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The Genetic Story.


Muscular Dystrophy Association - USA Research and Program
Services Department


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 happens.
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 X-linked recessive.


How does the autosomal dominant pattern of inheritance work?


Autosomal dominant inheritance chart

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?

Autosomal recessive inheritance chart

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 the disease.


How does inheritance work in X-linked recessive
diseases?

X-linked inheritance chart

X-linked diseases are those that come from gene defects on the X chromosome. These 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.

Reproduced from the pages of MDA USA


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