maternal-effect gene
A maternal-effect gene is one whose product the mother deposits into her egg before fertilization, so that the offspring's earliest development depends on the mother's genotype rather than its own. It is like a parent packing a lunchbox: what the child eats at first was prepared by the parent, regardless of what the child could have made.
In these cases the very early embryo runs on proteins and messenger RNAs that were loaded into the egg during its formation. Because the embryo's own genome has not yet switched on, a mutant mother can produce abnormal offspring even if those offspring carry a perfectly good copy of the gene — what matters is the mother's copies, not the embryo's.
This unusual inheritance pattern is a hallmark of genes that set up the egg's initial organization, such as its head-to-tail axis. Studying maternal-effect mutations revealed how a mother pre-loads positional information that the embryo later reads to begin building its body plan.
The fly gene bicoid is maternal-effect: a mother lacking it lays eggs that fail to form a proper head, even though the embryos themselves are not the deciding factor — the mother's egg-loading was.
The mother's genes shape the embryo's earliest steps.
A maternal effect is different from maternal (mitochondrial) inheritance: here a nuclear gene is inherited normally, but its product happens to be supplied by the mother to the egg.