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Single Gene Disorders
Genetic defects are a fact of life. Though the cause of some genetic disorders may be complicated and involve several genes, i.e. cancer, there are diseases that are caused by a single malfunctioning gene.
Collectively they are known as single gene disorders and can be traced back to defects in a particular gene. They can always be classified further into sex-linked or autosomal-linked diseases depending on the chromosome that carries the defective gene. In this article we will be looking at three diseases - Cystic fibrosis, Sickle cell anaemia, Huntington's disease and genetic defects behind them.
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Cystic fibrosis is caused by mutation/s of the CFTR gene (cystic fibrosis transmembrane conductance regulator). The gene codes for a chloride ion channel that plays a role in the secretion of sweat, mucus, and digestive enzymes. Since two copies of the mutant gene cause the disease phenotype, a person can survive with one functional copy, and as such they are termed carriers. Having one functioning copy is sufficient to prevent the disease condition.
However, when there are genetic mutations in both copies, the disease will be present. The condition is caused by the inability of the body to transport chloride ions into and out of cells due to the mutated CFTR gene. Gradually this leads to a blockage of the lungs and pancreas. The lung is further affected if infection is brought on by the build up of bacteria.
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Sickle-cell anaemia is another single gene disorder. It takes its name from the shape that the genetic mutation causes affected red blood cells to form.
The gene responsible for this disorder is the haemoglobin gene; there is a mutation in the beta-globin chain of haemoglobin. The defect is common in tropical and sub-tropical nations. There is a direct correlation with malaria, since carrying one copy of the sickle cell allele amongst the two copies of the gene offers resistance to the disease.
There are also variations where other mutations may occur in the healthy copy of the allele causing several different variants of the disease. The pathology of the disease is that the sickle-shaped cells cannot move freely in the capillaries and therefore create localized obstructions cutting off blood supply to vital organs, as well as causing pain. The spleen is an important organ that is severely affected by this condition.
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Huntington's disease is different from the two genetic disorders discussed above. It is an autosomal dominant genetic disorder. Inheriting a single defective copy of the gene is sufficient to cause the disease.
The condition occurs due to changes in the Huntingtin gene that codes for the Huntingtin protein (HTT). There is a trinucleotide repeat region in the gene that varies in length among individuals and generations. If the trinucleotide repeats (CAG - codes for guanine) exceed 27, then there is the possibility of the Huntingtin protein turning into a mutant form (mHTT). The Huntingtin protein is required for the functioning of the brain and therefore the disease affects brain functions. The mutant protein may also cause aggregations in cells affecting cellular function. Since the protein operates in the brain, the neuronal cells are affected causing severe debilitation of physical movement and cognitive abilities.