Developmental genetics studies the mechanisms how genes initiate and control the process by which a single cell can give rise to a mature organism. This includes mechanisms of early patterning, as well as later events that result in the formation and maturation of organ systems. Developmentally active genes exert their effects through many pathways and mechanisms including diffusing morphogens, cell migration, proliferation, and border formation. Transient structures such as the somites, the branchial arches and the apical ecto-dermal ridge serve as scaffold and signaling centers during embryogenesis. Gene defects frequently result in abnormal development with specific phenotypes that reflect the gene's essential functions during embryogenesis. In many instances this results in a combination of malformations that are characteristic for a specific syndrome.