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| What Is Muscular Dystrophy | ||||||||||||||||||||||||||
| Genetic Testing It is now possible to detect the abnormal gene for a number of the muscular dystrophies and other inherited muscle disorders. This can be done by examining the DNA extracted from a blood sample. In the future it is expected that this form of confirmation of the diagnosis will be possible for each of the muscular dystrophies, allowing more precise genetic counselling. What does hereditary mean? An hereditary characteristic is one which is determined by the genetic material which is put together when a sperm and an egg join at conception. That is true of both normal characteristics and hereditary disorders. The genetic material from the sperm and the egg produces inherited characteristics by directing the body to make certain proteins. Normal variations in those proteins produce the normal variability that we see in people. Inability to make an essential protein properly, because of a defect in the genetic material, may cause an hereditary disease like muscular dystrophy. What is a gene? There are about 100,000 different proteins whose manufacture is determined by the genetic material. A bit of the genetic material which is concerned with one protein is called a gene, so there are about 100,000 genes in the egg and a similar number in the sperm. The genes are made of deoxyribonucleic acid, usually abbreviated to DNA. The chemical structure of the DNA of a particular gene contains a code for the corresponding protein. Here is an example. Normally, humans have a gene which codes for a protein called dystrophin, essential for the integrity of muscle fibres. Normal people have dystrophin in their muscle fibres. Males with Duchenne type muscular dystrophy have something wrong with that gene. Their muscle fibres cannot make dystrophin properly and their muscles cannot function normally and suffer progressive damage. Is muscular dystrophy always inherited? By definition, all muscular dystrophies are genetic, but they are not always inherited. In other words there is not always a history of muscular dystrophy in a previous generation of an affected person's family for a number of reasons. First a person may be affected by a genetic defect which has just occurred in a family for the first time, in one egg or one sperm of his or her parent. This is called a new mutation. Secondly, a parent may have been so slightly affected by the disease that it was unnoticed and may have been undiagnosable even by an experienced physician. Thirdly, one important mode of inheritance, autosomal recessive, very seldom affects more than one generation. |
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