Of those children with detectable copy number variations, about 7.7 percent were inherited from the parents, while about 1.7 percent were "de novo," or new, variants, meaning the parents didn't have the abnormalities.
To look for overlap between ADHD and autism spectrum disorders (ASD), researchers also looked for copy number variants on 349 people with autism.
About 25 percent of the kids with ADHD had copy number variants that also showed up in the autism sample, although for about three-fourths, the copy number variants were "unique to ADHD," Schachar said.
Analyzed the other way around, researchers also found that a small percentage of ASD kids had copy number variants that showed up in the ADHD kids.
Overall, in four autism patients and two ADHD patients, the researchers discovered a cluster of rare copy number variations on chromosome 9 and around several genes necessary for the nervous system and brain development in mammals.
The same region on chromosome 9 is also thought to include copy number variations associated with bipolar disorder, intellectual disability and schizophrenia.
Dr. Andrew Adesman, chief of developmental and behavioral pediatrics at the Steven and Alexandra Cohen Children's Medical Center of New York, said parents shouldn't read the study to mean that kids with autism and ADHD necessarily have anything in common genetically.
"The study is reporting, using very sophisticated molecular genetic techniques, that a small percentage of children with ADHD have an identifiable, rare copy number variant, or a deletion or duplication of part of a gene," Adesman said.
Yet the study was only able to identify a copy number variation in one in 10 kids, which means for the other 90 percent, nothing abnormal showed up on the genetic test.
And then from that group, it was only a minority that shared a variant with the ASD kids and vice versa. "The vast majority of kids with both
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