A maternal gene that encodes for products (mRNA or protein) whose spatial distribution in the egg determines the orientation (polarity) of the egg. Its expression takes place even before fertilization.
Egg polarity genes are required for the normal development of the embryo. They produce products that establish the polarity well before fertilization. It was well studied and characterized in Drosophila embryogenesis.
Examples of these genes are bicoid, nanos and caudal.