Variation in the manner in which a trait is manifested. When there is variable expressivity, the trait may vary in clinical expression from mild to severe. For example, the condition neurofibromatosis type 1 may be mild, presenting with café-au-lait spots only, or may be severe, presenting with neurofibromas and brain tumors. OR Refers to the difference in the way that signs and symptoms of a genetic condition can show up in individual patients who have that condition. For example, some patients with neurofibromatosis type 1 may have only mild symptoms, such as brown spots and freckling on the skin, and others may have more severe symptoms, such as brain tumors and tumors on the nerves.