Take a. Look at the
in your neighborhood. Look at the kids in your school. Take notice of the around your town, at the mall, in church, and at sporting events. What do you notice? One thing that many Wonder Friends have noticed is that come in all sorts of colors. Why is that? noticed long ago that from parts of the world often had colors. For example, who lived in the usually had darker than who lived in. Over time, scientists who studied the human body learned that variations in appeared to be adaptive traits that were passed through genes from parents to children. These traits corresponded closely with geography and the Sun\’s ultraviolet (UV) radiation. A person\’s is determined by the amount of in his or her. is a dark brown to black produced by special cells called melanocytes. \’s purpose is to protect the from the. Ultraviolet radiation can cause. acts as a natural sunscreen by absorbing these UV rays. The more your has, the darker your will be and the more protection it will have against UV rays. with light have just a little. with darker have more. with yellow tones have more or another type of called. who live in the are exposed to more of the.
As a result, their tends to become darker as the body produces more to counteract the effects of the Sun\’s rays. Over successive generations, genes are passed down from parents to their children, including the tendency to produce a certain amount of given the area where they live. Likewise, tend to have lighter colors, because they don\’t receive as many of the Sun\’s harmful UV rays. As a result, their bodies do not need to produce as much, which makes their lighter. Also, lighter tones in northern areas allow more UV rays to penetrate the to help produce the essential amounts of that the body needs. The body must always strike a careful balance to make sure it receives just enough UV radiation to make essential vitamin D, while avoiding overexposure that can lead to. For example, in some northern areas, such as and Canada, you\’ll notice that native peoples tend to have darker than you might otherwise expect. This is usually because they eat a diet rich in seafood that provides all the vitamin D their bodies need. As a result, their produces more, which makes it darker. Of course, in today\’s modern world of international travel, of all races, ethnicities, and nationalities travel and live all over the globe.
Their individual bodies adapt to conditions where they live over time, and they pass on these traits to their children, which helps explain the tones we see all around us every day. People invest a great deal of time and money into the appearance of their hair. Although we know chemically why hair is colored the way it is, much remains to be learned about the genetics behind hair color. And the question of why humans exhibit the diversity of natural hair colors we see, from blond to black to brown to red, may hold keys to part of our evolutionary history. According to the geneticist Luigi L. Cavalli-Sforza, the variety of hair colors we see among people today may be the result of a force called sexual selection. Sexual selection is a force, like natural selection, that shapes evolutionary trajectories. But unlike natural selection, sexual selection focuses specifically on traits related to procuring mates. According to this theory, diversity in hair color may be the result of more eye-catching hair colors arising by chance, and those rare colors giving their owners an advantage when it came to attracting a mate.
Better success at attracting a mate would have meant better success at producing offspring, who would then carry the genes for new hair colors and pass them along to their own offspring. Hair color is determined by two types of pigment, eumelanins and pheomelanins, which together produce all the natural hair colors seen in humans. (БMelaninБ is the basic term for any pigment, or coloring, in the hair or skin. ) Pheomelanins produces the color red, and eumelanins can produce either black or brown pigments. Eumelanins determine how dark or light the hair will be. A person who produces very little brown eumelanin will have blond hair. Low concentrations of black eumelanin will result in gray hair. Lots of black or brown eumelanin will result in darker hair. Everyone has some pheomelanins (reddish) coloring in their hair as well. A person with true red hair will produce a high concentration of pheomelanins. Phenotypes are the physical expressions of a personБs genotype, or the unique sequence of DNA that determines a personБs makeup. But itБs not always straightforward to map physical traits directly onto the genes that produce them because genes often interact in complex ways.
Genetic complexity is the case with hair color, the underlying basis of which is not clearly understood. Theories for the genetic control of hair color include a multigene locus for control, and a dominant/recessive gene relationship. In a dominant/recessive gene relationship, a child must inherit two copies of the recessive allele for the gene (one from each parent) in order to express that trait (such as hair color) in her phenotype (or appearance). A dominant/recessive model would help explain how two dark-haired parents could produce a blond child, but this model canБt completely account for all the variations in human hair color that are seen today. Simply put, hair grays when the hair follicles stop producing melanin, specifically the eumelanins and pheomelanins discussed above. Each of us is born with a limited number of pigment cells in our follicles. The precise number is genetically determined. As we age, pigment production falls off and then stops, which results in gray hair. Poor diet, smoking and certain illnesses can speed the process of pigment loss and result in premature graying.