Genetic Disease (cont.)

Medical Author:

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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Medical Editor:

William C. Shiel Jr., MD, FACP, FACR

William C. Shiel Jr., MD, FACP, FACR

Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

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In this Article

• What is a genetic disease?
• What are the different types of inheritance?
• Single gene inheritance
• Multifactorial inheritance
• Chromosome abnormalities
• Mitochondrial inheritance
• What is the human genome?
• Genetic Disease Index

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.

The human genome is the entire "treasury of human inheritance." The sequence of the human genome obtained by the Human Genome Project, completed in April 2003, provides the first holistic view of our genetic heritage.. The 46 human chromosomes (22 pairs of autosomal chromosomes and 2 sex chromosomes) between them house almost 3 billion base pairs of DNA that contains about 20,500 protein-coding genes. The coding regions make up less than 5% of the genome (the function of all the remaining DNA is not clear) and some chromosomes have a higher density of genes than others.

Most genetic diseases are the direct result of a mutation in one gene. However, one of the most difficult problems ahead is to further elucidate how genes contribute to diseases that have a complex pattern of inheritance, such as in the cases of diabetes, asthma, cancer, and mental illness. In all these cases, no one gene has the yes/no power to say whether a person will develop the disease or not. It is likely that more than one mutation is required before the disease is manifest, and a number of genes may each make a subtle contribution to a person's susceptibility to a disease; genes may also affect how a person reacts to environmental factors.

REFERENCE:

National Human Genome Research Institute.
<http://www.genome.gov>

Previous contributing authors and editors: Barbara K. Hecht, Ph.D. and Frederick Hecht, M.D.