Genes expressed in maternal follicle cells whose products (messenger RNAs and proteins) diffuse into the egg cell to influence its early development. Gradients of the products are established in the egg cytoplasm; following fertilization and subsequent cell division of the zygote, these gradients influence zygotic gene expression and cause regional differentiation of the embryo by sequential expression of segmentation genes. For example, in many types of embryo, maternal effect genes are responsible for determining polarity, i.e. which end is the ‘head’ and which is the ‘tail’. See also pattern formation.