Great question! I would wager that both occur. Each disease will have slightly different genetics, but Huntington's disease provides a good example. There, we

Great question! I would wager that both occur. Each disease will have slightly different genetics, but Huntington's disease provides a good example. There, we know that some individuals have a predisposition to developing the disease - this bias results from having over a certain threshold of repeating DNA bases in the Huntingtin gene. Here's an accessible discussion of Huntington's and similar diseases: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3175376/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3175376/ .