Recessive Disorders

Two copies of the recessive allele must be present for a person to have the disorder. For this to happen it means both parents, while unaffected by the disorder, were both carriers for the disorder. There is a good illustration of this on the Mayo Clinic Autosomal Recessive Inheritance Pattern page. 

Examples of recessive disorders: cystic fibrosis, sickle cell anemia, Tay Sachs disease 

Dominant Disorders

Dominant autosomal disorders happen, but they do not happen very often. 

Example: Huntington's disease