The genes with products provided by the female to the egg are called maternal effect genes. The bicoid and nanos are maternal effect genes which are required zygotically.
The transcripts of these maternal effect genes are found in the egg at fertilization and they are the one which form morphogen gradients. These morphogen gradients are substances important for the pattern formation.
The regulation of maternal and zygotic gene activities is required for the formation of pattern and to maintain cellular identities in Drosophila embryo. The maternal effect genes encode transcription factors that regulate the expression of the gap genes.
Many zygotic genes are regulated by maternal effect gene Bicoid protein. The gap genes roughly divide the embryo along the Anterio-Posterior axis. The pair rule genes divide the embryo into pairs of segments. The segment polarity genes set the AP axis of each segment.
Bicoid, the maternal gene is responsible for the morphogenic gradient originating from the anterior pole of the egg. Maternal nanos activity serves as posterior located morphogen.
Maternal effect gene bicoid is required for head differentiation. Embryos developing in the absence of maternal bicoid, have an anterior duplication of posterior pattern. Hence bicoid function is very important for the distinction between head and tail determinative pathways.
The diffusion of Bicoid through cytoplasm establishes a protein concentration gradient. As a result, the anterior end has high concentration of bicoid protein and the concentration decreases gradually towards the posterior end. This helps in providing positional information along AP axis. Depending upon the concentration gradient, the zygotic gap protein hunchback is activated.
Deletions of Bicoid-binding sites in cis-acting regulatory element alter the expression of the reporter. To create sharp boundary of reporter expression, more than one bicoid site must be occupied. Before the gene expression is activated, threshold concentration of Bicoid protein is required to occupy multiple sites.
The expression of several gap genes is altered in embryos isolated from bicoid mutant mothers.