Many diseases occur as a result of mutations in certain genes. However, inheriting a gene with a mutation from only one parent doesn’t mean that you are at risk for the disease. Everyone has two copies of genes one copy from their mother and one from their father. Sometimes it only takes one bad copy of a gene to cause disease to their son while other times it takes both cells to infect him. In fact, there are many different ways to inherit diseases and other traits.
One example of this is hair color. If a person has one version of a hair gene that results in blond hair and one that results in brunette hair, the person will have dark hair. The same can happen with diseases.
If the parents know they have any hereditary diseases they should go to check their kids to know if he got the same disease as they do, if he doesn’t have a disease it doesn’t mean he will not develop it in the future so he must be very carefull later in his life.
Gianluca Grazioso 