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Human Disease Sickle-cell: For a specific example of the way in which genetic information contained in a particular gene is expressed in an organism, let us consider the case of the human disease sickle-cell anemia. To understand how this disease comes about, one must first look at the function of hemoglobin, which is the protein in red blood cells that picks up oxygen in the lungs and carries it to the body tissues.
Finally, one might ask, why is this mutation not eliminated by natural selection? In Africa, where the disease is common, the mutation has been preserved, most likely because individuals with sickle-cell trait, that is, having only one copy of the mutant gene, have an increased resistance to malaria. We still do not know exactly why. Inasmuch as malaria is very prevalent in those districts where the sickle-cell mutant is commonly found, it appears that the mutant offers a selective advantage to those bearing it.
Muscular dystrophy is another: it is a disorder believed to be an inherited abnormality in the chemistry of the body resulting in a degeneration of the muscle cells. As described by Francis H. C. Crick in the feature article "The Language of Life," sickle-cell anemia is an inherited disease of the blood that affects the chemical composition of the hemoglobin in red blood cells. Hemophilia, another inherited blood disease, is characterized by the inability of the blood to clot properly. The "bleeder's disease," as hemophilia has been popularly called, has run through many of the royal families of Europe, including among its victims the young son of the last czar of Russia.
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