Sickle1 - oxygen stress. Achondroplasia Achondroplasia, a...

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Sickle-Cell Disease The most common inherited disease among people of African descent is  sickle-cell disease,  which affects one of  400 African- Americans. o Sickle-cell disease is caused by the substitution of a single amino acid in hemoglobin. o When oxygen levels in the blood of an affected individual are low, sickle-cell hemoglobin aggregates into long  rods that deform red blood cells into a sickle shape. o This sickling creates a cascade of symptoms, demonstrating the pleiotropic effects of this allele, as sickled cells  clump and clog capillaries throughout the body. Doctors can use regular blood transfusions to prevent brain damage and new drugs to prevent or treat other  problems. At the organismal level, the nonsickle allele is incompletely dominant to the sickle-cell allele. o Carriers are said to have  sickle-cell trait. o These individuals are usually healthy, although some may suffer symptoms of sickle-cell disease under blood-
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Unformatted text preview: oxygen stress. Achondroplasia Achondroplasia, a form of dwarfism, has a prevalence of one case in 25,000 people. o Heterozygous individuals have the dwarf phenotype. o The 99.99% of the population who are not achondroplastic dwarfs are homozygous recessive for this trait. o Achondroplasia is another example of a trait for which the recessive allele is far more prevalent than the dominant allele. Lethal dominant alleles are much less common than lethal recessive alleles. o If a lethal dominant allele kills an offspring before he or she can mature and reproduce, the allele will not be passed on to future generations. o In contrast, a lethal recessive allele can be passed on by heterozygous carriers who have normal phenotypes....
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