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Each year a number of children are born with biological defects that impair normal function For THREE of the following conditions discuss such aspects as the biological cause the methods of treatment and possible means of detection andor prevention One lethal disorder inherited as a recessive allele is Tay-Sachs disease This is caused by a dysfunctional enzyme that fails to break down brain lipids of a certain class The symptoms usually become manifest a few months after birth Some symptoms are seizures blindness and degeneration of motor and mental performance Death is the result of this disease in children With Tay-Sachs disease the brain cells of a baby are unable to metabolize gangliosides a type of lipid because a crucial enzyme does not work properly As the lipids accumulate in the brain the brain cells gradually cease to function normally Only children who inherit two copies of the Tay-Sachs allele qualifies as a recessive At the biochemical level we observe an intermediate phenotype characteristic of incomplete dominance The enzyme deficiency that causes Tay Sachs disease can be detected in heterozygotes who have an activity level of the lipid-metabolizing enzyme that is intermediate between individuals homozygous for the normal allele and individuals with Tay-Sachs disease Heterozygotes lack symptoms of the disease apparently because half the normal amount of functional enzyme is sufficient to prevent lipid accumulation in the brain In fact heterozygous individuals produce equal numbers of normal and dysfunctional enzyme molecules At the molecular level the normal allele and the Tay-Sachs allele are codominant Sickle-cell disease is caused by the substitution of a single amino acid in the hemoglobin protein of red blood cells When the oxygen content of an affected individuals blood is low the sickle-cell hemoglobin deforms the red cells to a sickle shape Sickling of the cells in turn can lead to other symptoms The multiple effects of a double dose of the sickle-cell allele exemplify pleiotropy which is the ability of
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