ADHD, or attention deficit hyperactivity disorder, is one of the most common childhood psychiatric diagnoses, characterized by hyperactivity, inattention, and impulsivity. While the causes of ADHD are still not fully understood, it is highly likely that many different types of problems can result in the behavioural characteristics that we describe as "ADHD". One of these is very likely to be genetic (i.e. passed in the genes from the parents to the children).
Evidence for a genetic basis for ADHD is suggested by comparing the rates of ADHD between idential twins to the rates of ADHD between non-identical twins. When one identical twin has ADHD, there is around a 90% chance that the other twin will have ADHD as well. For non-identical twins, when one has ADHD, the other has only a 30% chance of having ADHD. Since identical twins have identical genes, this is strongly suggestive (although not scientific proof) that there is a genetic basis for ADHD.
There are many genes which are being investigated currently as potential causes of ADHD, but none of these genes has been yet definitively established to be a cause. Furthermore, there is evidence that having "ADHD genes" is probably not enough to cause someone to have ADHD - rather, it is probably a combination of genes and the developing environment that leads to the final diagnosis. Because the causes of ADHD are still not clearly known, there is no test currently available to tell if you have an "ADHD gene," although we can say that genes are likely to play a role in the development of ADHD. Therefore, parents with ADHD are probably more likely to have a child with ADHD.