A U wave as seen on ECG. An electrocardiogram of an 18-year-old man showing U waves, most evident in lead V3. The U wave is a wave on an electrocardiogram that is not always seen. It is typically small, and, by definition, follows the T wave. U waves are thought to represent repolarization of the papillary muscles or Purkinje fibers.
HCP is caused by mutations in the CPOX gene, which codes for the enzyme coproporphyrinogen oxidase. This enzyme is responsible for the sixth step in the heme biosynthetic pathway, converting coproporphyrinogen III to protoporphyrinogen IX.
Richard's answer is actually quite wrong. *Pleiotropy* is when a single gene controls/influences two or more seemingly unrelated phenotypic traits. You were looking for ~polygenetic inheritance~ in which a single phenotypic trait is controlled by multiple genes. Some common examples include height, hair and eye color, and predisposition to multifactorial diseases like diabetes or heart disease.