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What is a genetic disease? How is it defined?
A genetic disease is any disease that is caused by an abnormality in an individual's genome, the person's entire genetic makeup. 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 can occur either randomly or due to some environmental exposure.
What are the types of genetic inheritance?
There are a number of different types of genetic inheritance including:
- Single gene inheritance
- Multifactorial inheritance
- Chromosome abnormalities
- Mitochondrial inheritance
Single gene genetic inheritance
Single gene inheritance, also called Mendelian or monogenetic 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. These disorders are known as monogenetic disorders (disorders of a single gene).
Some examples of monogenetic disorders include:
Single-gene disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked.
Multifactorial genetic inheritance
Multifactorial inheritance, which 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. Abnormalities in chromosomes 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 disorder that occurs when a person has three copies of chromosome 21. There are many other chromosome 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:
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.
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Asperger's syndrome (AS, Asperger syndrome, Asperger disorder) is an autism spectrum disorder. Asperger's syndrome is characterized by a degree of impairment in language and communication skills, and repetitive or restrictive thoughts or behaviors. The most common symptom of Asperger's syndrome is the obsessive interest in a single object or topic. Other conditions that may co-exist with Asperger's syndrome include:
- tic disorders,
- anxiety disorders,
- and OCD.
Diagnosis of Asperger's syndrome is complicated, and treatment is generally directed toward minimizing the symptoms of the syndrome and behavioral and educational interventions.
Atopic DermatitisEczema is a general term for many types dermatitis (skin inflammation). Atopic dermatitis is the most common of the many types of eczema. Other types of eczema include: contact eczema, allergic contact eczema, seborrheic eczema, nummular eczema, stasis dermatitis, and. dyshidrotic eczema.
Basal Cell Nevus Syndrome PictureNumerous basal cell epitheliomas on the neck of a child. See a picture of Basal Cell Nevus Syndrome and learn more about the health topic.
Bipolar SlideshowBipolar disorder (once called manic depression) causes extreme mood shifts and can be disorienting. Our experts define bipolar disorder, discuss bipolar symptoms, and describe bipolar medications that can help.
Blood Disorders QuizExactly what is sickle cell anemia? Learn about sickle cell and other diseases by testing your IQ with the Blood and Bleeding Disorders Quiz.
Young Women & Breast CancerIs breast cancer genetic? Should I get tested for the BRCA gene? What every young women should know about breast cancer. Discover the signs and symptoms of breast cancer and other crucial breast cancer facts.
Deep Vein Thrombosis
Deep vein thrombosis (DVT) is a blood clot in the deep veins, and can be caused by broken bones, trauma to a limb, immobility, medications, smoking, cancer, genetic predisposition, and cancer. Symptoms of a deep vein thrombosis in a leg are swelling, tenderness, redness, warmth, and pain.
Treatment for DVT include medications and surgery.
Dementia SlideshowWhat is dementia? Learn about dementia disorders such as Lewy Body Dementia, Alzheimer's disease (AD), Vascular (multi-infarct) dementia (MID), and more. Discover dementia stages, signs of dementia, causes, diagnosis, treatments, and medications.
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Diabetes MellitusDiabetes is a chronic condition characterized by high levels of sugar (glucose) in the blood. The two types of diabetes are referred to as type 1 (insulin dependent) and type 2 (non-insulin dependent). Symptoms of diabetes include increased urine output, thirst, hunger, and fatigue. Treatment of diabetes depends on the type.
HemoglobinHemoglobin is the oxygen-carrying protein molecule in the blood, specifically in the red blood cells. High hemoglobin count may be caused by dehydration, smoking, emphysema, tumors, or abuse of Epogen. Low levels of hemoglobin may be caused by anemia, blood loss, nutritional deficiency, bone marrow problems, chemotherapy, kidney failure, or sickle cell disease.
Elevated homocysteine levels in the blood called hyperhomocysteinemia, is a sign that the body isn't producing enough of the amino acid homocysteine. is a rare and serious condition that may be inherited (genetic). People with homocystinuria die at an early age. Symptoms of hyperhomocysteinemia include developmental delays, osteoporosis, blood clots, heart attack, heart disease, stroke, and visual abnormalities.
There are other causes of hyperhomocysteinemia, for example, alcoholism.
Supplementing the diet with folic acid and possibly vitamins B6 and B12 supplements can lower homocysteine levels. Currently there is no direct proof that taking folic acid and B vitamins lower homocysteine levels and prevent heart attacks and strokes. Talk to your doctor if you feel you need to have your homocysteine blood levels checked.
ParathyroidectomyParathyroidectomy is the removal of one or more of the parathyroid glands to treat hyperparathyroidism. Risks of parathyroidectomy include:
- paralysis of the vocal cords,
- difficulty swallowing thin liquids,
- difficulty breathing,
- and drug reactions.
- damage to the recurrent laryngeal nerve,
- bleeding or hematoma,
- problems maintaining calcium levels in the blood,
- need for further and more aggressive surgery,
- need for a limited or total thyroidectomy,
- prolonged pain,
- impaired healing,
- and recurrence of the tumor.
PCR (Polymerase Chain Reaction)PCR (polymerase chain reaction) is a method to analyze a short sequence DNA or RNA. PCR (polymerase chain reaction) has many uses, for example, it is used to diagnose genetic diseases, establish paternity or biological relationships, DNA fingerprinting, DNA forensics, and finding bacteria and viruses.
Polycythemia (High Red Blood Cell Count)
Polycythemia (elevated red blood cell count) is a rare blood disease in which the body produces too many red blood cells. Causes of polycythemia are either primary (acquired or genetic mutations) or secondary (diseases, conditions, high altitude). Examples of primary polycythemia include:
- Excessive sweating
- Unintended weight loss
- Gouty arthritis, usually in big toe
- Shortness of breath
- Feeling pressure or fullness on the left side of the abdomen where the spleen is located.
- Vision problems
- Heavy bleeding from minor cuts
- Bleeding from the gums
- Redness in the face
- A burning feeling in the hands and feet
Complications of a high red blood cell count include blood clots, heart attack, stroke, enlarged liver and spleen, angina (heart pain), AML leukemia, and heart failure. Blood clots in the liver or kidney can cause sudden, intense pain. Treatment goals for patients with polycythemia are to manage symptoms and reduce the risk of complications like heart attack and stroke.
REFERENCE: NIH. National Heart, Lung, and Blood Institute. "What Causes Polycythemia Vera"? Updated: Mar 20, 2011.
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