Genetic Diseases

A genetic disease is any disease that is caused by an abnormality in an individual's genome. The abnormality can range from minuscule to major -- from a discrete mutation in a single base in the DNA of a single gene to a gross chromosome abnormality involving the addition or subtraction of an entire chromosome or set of chromosomes. Some genetic disorders are inherited from the parents, while other genetic diseases are caused by acquired changes or mutations in a preexisting gene or group of genes. Mutations occur either randomly or due to some environmental exposure.

There are a number of different types of genetic inheritance, including the following four modes:

Single gene inheritance

Single gene inheritance, also called Mendelian or monogenetic inheritance. This type of inheritance is caused by changes or mutations that occur in theDNA 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. These disorders are known as monogenetic disorders (disorders of a single age).

Some examples of monogenetic disorders include:

• 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

Examples of multifactorial inheritance include:Multifactorial inheritance, also called complex or polygenic inheritance. Multifactorial inheritance disorders are 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.

• 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. Abnormalities in chromosomes typically occur due to a problem with cell division.

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), and
• cri du chat syndrome, or the "cry of the cat" syndrome (46, XX or XY, 5p-).

Diseases may also occur because of chromosomal translocation in which portions of two chromosomes are exchanged..

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. Since egg cells, but not sperm cells keep their mitochondria during fertilization, mitochondrial DNA is always inherited from the female parent.

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.