Genetic diseases are conditions that are carried and possibly affected by a person's genome. Diseases such as Huntington's disease and Down's syndrome are strictly genetic and caused entirely by the individual's genome. Certain cancers, diabetes and even obesity are also genetic diseases, insofar as their occurrence and severity are affected by distinct genetic traits.
Causes
Genetic disorders can be inherited from parents. If one or both parents carry the gene for a disorder, it can be passed on to their children. Some diseases require two copies of the gene to manifest, meaning both parents must be carriers, while others require only a single gene. Other genetic disorders can manifest as a result of favorable environmental factors, such as lifestyle. Lifestyle can also impact an individual's genome and cause genetic mutations during life, leading to a developed genetic disease.
Types
Genetic diseases are grouped into three categories: single gene, multifactorial and chromosomal. Single gene disorders are inherited from parents and result from a damaged or mutated gene's inability to function properly. Multifactorial diseases, as the name implies, are caused by genetics and other factors, such as smoking, overeating or environment. Chromosomal diseases result from having either extra or missing chromosomes, such as in Down's syndrome in which the individual has three copies of the chromosome 21 instead of the normal two.
Recessive and Dominant Conditions
Every human being has two copies of each gene, which they inherit from each parent. In recessive conditions, a type of single gene disorder, each copy of the gene must be affected for the disease to manifest. If only one parent passes on a recessive condition, the child will only be a carrier; however, if both parents pass on the gene, the child will be affected. Dominant conditions manifest even if only one parent passes on the mutated gene.
X-Linked Conditions
X-linked conditions, another form of single gene disorder, result from mutations to genes associated with the X chromosome. This chromosome determines gender. Women have two X chromosomes, while men have both an X and a Y. Because men have only one copy, they are more likely to be affected by X-linked conditions, while women are more likely to be carriers. Many X-linked conditions, such as Duchenne muscular dystrophy, are carried on the mother's side, as they are fatal or debilitating to men early in life.
Environmental Factors
Multifactorial conditions arise from exposure to the appropriate environmental factors. Current methods of genetic testing can detect an individual's propensity for developing a condition, such as certain cancers, based upon the presence of mutated genes. While individuals with the mutated genes are at a higher risk for developing these conditions, the disease will not activate without the proper external influence. For example, heart disease is a multifactorial disease that will not manifest unless external factors, including poor diet and little exercise, promote it.
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