Abnormal genetic traits are often caused by new genetic mutations rather than by inheritance from the parents. When one parent has an abnormal trait and the other does not, each child has a 50 percent chance of inheriting the abnormal trait and a 50 percent chance of not inheriting it. A person who doesn't have the abnormal trait, even though his siblings have it, doesn't carry the gene and can't pass the trait on to his offspring. Males and females are equally likely to be affected. The abnormality can, and usually does, appear in every generation.
Mutations are highly unlikely to result in expression of the trait.
If the parent without the trait doesn't have the abnormal recessive gene, none of their children will have the trait, but all of their children will inherit an abnormal gene that they can pass on to their offspring. A person who doesn't have the abnormal trait but whose siblings do have it is likely to carry one abnormal gene. Males and females are equally likely to be affected. The abnormality can appear in every generation but usually doesn't unless both parents have the trait. Dominant genes that cause severe diseases are rare.