Question: explain how small changes in developmental genes can lead to major changes in body plan

title: Evolutionary developmental biology
text: A small fraction of the genes in an organism 's genome control the organism 's development . These genes are called the developmental - genetic toolkit . They are highly conserved among phyla , meaning that they are ancient and very similar in widely separated groups of animals . Differences in deployment of toolkit genes affect the body plan and the number , identity , and pattern of body parts . Most toolkit genes are parts of signalling pathways : they encode transcription factors , cell adhesion proteins , cell surface receptor proteins and signalling ligands that bind to them , and secreted morphogens that diffuse through the embryo . All of these help to define the fate of undifferentiated cells in the embryo . Together , they generate the patterns in time and space which shape the embryo , and ultimately form the body plan of the organism . Among the most important toolkit genes are the Hox genes . These transcription factors contain the homeobox protein - binding DNA motif , also found in other toolkit genes , and create the basic pattern of the body along its front - to - back axis . Hox genes determine where repeating parts , such as the many vertebrae of snakes , will grow in a developing embryo or larva . Pax - 6 , already mentioned , is a classic toolkit gene . Homeobox genes are also found in plants , implying they are common to all eukaryotes . 

Answer: answer: A small fraction of the genes in an organism's genome control the organism's development.  These genes are called the developmental - genetic toolkit.  Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts.  Together, they generate the patterns in time and space which shape the embryo, and ultimately form the body plan of the organism.