Experts do know that ADHD has a strong genetic component. They think, aren't sure, that the genes that control the levels of certain chemicals in the brain called neurotransmitters seem to be different in those with ADHD. ADHD tends to run in families, studies have shown certain genetic characteristics that occur with high frequency in families where one or more family member has ADHD, if one or both parents have ADHD, their children are more likely to develop the condition, and at least one-third of all fathers who had ADHD in their youth as your fiance did, do have children with ADHD.
In some cases, though, there is no genetic link to ADHD, there are other factors to consider, if the mother smoked or drank during pregnancy, had a difficult pregnancy, the child sustains a head injury...there are so many factors but they do not ensure a child will develop ADHD by any means.
http://www.webmd.com/add-adhd/guide/adhd-causes
And IF your daughter does get diagnosed with it, it's not the end of the world by any means, just a challenge to be met, relax : )