The dorsal-ventral patterning gene snail (shown in red) is one of the few genes that are active before as well as after the midblastula transition. The segmentation gene paired (shown... When egg and sperm combine, the new embryo bustles with activity. Its cells multiply so rapidly they largely ignore their DNA, other than to copy it and to read just a few essential genes. The embryonic cells mainly rely on molecular instructions placed in the egg by its mother in the form of RNA.