3.4 - E - Genetic Diseases
Genetic diseases are illnesses that are caused by a gene. Most commonly, they are caused by a recessive allele of a gene and so only develop in individuals that inherit two recessive copies. Individuals that are heterozygous for the gene possess one dominant copy and so do not show the disease. Instead, they are referred to as carriers and can pass either the dominant or recessive allele to their children. Recessive genetic diseases are usually unexpected as both parents must be carriers. Since the parents do now show symptoms of the disease, they are typically unaware that they are carriers at all. Improvement in technology and understanding has made genetic testing possible for parents prior to reproducing.
Other genetic diseases are caused by dominant alleles, but they are less common. Since they are dominant, it is not possible to be a carrier as having just one copy of the alleles will result in symptoms. As shown in the diagram to the right, if one parent is heterozygous for a dominant genetic disease, there is a 50% chance that each child will inherit it. If one parent is homozygous for the disease, then 100% of children will show symptoms. Very few diseases are caused by co-dominant alleles, an example which is sickle-cell anemia.
Overall, many genetic diseases have been identified in humans, but most are very rare.
Genetic Diseases Examples
Cystic fibrosis is a common genetic disease, which is caused by a recessive allele of the CFTR gene located on chromosome 7. It codes for a chloride ion channel involved in secreting sweat and mucus. When it is mutated, digestive juices and mucus are secreted with insufficient amounts of of NaCl. This prevents the movement of water via osmosis so sticky mucus builds up in the lungs and the pancreatic duct is usually blocked.
Huntington's diseases is caused by a dominant allele of the HTT gene located on chromosome 4. It codes for the hungtingtin protein, whose function is still being researched. The dominant allele of this gene results in degeneration in the brain, usually starting between age 30-50. Life expectancy after the start of symptoms is ~20 years. Eventually, full nursing care is necessary and persons typically die of heart failure, pneumonia or other infectious diseases.