Like a lot of disorders, ADHD is highly heritable within families with an estimated 3-5 times greater risk in first-degree relations . So, what this means is if you are a parent with ADHD you will have a better than 50% chance of having a child with ADHD. Similarly, about 25% of children with ADHD have parents who meet the formal diagnostic criteria for ADHD . There is a considerably higher heritability of ADHD among twins with a mean heritability estimate of between 76%  and 80% . This makes ADHD among the most heritable of psychiatric disorders . It is often the case that one of the parents of a child with ADHD are unaware they may also have the disorder until their child is diagnosed. That is not to say that every parent of a child with ADHD has ADHD, only that it is more likely.