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Genetics
- Some Facts
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Chromosomal
disorders affect about 7 of 1000 live-born infants and account
for about half of all spontaneous first-trimester abortions. |
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Some genetic defects affect whole
chromosomes or segments of chromosomes. As in case of Downs
syndrome three copies of chromosome 21 are present and no
individual gene is abnormal. |
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Chromosomal disorders can involve duplication, loss, breakage, or
rearrangement of chromosomal material.
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Artificial chromosomes were created in yeast in the early 1980s,
and the first artificial human chromosomes were created in the
same medium in 1997. Medical applications of the artificial
human chromosomes may include their use as vectors for moving
therapeutic genes into cells to treat such diseases as
sickle-cell anemia, hemophilia, and immune deficiencies.
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Multifactorial
disorders - In these there is no single error in genetic
information but a combination of small variations that
together can produce or predispose an individual to a serious
problem. Such disorders run in families. Environmental
influences, diet and other lifestyle factors predispose the
person to such disorders like - Coronary artery disease and
diabetes mellitus.
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Single
gene disorders - Some genetic disorders result from a mutation in
a single gene. Most single gene disorders have a recessive
inheritance, which means that both copies of the same gene
(one inherited from each parent) must be defective for the
disease to be apparent. In such cases, the parents may not be
affected themselves, but are described as carriers of the
disease. Example - Cystic Fibrosis.
A single gene disorder with a dominant inheritance
requires only one copy of a defective gene for the disease to
develop. An example of this is Huntington’s
disease.
Recessive Single gene disorders affecting the X
chromosome are more likely to affect the males because males
have only one X chromosome and females have two. Example of this
is Color Blindness.
Other examples of single gene disorders are: -
Duchenne Muscular dystrophy
Familial hypercholesterolemia - high cholesterol levels
Hemophilia
Neurofibromatosis type 1
Phenylketonuria
Sickle cell anemia
Tay-Sachs disease
Thalassemia
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A special type of single gene disorder is also seen.
This is the case when the mutation has occurred in the gene of
mitochondria. Mitochondria are small bodies in the cells. During
fertilization of ovum the fertilized embryo has the mitochondria of
the egg and none from the sperm. Thus the genetic
disorders transmitted through the mitochondria thus affect all the
children of an affected woman but none of the children of an affected
man. Example of this is Leber’s hereditary optic neuropathy,
a disorder characterized by atrophy of the optic nerve.
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