<P> The puzzle of how embryonic development was controlled began to be solved using the fruit fly Drosophila melanogaster as a model organism . The step - by - step control of its embryogenesis was visualized by attaching fluorescent dyes of different colours to specific types of protein made by genes expressed in the embryo . A dye such as green fluorescent protein, originally from a jellyfish, was typically attached to an antibody specific to a fruit fly protein, forming a precise indicator of where and when that protein appeared in the living embryo . </P> <P> Using such a technique, in 1994 Walter Gehring found that the pax - 6 gene, vital for forming the eyes of fruit flies, exactly matches an eye - forming gene in mice and humans . The same gene was quickly found in many other groups of animals, such as squid, a cephalopod mollusc . Biologists including Ernst Mayr had believed that eyes had arisen in the animal kingdom at least 40 times, as the anatomy of different types of eye varies widely . For example, the fruit fly's compound eye is made of hundreds of small lensed structures (ommatidia); the human eye has a blind spot where the optic nerve enters the eye, and the nerve fibres run over the surface of the retina, so light has to pass through a layer of nerve fibres before reaching the detector cells in the retina, so the structure is effectively "upside - down"; in contrast, the cephalopod eye has the retina, then a layer of nerve fibres, then the wall of the eye "the right way around". The evidence of pax - 6, however, was that the same genes controlled the development of the eyes of all these animals, suggesting that they all evolved from a common ancestor . Ancient genes had been conserved through millions of years of evolution to create dissimilar structures for similar functions, demonstrating deep homology between structures once thought to be purely analogous . This has caused a radical revision of the meaning of homology in evolutionary biology . </P> <P> 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 . </P> <P> The protein products of the regulatory toolkit are reused not by duplication and modification, but by a complex mosaic of pleiotropy, being applied unchanged in many independent developmental processes, giving pattern to many dissimilar body structures . The loci of these pleiotropic toolkit genes have large, complicated and modular cis - regulatory elements . For example, while a non-pleiotropic rhodopsin gene in the fruit fly has a cis - regulatory element just a few hundred base pairs long, the pleiotropic eyeless cis - regulatory region contains 6 cis - regulatory elements in over 7000 base pairs . The regulatory networks involved are often very large . Each regulatory protein controls "scores to hundreds" of cis - regulatory elements . For instance, 67 fruit fly transcription factors controlled on average 124 target genes each . All this complexity enables genes involved in the development of the embryo to be switched on and off at exactly the right times and in exactly the right places . Some of these genes are structural, directly forming enzymes, tissues and organs of the embryo . But many others are themselves regulatory genes, so what is switched on is often a precisely - timed cascade of switching, involving turning on one developmental process after another in the developing embryo . </P>

Explain how small changes in developmental genes can lead to major changes in body plan
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