From $m_d=m_e$ to Realistic Mass Relations in Quark-Lepton Symmetric Models

In recent years a new potential symmetry of fundamental particle physics has been investigated — discrete quark-lepton symmetry. When this symmetry is implemented, however, it often leads to either of the unrealistic predictions $m_u=m_e$ or $m_d=m_e$. This paper considers two possible ways models based on $m_d=m_e$ can be made realistic.