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Maternal effect genes encode the substances that initially determine the polarity or axis of the drosophila body. If there is a mutation, then the resulting offspring also get a mutation. The products of these maternal genes are mRNA and proteins that determine the egg's axis in the mother’s body.

An embryo is an early stage forming after the development of the single cell (Zygote) that, after many developmental processes, alters into a complete organism.

It is the part of the life cycle that undergoes many cell divisions to form a blastula. After rearrangements of layers, the blastula emerges as the gastrula. These layers give rise to many organ systems in multicellular organisms like drosophila.

When the egg fertilizes in the female drosophila, it starts to divide to form the embryo. The effect of maternal genes decides the polarity or axis of this egg. These genes encode for some products like proteins in the egg's cytoplasm that decide the polarity in the egg or embryo.

These maternal effect genes are called egg polarity genes. These are divided into two groups. One group decides the anterior-posterior axis, and the second decides the dorsal-ventral axis of the embryo.