We, at least most of us, have 24 chromosomes plus 2 sex chromosomes, xx or xy for most of us, half from our mother and half from our father. The genes are carried on these chromosomes. Science has been searching for which variation of which gene causes which characteristic, and specifically which disease. There has been some success; for example, if you have particular form of the HTT gene, you definitely will develop Huntington’s disease; if you don’t have it , you won’t. (This leads to a lot of discussion about ethical issues, and do you want to know or not?)
But in general, the search for the disease genes has been frustrating, as it turns out that most illnesses (schizophrenia or depression, for example, and probably ADD or ADHD) are caused by small effects from various particular combinations of many genes, which then make one susceptible to developing the disorder.
But it is even more complicated, because here is where the environment comes in, your parents and their form of child rearing, exposure to trauma or to toxins, head injury, stress, etc. It seems that the environment has a large effect on which genes are turned on or turned off, how much and when, and that determines whether the tendency to some disorder actually produces it or not. These factors can function either by turning genes on or off, or by making them function more or less, or functioning at the right time during your development, from embryo to adulthood (and beyond?) This is called epigenetics.
It does not seem accurate to say that parents cause ADD or ADHD, but how they manage the child with it can make a big difference in the long term outcome, through both psychological and genetic effects.
The positive – in any event, strategies can help.