADHD Genetics

Is ADHD genetic or developmental?

One of my patients asked the question in the title of this post.   He also asked if the genetic cause was a mutation.  After I answered him, I thought that my readers might be interested in my reply, so I have cut and pasted it below. 
 
ADHD is clearly inherited genetically since it is passed down from one generation to the next as reliably as height is.  It is a general rule that boys grow to be about 4 inches taller than the average of their parents' heights and girls to be 4 inches shorter.  In the case of height, there are known exceptions, some because of genetic issues (like dwarfism) and others because of developmental or environmental events like accidents or malnutrition.
 
Several genes have been associated with ADHD.  They are not mutations any more than brown eyes are mutations of green eyes.  The one gene that I remember is called DR4.  It is one of at least 6 different sequences that code for a Dopamine Receptor.  More people with ADHD have this particular gene than do people without ADHD.  However, not everyone who has this gene also has ADHD and not everyone with ADHD has this gene.
 
This is probably where the developmental part comes in.  It has been shown that young children have more symptoms of ADHD immediately after watching Spongebob Squarepants than they do after watching Sesame Street.  It is possible that young children who are exposed to a lot of electronic media might be more likely to have enough symptoms to be diagnosed with ADHD than children who spend a lot of time reading or playing with blocks.
 
There is also the fact that while only 4.4% of adults can be diagnosed with ADHD, about 50% of the population in prison can be so diagnosed.  This means more people who made the choice to do illegal activities (and got caught) exhibit ADHD than those who chose to obey the law.  Most of these choices are made during adolescence and can be considered "late developments."  It is clear that ADHD is associated with conduct disorder (a developmental issue that leads to problems with law enforcement) but conduct disorder can also occur without ADHD, so there may be reciprocal effects.  This would be another example of development affecting the expression of a genetic tendency to ADHD.
 
To summarize, genes are responsible for the possibility of having ADHD symptoms.  Environment and development probably determine whether an individual will have enough of those symptoms to match the full requirements of the diagnosis. There are
many genes involved and it is likely that certain variations of these genes are more common in ADHD.  Therefore if more of these genes are the ADHD type in a given person, then that person will have more symptoms of ADHD.

More genetics and ADHD

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?
 

New evidence of ADHD genetics

I just read a report on an article in The Lancet, a British medical journal.  It documents a study comparing the genes of about 400 children with ADHD to those of more than a thousand kids without symptoms of ADHD.  They found that genes from those with ADHD were twice as likely to have extra copies or fewer copies of certain genes associated with brain function.  This is the first direct genetic evidence for ADHD and defines it as a neurodevelopmental condition and NOT a behavior disorder!
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