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A trait that is determined by several genes.
Variation of a trait that shows up in a population of people, body height for instance.
A gene that is partially controlled by genetics and partially controlled by the environment. For example, body height is genetic but nutrition growing up also influences height.
The degree to which an inherited allele is expressed in the phenotype. An example would be achondroplastic dwarfism; if a person inherits the allele and it presents, they will be a dwarf.
When one gene influences multiple phenotypic traits. Examples would be sickle-cell anemia and phenylketonuria (PKU).
An example of pleiotropy, a person who inherits the mutated HBs gene will contract sickle-cell anemia which damages erythrocytes, the spleen and many other organs of the body.