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Human Mitochondrial DNA Inheritance
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Every cell contains organelles known as mitochondria. These are responsible for
important cellular activities, of which production of energy for the cell is the
main one. Each cell has on average 100 mitochondria. Most of the cell's genetic
information is concentrated in the cell nucleus, but mitochondria also contain a
small number of genes. The mitochondrial genes
encode proteins that only act in the mitochondrion to provide energy to the cell.
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After fertilization, only the mother contributes mitochondria to the fetus, since
the sperm cell contains only a nucleus and no mitochondria. This means that an abnormality
in the paternal mitochondrial genes will not be transmitted to the next generation
because all the mitochondria in a person (whether male or female) are inherited
from the mother. This is called mitochondrial inheritance.
The clinical expression of an abnormality in a mitochondrial gene depends, among
other factors, on the percentage of mitochondria containing the abnormality. A woman
who has only a few mitochondria with abnormal genes will be healthy, but there is
a risk that she may transmit a large number of these mitochondria to some of her
children, who will therefore have the disease.
Mitochondrial diseases, which arise from abnormalities in the mitochondrial genes,
are always transmitted by the mother and never
by the father. It must be noted, however, that there are diseases that manifest
as disorders of mitochondrial functioning, but where the source of the abnormality
is in the protein encoded by a gene in the cell nucleus. In this type of condition,
the inheritance pattern will be one of the regular ones (autosomal recessive, autosomal dominant, X-linked).
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