Genetic disorders can happen both from random mutation that happens just to one person, and inherited mutation passed down from the original mutated ancestor to descendents. An inheritable mutation must happen in an egg or sperm originally to be passed down, such as sicle cell anemia.

Random mutations otherwise, such as bone cancer in example, are not passed down. Random mutations appear the same a lot of the time because our genes can be broken in some ways more easily that others that cause rare diseases.

Our genome is a patchwork thing from several different human and human ancestor lines and it's amazing it all works together as well as it does.