How is localization of Gap gene expression achieved in the Drosophila embryo? (what is the mechanism?) Solution Drosophila has segmented body pattern, along the longitudinal axis. Gap gene is involved in development of the segmented embryos. Specialized transcription factors (mostly zinc fingers) are coded by gap genes which directly control the expression of set of genes (the pair rule genes) involved in segmentation. There is a cascade of transcription factors, which establish the segemented body pattern of drosophila. This starts during early development stage, wherein, the maternal gene products gets localized at the polar ends of the egg. Zygotic segmentation genes has the information for positioning of transcription factors. This regulatory hierarchy is led by gap gene. Along the longitudinal axis, expression of gap genes occurs in preblastoderm. Its discrete expression pattern defines specific and overlapping sets of segment primordia. The transcription factors encoded by gap genes form concentration gradients which are overlapping in nature, and the protein gradients thus formed provide cues to form repeated patterns. Thus the pattern of segments from blastoderm embryo are copied. Therefore, basically it is the maternal transcription factors which regulate the region-specific expression of zygotic transcription factors. The transcription factors encoded by gap genes regulate the expression of pair-rule genes and homeotic genes. Thus, it is the localized gene expression of gap gene which ultimately leads to the formation of the periodic pattern; mechanism known as the pair-rule gene expression. Studies of gap gene expression has demonstrated that from drosophila blastoderm, organisms with similar phenotype can be developed, despite of varitions occurring in the genotype, the phenomenon known as canalization..