A pleiotropic gene exerts an effect on more than one trait. As iain notes, sometimes a single gene will have multiple negative effects--a single genetic mutation is responsible for all the symptoms of Huntington's disease, for example. In other cases, though, a gene will have both positive and negative effects. For example, the apoe-2 allele decreases the risk of Alzheimer's disease but increases the risk of heart disease; the apoe-4 allele has the opposite effect. Along the same lines, people with sickle cell anemia show greater resistance to malaria. Pleiotropy therefore raises problems for genetic engineering and eugenics--you can't easily produce a perfect organism, because optimizing one trait may simultaneously weaken another.