Not all genes are expressed and you can't tell by looking at someone what genes they carry. I have brown hair and brown eyes but it is entirely possible, based on family history, that I carry the genes for blonde hair and blue eyes. If I had children they could potentially look nothing like me. I could mate with a brown haired, brown eyed partner and if he is also a carrier then we could still have blond haired/blue-eyed children.
It's unlikely but I could also be carrying the gene for Cystic Fibrosis or Hemophilia. As a female carrier I won't show symptoms but if I pass it on to a son he would suffer from it. Luckily I don't have a family history of it but some families do. That's why there are now genetic carrier tests so that potential parents will be aware of what diseases they might pass onto their children. Because you can't tell by looking!
As a famous human example, people of Ashkenazi Jewish descent are recommended to be screened for several different diseases:
Ashkenazi Jewish Carrier Testing | Patient Education | UCSF Medical Center