Mutations can happen because of damage by environmental factors, such as radiation and chemicals. Viruses may also cause DNA damage. Mutation can also occur during DNA replication due to errors. When DNA is changed the protein it codes may become non-functional or it may change its shape. Some genes control other genes. For example, the Hox genes are responsible for controlling other genes during development. They specify where the head, body and legs may be located, and also where where your nose, ears and eyes are located on your head. A mutation in the Hox genes, for example, may cause big changes, for example, a wing that grows out of the eyes of a fruit fly is due to a mutation in the Hox gene. Hox genes do not tell how wings are formed, just where it may be formed. If a gene that controls the development of the wing or legs or nose has a mutation then it may change the shape of the wing, leg, or nose.
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There are also some genes that do not govern the development of body parts, but they nevertheless have influence on traits. For example, the genes that govern the manufacture of eumelanin, if they malfunction, may result in such traits as light skin, blue eyes, and/or blond hair, because eumelanin is the pigment that gives hair, skin and eyes their dark brown to black color. An albino African, for example, will have light skin and even blue eyes and blond hair, because his/her genes for manufacturing eumelanin is defective. Some other genes control other traits but their effects are not as visible. For example, a mutation in some people's genes make them resistant to the HIV virus and the Bubonic plague, but such a trait is not visible from the outside.</span>
