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If, as a result of the point mutation or partial deletion, the reading frame is shifted, the consequence is that entirely different codons, specifying different amino acids are produced after the mutation. This type of mutation is called a frameshift mutation. The effects of a frameshift mutation can be severe.
Although some human molecular diseases have a typical type of mutation in a specific gene, many single gene defects arise due to several mutations of different types and in differing places. Some of these types of mutations can be seen in the examples that are going to be discussed. An example of each of a single gene defect has been chosen for each of the categories of; autosomal recessive and dominant and sex-linked. .
X-Linked Muscular Dystrophies.
There are two main types of x-linked muscular dystrophies; Duchenne and Becker, although there are many less common forms. The gene involved in this condition is the dystrophin gene. The dystrophin gene is the largest gene known, at about 2.3 million base pairs long. The gene is situated on the short arm of the X chromosome and encodes a 14 kb mRNA transcript which translates a 427 kDa protein called dystrophin1. which is present not only in the muscle but also in the brain. The protein dystrophin is found very close to the muscle membrane, it links the extracellular lamina (connective tissue) with the intracellular actin, this is achieved by binding to a glycoprotein complex in the muscle membrane, as can be seen in figure 1. This complex is required for the maintenance and function of muscle cells.
Figure 1. . A representational diagram showing the predicted structure of the dystrophin protein, a dimmer, linking the extracellular laminin with the intracellular actin.
(Taken from Emery's Elements of Medical Genetics' by Robert S. Mueller and Ian D. Young.).
60-70% of X-linked muscular dystrophy conditions are caused by deletions within the gene, these arise due to unequal crossing over.