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Genetic Disorders

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 and/or 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 a gene to affect an organism in many ways. Regular blood transfusions could be used to ward off brain damage in children with sickle-cell disease. Heterozygotes with the sickle-cell allele may suffer some symptoms of the disease when there is a reduction of blood oxygen. Since the two alleles are codominant at the molecular level; both normal and abnormal hemoglobins are made. Only individuals who are homozygous for the sickle-cell allele suffer from the disease.Down syndrome is an aneuploid condition, meaning that if either of the aberrant gametes unites with a normal one at fertilization, the offspring will have an abnormal chromosome number. Down syndrome is usually the result of an extra chromosome, so that each body cell has a total of 47 chromosomes. In chromosomal terms, the cells are trisomic for chromosome 21. Its trisomy severely alters the individuals phenotype. Down syndrome includes characteristic facial features, short stature, heart defects, susceptibility to respiratory infection, and mental retardation. Most cases of Down syndrome result from nondisjunction during gamete production in one of the parents. Nondisjunction, being something in which the members of a pair of homologous chromosomes do not move apart properly during meiosis I, or in which sister chromatids fail to separate during meiosis II. In these cases, one gamete receives two of the same type of chromosome and another gamete receives no copy. There is a correlation of Down syndrome with maternal age, the hypothesis that older women are more likely than younger women to carry Down babies to term rather than spontaneously aborting the trisomic embryos.One way to detect this is to use an amniocentesis, which can determine whether a developing fetus has these diseases. Tests for disorders are performed on cells grown in the laboratory from the fetal cells that had been sloughed off into the amniotic fluid. In an alternative technique called chorionic villus sampling, the physician suctions off a small amount of fetal tissue from the placenta. These results can be found quicker, within 24 hours.

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