Testing for Genetic Diseases

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Genetic disease is a disorder or condition caused by abnormalities in a person's genome. Types of genetic inheritance include single inheritance (for example, cystic fibrosis, sickle cell anemia, Marfan syndrome, and hemochromatosis), multifactoral inheritance (for example, high blood pressure, Alzheimer's disease, cancer, arthritis, and diabetes), chromosome abnormalities (for example, Turner syndrome, and Klinefelter syndrome), and mitochondrial inheritance (for example, epilepsy and dementia).

Single gene inheritance -- Also called Mendelian or monogenic inheritance. This type of inheritance is caused by changes or mutations that occur in the DNA sequence of a single gene. There are more than 6,000 known single-gene disorders, which occur in about 1 out of every 200 births.

Some examples of single gene inheritance are cystic fibrosis, sickle cell anemia, Marfan syndrome, Huntington's disease, and hemochromatosis. Single-gene disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked.

Multifactorial inheritance -- Also called complex or polygenic inheritance. This type of inheritance is caused by a combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer susceptibility have been found on chromosomes 6, 11, 13, 14, 15, 17, and 22. Some common chronic diseases are multifactorial disorders.

Examples of multifactorial inheritance include heart disease, high blood pressure, Alzheimer's disease, arthritis, diabetes, cancer, and obesity. Multifactorial inheritance also is associated with heritable traits such as fingerprint patterns, height, eye color, and skin color.

Chromosome abnormalities -- Chromosomes, distinct structures made up of DNA and protein, are located in the nucleus of each cell. Because chromosomes are the carriers of the genetic material, abnormalities in chromosome number or structure can result in disease.

For example, Down syndrome or trisomy 21 is a common disorder that occurs when a person has three copies of chromosome 21. There are many other chromosome abnormalities including Turner syndrome (45,X), Klinefelter syndrome (47, XXY), the cat cry syndrome (46, XX or XY, 5p-), and so on.

Mitochondrial inheritance -- This type of genetic disorder is caused by mutations in the nonchromosomal DNA of mitochondria. Mitochondria are small round or rod-like organelles that are involved in cellular respiration and found in the cytoplasm of plant and animal cells. Each mitochondrion may contain 5 to 10 circular pieces of DNA.

Examples of mitochondrial disease include an eye disease called Leber's hereditary optic atrophy; a type of epilepsy called MERRF which stands for Myoclonus Epilepsy with Ragged Red Fibers; and a form of dementia called MELAS for Mitochondrial Encephalopathy, Lactic Acidosis and Stroke-like episodes.

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