- What are the four types of genetic disorders (inherited)?
- Single gene genetic inheritance
- Multifactorial genetic inheritance
- Chromosomal abnormalities
- Mitochondrial genetic inheritance
What is a genetic disease? How is it defined?
A genetic disease is any disease caused by an abnormality in the genetic makeup of an individual. The genetic 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 chromosomal abnormality involving the addition or subtraction of an entire chromosome or set of chromosomes. Some people inherit genetic disorders from the parents, while acquired changes or mutations in a preexisting gene or group of genes cause other genetic diseases. Genetic mutations can occur either randomly or due to some environmental exposure.
Single gene genetic inheritance
Single gene inheritance is also called Mendelian or monogenetic inheritance. Changes or mutations that occur in the DNA sequence of a single gene cause this type of inheritance. There are thousands of known single-gene disorders. These disorders are known as monogenetic disorders (disorders of a single gene).
Single-gene disorders have different patterns of genetic inheritance, including
- autosomal dominant inheritance, in which only one copy of a defective gene (from either parent) is necessary to cause the condition;
- autosomal recessive inheritance, in which two copies of a defective gene (one from each parent) are necessary to cause the condition; and
- X-linked inheritance, in which the defective gene is present on the female, or X-chromosome. X-linked inheritance may be dominant or recessive.
Some examples of single-gene disorders include
Multifactorial genetic inheritance
Multifactorial inheritance is 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.
Examples of multifactorial inheritance include
Multifactorial inheritance also is associated with heritable traits such as fingerprint patterns, height, eye color, and skin color.
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. Chromosomal abnormalities typically occur due to a problem with cell division.
For example, Down syndrome (sometimes referred to as "Down's syndrome") or trisomy 21 is a common genetic disorder that occurs when a person has three copies of chromosome 21. There are many other chromosomal abnormalities including:
- Turner syndrome (45,X0),
- 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 genetic inheritance
This type of genetic disorder is caused by mutations in the non-nuclear 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
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What is the human genome?
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.
Health Solutions From Our Sponsors
United States. CDC. "Facts About Down Syndrome." <https://www.cdc.gov/ncbddd/birthdefects/downsyndrome.html>.
United States. National Heart, Lung, and Blood Institute. "Cystic Fibrosis." <https://www.nhlbi.nih.gov/health-topics/cystic-fibrosis>.
United States. National Human Genome Research Institute. "Frequently Asked Questions About Genetic Disorders." <https://www.genome.gov/19016930/faq-about-genetic-disorders/>.
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A common form of short stature, achondroplasia (dwarfism) is a genetic condition causing a disorder of bone growth. Complications of achondroplasia that need monitoring include (this is not all inclusive) stenosis and compression of the spinal cord, a large opening under the skull, lordosis, kyphosis, spinal stenosis, hydrocephalus, middle ear infections, obesity, and dental crowning. Achondroplasia is caused by mutations of the FGFR3 gene.
Alpha 1 Antitrypsin Deficiency
Alpha-1 antitrypsin deficiency (AATD) is an inherited disorder caused by mutations in the SERPINA1 gene. People with the condition are at risk for developing serious lung and liver disease. Symptoms and signs of lung disease caused by this condition include:
The earliest symptoms and signs of lung disease usually develop between 20 and 50 years of age, and are
- The reduced ability to exercise
- Shortness of breath (dyspnea) following mild activity
Other symptoms and signs of alpha-1 antitrypsin deficiency are:
- Rapid heartbeat when going from sitting to standing
- Recurring respiratory infections
- Unintentional weight loss
Lung disease: People with this condition often develop emphysema, with symptoms of a hacking cough, barrel-shaped chest, and difficulty breathing. If you have this condition and smoke or are exposed to tobacco smoke, it accelerates the appearance of emphysema symptoms and lung damage.
Liver disease: Alpha-1 antitrypsin deficiency also cause liver disease in some people with the condition, that include liver cancer, cirrhosis of the liver, an abnormally large liver (hepatomegaly), liver failure, and hepatitis. Liver damage from alpha-1 antitrypsin deficiency causes symptom of a swollen abdomen, swollen legs or feet, and jaundice.
Treatment of AATD depends upon the severity of symptoms. FDA approved drug for AATD is an orphan product called alpha-1-proteinase inhibitor (human), sold under the brand name "Prolastin."
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