Examples of traits and disorders in each category are shown below. Traits and disorders are established to be heritable by family studies, twin studies and animal studies. By no means have all human traits been examined for heritability. Traits and disorders which are partially heritable are also influenced by nongenetic or environmental factors. As examples, diet influences height, and cigarette smoking influences lung cancer.
Dominant inheritance means one copy of the abnormal gene is sufficient to cause disease.
Examples of dominant disorders include Huntington's disease, some forms of breast cancer and some forms of Alzheimer's disease. Recessive inheritance means two abnormal copies of the relevant gene must be present in the affected individual. Examples of recessive disorders include cystic fibrosis, sickle cell anemia and many forms of deafness. When an abnormal gene responsible for a recessive disorder is located on the X chromosome, then only males are primarily affected. Complex inheritance often means that several genes combine to influence a single trait or disease and that nongenetic factors also play a role.