A maternal effect, in genetics, is the phenomena where the genotype of a mother is expressed in the phenotype of its offspring, unaltered by paternal genetic influence. This is usually attributed to maternally produced molecules, such as mRNAs, that are deposited in the egg cell. Maternal effect genes often affect early developmental processes such as axis formation.
This term may also be used to describe maternal inheritance, in which some aspect of an offspring's genotype is inherited solely from the mother. This is often attributed to maternal inheritance of mitochondria or plastids, which each contain their own genome. This phenomenon is distinct from the first phenomenon because in maternal inheritance the individual's phenotype reflects its own genotype, not necessarily the genotype of its parent.