Genetic Disorders

2 Pages 608 Words

Each year a number of children are born with biological defects
that impair normal function. 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 final 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 individual's 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 i...