dominant

dominant A genetic trait that is evident when only one copy of that gene for that trait is present. (As opposed to a recessive trait, which is usually expressed only when two copies of the gene for that trait are present.) Most dominant traits are due to genes located on the autosomes (the nonsex chromosomes). Diseases inherited in an autosomal dominant manner typically affect both males and females and each of their children run a 50 percent chance to receive their disease gene. Autosomal dominant diseases include achondroplasia (dwarfism with short arms and legs), Huntington disease (a form of progressive dementia), and neurofigromatosis (a neurologic disorder with an increased risk of malignant tumors). X-linked dominance is due to genes on the X chromosome. A single copy of the mutant gene on the X chromosome causes the disease in a female. An example is a type of hereditary rickets called hypophosphatemic rickets. See also autosomal dominant trait; X-linked dominant.