How do gene defects cause different forms of muscular dystrophy?
There are probably about 3,000 muscle proteins, and the recipe for each is contained in a separate gene. Some muscle proteins are part of the structure of muscle fibers, while others influence chemical reactions in the fibers.
The precise defect in a muscle protein gene can influence the nature and severity of a muscle disease. For example, some defects in the gene for dystrophin cause the severe, progressive muscle wasting known as Duchenne muscular dystrophy, while other defects in the
same gene cause the less severe dystrophy, Becker. Many different defects in a particular gene can occur, and, of course, defects can occur in many different genes.
How are muscular dystrophies inherited?
All genes are inherited, half coming from a child's
mother, the other half from the father. Flaws in genes
can be inherited along with the parents' genes, although
they can also occur for the first time in a child.
Scientists use the term spontaneous mutation when this
Doctors use the term familial for a disease that
occurs with a family history and sporadic when there's no
family history. Sporadic diseases can nevertheless stem
from gene flaws.
Genes are found in virtually every cell in the body,
including muscle cells, on structures called chromosomes.
Each person has 46 chromosomes. Of these, 44 are
actually 22 matched pairs of chromosomes, or autosomes,
which are made up of identical sets of genes. One
chromosome out of each matched pair is inherited from
the mother, the other from the father.
The remaining two chromosomes, the X and Y
chromosomes, determine whether the individual is male or
female. A male has an X chromosome, which he inherits
from his mother, and a Y chromosome, which he inherits
from his father. A female has two X chromosomes, one
from each parent.
The different forms of muscular dystrophy can be
passed from parent to child by one of three inheritance
patterns -- autosomal dominant, autosomal recessive and
How does the autosomal dominant pattern of
A disease governed by the
autosomal dominant pattern
is developed when only one
parent passes on one
defective gene. Offspring
have a 50 percent chance
of inheriting an autosomal
dominant disease. These
diseases can also occur because of spontaneous mutations.
Once a spontaneous mutation occurs, it can then be
transmitted to future generations.
How is autosomal recessive inheritance different?
Both parents must pass on
the same or similar gene
defects for children to
inherit a recessive disease.
If one gene in a pair is
normal and the other
defective, the person is a
carrier. Carriers of genetic
defects have minimal, if
any, disease symptoms.Children of parents who each
carry a gene for the same recessive disease have a 25
percent chance of inheriting two flawed genes and
therefore developing the disease.
These diseases are unlikely to be due to spontaneous
mutations, since two mutations have to occur to produce
How does inheritance work in X-linked recessive
X-linked diseases are those
that come from gene
defects on the X
disorders affect males
almost exclusively, because
males have only one X
chromosome. This means
they have no "backup" copy
of a normal gene on a
second X chromosome.
Females have two X chromosomes, so a gene defect on
one of them usually doesn't pose much of a problem. A
normal gene on the female's other X chromosome serves
as a "backup" copy.
A woman with an X-linked gene defect usually doesn't
show any signs of an X-linked disease. However, she's a
carrier of the disease and can pass it to her sons. Her
sons each have a 50 percent chance of inheriting the
abnormal gene and getting the disease. Her daughters
each have a 50 percent chance of inheriting the gene and
being carriers of the disorder.
Spontaneous mutations in X-chromosome genes can
lead to X- linked recessive diseases in boys.