I'll share what my parents did with me and my siblings because it was effective for us.
First, we never had a "come sit down, we are going to talk about this" type conversation. It mostly came up organically. For example, we have some eye problems in my family. So my parents took us to eye doctors as a matter of course. And during those visits, my mom wasn't shy about making sure the doctors knew that those things were possibilities for us. And that usually led to questions later about what those things were and why she was worried about them. Then she would explain to us what those things were and why we should be aware of them. She never impressed upon us that we should be fearful, just aware.
Also, these things also came up as we visited with grandparents and such. Children tend to be inquisitive and I was no different. When I would see my grandparents dealing with something (or my parents would tell me that grandma had to go to the hospital for such-and-such), it prompted questions. My parents were good about providing answers to my questions without inundating me with information and worry. The tone was empathetic, understanding and matter-of-fact. This made those things feel like unfortunate things that should cause us to have love and understanding towards that person. But they also showed us that it was something that could be dealt with, managed and handled.
Second, my parents always showed us that they were supportive of us. That even if one of us had one of these diseases or conditions, that they would do everything in their power to help us and make it as much of a non-issue as possible. And of we have had to deal with some of those things. And they were true to their word. They loved us and helped us thru our problems, no matter when we needed it. Knowing that I had that support network in my family has been fantastic because no matter what I have to deal with, I know they will help me. And because of that I don't have to worry about how to deal with it myself. Or who I could talk to. Or a ton of other things. In short, I can worry less.
Third, my parents always showed us that we could overcome any of those things. Genetic diseases have a tendency to impose limitations on us in terms of what we can do. Allergies can limit what we can eat or the medicines we can take. Other diseases can restrict the physical things we can do. My parents made it crystal clear to us when we had to deal with one of those things what those limitations were and how serious it was to respect those limits. But beyond that, they helped us learn how to get around those limits and live normal lives. Sure, there are certain things I just plain can't or shouldn't do. But everything else is free game. And I don't need to let any of those things stop me.
Finally, teach kids that they are strong. Anyone who has to deal with something like what you described has a choice to make. Either they can sit down and feel sorry for themselves or they can live life. Those diseases may mean that doing certain activities may need extra planning, preparation, effort or even creative solutions. Teach them that life is worth living. Show them all the good things that you love and share your passions with them. Make sure they have their own passions. Instill in them that those diseases don't define them. They are part of their lives, sure. But they don't have to control their lives or change who they are.
Even if they don't end up having one of those diseases, you will have raised a strong, passionate, loving child who can help others and has a strong support network in place for any kind of problem. They won't worry about things because they know that they can handle it, possibly with help from those around them.
TL/DR: Don't avoid the topic. It will come up as you go to doctors and they see grandparents or whoever dealing with things. Explain to them things to be aware of and how serious those things are. Love them and support them so they don't have to worry. Raise them so they know that those things shouldn't stop them from living and that they can handle whatever life throws at them.