13. e. pleiotropy. Pleiotropy is the genetic phenomenon in which a gene at a single locus affects more than one trait (has several effects). Crohns disease, phenylketonuria (PKU), and systemic lupus erythematous (SLE) are pleitropic conditions.
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Pleitropy should not be confused with polygenic inheritance: In pleiotropy a gene at a single locus is responsible for various phenotypic experessions. But in polygenic inheritance several genes at different loci contribute to the phenotype. Examples of polygenic expressions in humans are body height, skin color and blood pressure. Diseases like essential hypertension and pyloric stenosis are also polygenic conditions.