Huntington disease is an example of a dominant gene disorder. This means that if you have the gene for Huntington disease, you will get the disease. A genetic test can show whether you have an abnormal copy of the Huntington gene and will get the disease. Most genetic diseases we know about today are recessive disorders. In a recessive condition, there must be two abnormal copies of a gene for it to be expressed.
Cystic fibrosis is an example of a recessive disorder.
A person who has cystic fibrosis has two abnormal copies of the cystic fibrosis gene. That means that one copy of the cystic fibrosis gene was inherited from the person's mother, and one copy was inherited from the person's father. A person who has only one abnormal copy of a gene for a recessive condition is known as a carrier. A carrier of a genetic condition won't get the disease, but could pass the gene for that disease along to children. If both parents are carriers for a specific genetic condition, like cystic fibrosis, their children will have a 1 in 4 chance of getting that condition, and a 1 in 2 chance of carrying one abnormal gene copy.