When a single gene controls two or more characters (traits) it is called pleiotropy or pleiotropism. The gene is called a pleiotropic gene.

As per Mendel's principle, one gene controls one character but this is a different way of expression. For example, Marriage between two carriers of Sickle cell will produce normal, carriers and sickle-cell anaemic children in a 1: 2: 1 ratio. Sickle cell anemic dies and we have only carriers and normals in the ratio of 2: 1.

Thus, pleiotropy shows 2:1 ratio.