Homeobox genes are a group of genes fundamental to body development in living things. Genes are the basic unit of heredity. They carry the chemical instructions that determine the form and function of each cell in an organism. Genes are encoded in the structure of the genetic material DNA (deoxyribonucleic acid). Homeobox genes control body development by regulating the activity of other genes in various parts of the body. The genes are found in eukaryotic organisms (living things whose cells have a nucleus). Scientists have identified hundreds of homeobox genes in organisms ranging from fungi to plants and animals.
Two independent teams of scientists—one in Switzerland and the other in the United States—first identified homeobox genes in 1983. The scientists were studying homeotic mutations in fruit flies. Homeotic mutations are changes in genetic material that cause one body part to take on the identity of another. For example, fruit flies with a homeotic mutation may have an extra set of wings or legs in the place of antennae on the head. The scientists realized that the mutated genes were important in establishing the body plan of a fruit fly as the creature develops from an embryo. Scientists called these genes homeobox genes for the homeotic mutations that result if they are altered.
Homeobox genes play an important role in differentiation. Differentiation is the process by which cells develop into different types, including skin cells and muscle cells. The type of cell that develops during differentiation depends on which genes are active in the developing cell. Homeobox genes carry the genetic instructions to produce homeodomain proteins. These proteins bind to segments of DNA, helping to control the action of other genes. By switching genes on or off, homeodomain proteins help determine what type of cell will develop.
One well-known group of homeobox genes are called Hox genes. Similar Hox genes are found in all animals where they control the pattern of the body. For example, Hox genes are responsible for developing a fly’s antennae on a segment in front of the segment that has wings. Likewise, they determine the order of vertebrae (bones that make up the spine) in animals with a back bone. Hox genes thus largely control the development of the overall body plan.
Homeobox genes are so fundamental to development that they are remarkably similar in different organisms. Scientists say such genes that are similar across diverse species are conserved. For this reason, homeobox genes are of particular interest to scientists in a branch of study called evolutionary developmental biology, often abbreviated evo-devo. These scientists study homeobox genes to learn how changes in developmental processes have led to the wide variety of living things that exist today.
