Maternal effect genes are transcribed, and their mRNA translocated into the egg, during oogenesis. Maternal effect genes acts before any zygotic genes, which are encoded by the embryonic genome itself. The maternal phenotype determines the zygotic phenotype. Thus, a MEG+/- female will have all normal children even if the child's phenotype is MEG-/- (due to mating with a MEG+/- male or MEG-/-). However, a MEG-/- female will be unable to have normal children because embryonic development will be defective. There are forty maternal effect genes in Drosophila, including:
|Region||Description||Maternal Effect Genes|
|Acron||Part of the head.||torso|
|Anterior||Head and thorax.||bicoid (bcd)|
A spacially restricted ligand activates the Torso RTK, which then initiates a phosphorylation cascade -- involving Ras, Raf-1, MAP kinase kinase (MAPKK, aka MEK) and then MAP kinase (MAPK) -- that inactivates transcriptional repressors at the two poles of the embryo. This is critical for terminal formation. Where RTK is most activated, huckebein and tailless transcription is activated; extending to where activation is reduced, tailless transcription (encoding tailless) is still activated.