✍ Scribed by Martin Hülskamp; Diathard Tautz
No coin nor oath required. For personal study only.
The Genetic Hierarchy Systematic genetic analysis of embryonic lethal mutations has allowed the identification of most of the genes required for the specification of early dcvelopment in Drosoyhild3). The genes can be grouped according to their mutant phenotypes into several classes with apparently similar functions. Three general categories of genes can be recognized: the maternallyexpressed genes, the zygotically-expressed scgmentation genes and the zygotic homoeotic gencs. The maternal genes themselves fall into three classes: The so called anterior, posterior and tcrminal classes, exemplified by hicoid; Y I U I I O S and torso(') (Fig. l), while the segmentation genes can be further subdivided into the gap genes, the pair-rule genes and the segmcnt polarity genes. Mutations of gap genes result in the loss of several adjacent segments, while mutations of pairrule genes effect evcry other segment. Mutations in segment polarity genes lead to losses of structures in each segment, which is often accompanied by inverted duplications of the remaining pattern elements(3). Finally, the group of hoinoeotic genes determines the identity of the segments. These three groups of genes form a hierachy of molecular interactions which lead apparently step-by-step to a successive subdivision of the embryo (Fig. 2). In this sequence the gap genes play a particularly crucial role. They interpret the crude maternal positional information and cause the first zygotic subdivision of the embryo, which serves as a positional reference system for the subsequent expression o f the other groups of genes in their cliaractcristic spatial domains. The direct target genes of the gap genes include both the pair rule enes, and, at least, some of the homoeotic genes .
j l l -l f ) Summary Gap genes stand at the top of the zygotic segmentation hierarchy in Drosophila. Their expression domains are set up by a combination of maternal regulatory signals and interactions among themselves. In addition, these regulatory pathways are partially redundantly specified by the maternal and zygotic genomes, or by multiple zygotic gene products. The gap genes code for transcription factors which exert their function by forming short-range morphogenetic gradients; differential concentrations of these transcription factors can either act as activators or as repressors for the expression domains of neighbouring genes. It is possible to view the gap genes as a system of genes that subdivides the embryo by forming an activation-repression cascade proceeding from anterior towards posterior.
📜 SIMILAR VOLUMES
### Amazon.com Review _Punisher_ is on the run from Billingate Space Station, as well as other predators that follow: UMCP Enforcement Division director Min Donner aboard a crippled _Punisher_ , Nick's archenemy (and slave to the aliens) Sorus Chatelaine aboard _Soar_ , and the mysterious hired gu
Gap junctions are cell junctions found between most cells and tissues. They contain membrane channels that mediate the cell-to-cell diffusion of ions, metabolites, and small cell signaling molecules. Cellcell communication mediated by gap junctions has been proposed to have a variety of functions, i
After Dash gets accepted into Oxford University and Lily stays in New York to take care of her dog-walking business, the devoted couple struggle to make a long-distance relationship work. And when Dash breaks the news that he won’t be coming home for Christmas, Lily makes a decision: if Dash can’t r