In fruitfly embryo, the bicoid gene determines which end is the front end of the embryo. A defective bicoid gene results in the lack of anterior segments, such as mouthparts, head, and the first two thoracic segments. It also results in a larva developing two posterior ends (hence the name bicoid).
The Bicoid (as a gene product) is synthesized not by the zygote but by its mother. The zygote inherits the Bicoid directly from its mother that started synthesizing the product even before fertilization.
See also: maternal-effect gene.