Responses to the Concentration Gradient
Coordinate genes such as bicoid lay down the grand plan, so to speak, upon which the genes downstream will act. The pattern of the developing embryo arises as these downstream genes are activated or repressed.
Like many of the other coordinate genes, bicoid encodes a transcription factor; thus, there is a concentration gradient of a transcription factor. The next genes in this developmental cascade, the "gap genes," possess binding sites for this transcription factor. Gap genes are so named because mutations in these genes can produce larvae with "gaps" (missing several segments). These genes differ in how many bicoid binding sites they have and, thus, vary in their sensitivity to this transcription factor. Some gap genes will become active at low concentrations of bicoid, while the activation of others will require higher concentrations. Due to the concentration gradient, different regions of the developing embryo will activate different gap genes.
Unlike the coordinate genes, the gap genes are not maternal effect genes. The activities of the embryo's gap genes (and not those of the mother's genes) determine the phenotype. Gap genes also encode for transcription factors, and these affect the transcription of genes that further refine the patterning of the Drosophila embryo (Fig. 4).
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