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There are many theories about why genetic diseases occur. We do know that both environmental and genetic factors play a role in their development, but we don’t understand exactly how.
A genetic disorder is caused by abnormalities in an individual’s genetic material (the DNA, or the genome). There are four different types of genetic disorders.  They are :

(1)    Single-gene is mutated
(2)    Multiple genes are mutated
(3)    Chromosomal changes-entire areas of the chromosome can be missing or misplaced.
(4)    Mitochondrial. The maternal genetic material in mitochondria can mutate as well. 
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We know that some ethnic groups are pre-disposed for certain genetic disorders (people originating in the Mediterranean areas of Europe, for example are more likely to have a form of anemia that is genetic—the thalassemias).  We also know that some genetic disorders, sickle cell anemia (SCA), for example, are an attempt by the body to combat stressors in the environment.  In SCA, the mutation that causes the red blood cells to change shape also helps minimize infection with the malarial parasite. We also know that a woman is born with all her eggs in her ovaries—and the longer she waits to have children, the greater the risk of a genetic disorder in that child.  We can also test for some genetic diseases such as Cystic Fibrosis, Down Syndrome, Fragile X Syndrome, Inherited Thrombophilias (Factor V Leiden, Factor II), Bloom Syndrome, Canavan Disease, Cystic Fibrosis, Familial Dysautonomia, Fanconi Anemia, Gaucher Disease, Mucolipidosis Type IV, Niemann-Pick Disease, Tay-Sachs Disease, Klinefelter Syndrome, Open neural tube defects, Sickle Cell Disease, Thalassemia, some trisomic disorders and Turner Syndrome

Cystic Fibrosis

Cystic fibrosis (CF) is among of the most common inherited diseases in Caucasians  and is caused by the lack of a protein that controls the balance of chloride in the body. The effect of this is breathing difficulties and recurrent lung infections.  It also involves problems in digestion and in reproduction. The symptoms vary from relatively mild to severe. Both parents must be carriers for a child to be affected—each child then has a 25% (1 in 4) chance of being affected.  CF is most common in Caucasians, particularly in Ashkenazi Jews, but is found in all ethnic groups.

Down syndrome

Down syndrome is a common chromosomal abnormality caused by an extra copy of genes found on chromosome 21. Down syndrome affects about 1 out of 800-1000 newborn babies.   It can be detected by pre-natal testing.  There is a pattern of features some of which are usually immediately apparent at birth—these include facial characteristics, decreased muscle tone, heart and digestive system defects and developmental delays. Children with Down Syndrome are variably affected, the affects ranging from mild to moderate to severe.  Any ethnic group can be affected and it is most often associated with increased age of the mother.

Fragile X syndrome

Fragile X syndrome is associated with the most common type of inherited developmental delays and mental retardation.   The delays and cognitive difficulties can range from very mild to severe and are sometimes associated with autism. About 1/1500 males and 1/2500 females have fragile X syndrome.4 Normally, men have one X chromosome and women have two. In Fragile X syndrome, part of the X chromosome can break apart. The area on the X chromosome that causes the fragility can be repeated on that chromosome—the more the number of repeated areas, the greater the fragility and the more serious the syndrome.

Inherited Clotting Problems

The blood clotting process is one of the more complicated biochemical pathways in the body and there are a number of different inherited clotting problems. These clotting problems can result in too much bleeding and the formation of abnormal clots throughout the body, most commonly in the veins. The most common is the Factor V Leiden abnormality and can be particularly a problem in pregnancy, leading to pre-eclampsia, small-for-gestational-age babies, stillbirths and problems with the placenta.

There are other inherited clotting problems including prothrombin (Factor II) deficiency, Protein S deficiency, Protein C deficiency, and antithrombin III deficiency.

Hemophilia is a well known clotting disorder—the most common types are Hemophilia A (where there is a lack of clotting factor VIII), Hemophilia B (where there is a lack of clotting factor IX) and Von Willebrand disease (where there is a lack of the Von Willebrand clotting factor).  Symptoms of all these clotting disorders include:

•    Excessive bleeding of the gums, nose, gastrointestinal system and bleeding into the joints
•    Abnormal menstrual bleeding
•    Excessive Bruising
•    Skin rashes

Familial combined hyperlipidemia and Familial hypercholsterolemia

These are inherited disorders that result in an increase of blood lipids and cholesterol .  These disorders predispose to obesity, glucose intolerance and diabetes, strokes and heart disease. Treatment includes lifestyle and dietary changes.

Huntington's disease

This is an inherited disease which causes certain nerve cells in the brain and central nervous system to degenerate.  Loss of these nerve cells causes symptoms such as behavior changes, unusual, snake-like movements (chorea), uncontrolled movement, difficulty walking, loss of memory, speech and cognitive functions and difficulty in swallowing.  Treatment aims to limit the course of the disease. There is an adult-onset Huntington’s Disease (HD), which is most common and usually develops when a person is in the 30’s and 40’s.  There is also an early-onset form which begins in childhood. HD is an autosomal dominant disorder, meaning that if one parent has HD, you have a 50% chance of having the disease—and if you inherited the gene, you will develop the disease and can pass it on to your own children.

The muscular dystrophies

Many of the muscular dystrophies—characterized by worsening muscle weakness—are inherited disorders.  The most common are:

Duchenne muscular dystrophy:

In Duchenne muscular dystrophy, the symptoms usually are apparent before the age of 6 and may appear even earlier.  Those symptoms include: Fatigue, possible mental retardation and muscle weakness, beginning in the legs and then to the upper body. There can also be associated heart problems, respiratory problems and deformities of the chest and back. The muscle weakness makes it progressively more difficult to walk and get around.  That weakness gets progressively worse—by the age of 12, most kids are confined to a wheelchair. Boys are more likely to inherit this disorder.

Becker muscular dystrophy:

In Becker muscular dystrophy, the symptoms are similar to Duchenne muscular dystrophy, but are slower to appear and slower to worsen. Those symptoms include: Fatigue, possible mental retardation and muscle weakness, beginning in the legs. The muscle weakness in the upper body is not as severe as in Duchenne. Again, boys are more likely to have the disorder and are often confined to a wheelchair by the age of 25-30.

Sickle Cell Anemia

Sickle cell anemia (SCA) is an inherited disease where the red blood cells, normally shaped like discs, instead form a crescent or sickled shape.  This reduces the red blood cell’s ability to carry oxygen to all the tissue of the body—this can result in painful sickle cell crises and can occur just a few times in a person’s life or many times.  Besides pain (abdominal, chest and in the bones), other symptoms include fatigue, shortness of breath, increased heart rate, delayed growth and puberty, fever and leg ulcers. Treatments include pain medication, hydroxyurea to decrease the number of pain episode, folic acid to help support the red blood cells, blood transfusions and kidney dialysis. Sickle cell disease is more common in people of African and Mediterranean backgrounds, but it is also found in people from South and Central America, the Caribbean, and the Middle East.

Thalassemias

The thalassemias (beta-thalassemia is the most common, alpha-thalassemia is less common) is a group of inherited blood disorders where hemoglobin, the oxygen-carrying molecule, is not properly synthesized by the red blood cells.  This results in an anemia—where the usual symptoms are fatigue, an enlarged spleen, easily broken bones, bone pain and shortness of breath. People with thalassemia may also have poor appetite, darkened urine and jaundice (a yellowish discoloration of the skin or the white parts of the eyes—jaundice is a signal for liver dysfunction).  Infections are common in people with thalassemia.

Mutations affecting Biochemical pathways:

Phenylketonuria (PKU) is the result of a lack of a liver enzyme that is needed to convert an amino acid, phenylalanine to another amino acid, tyrosine.  If it is not caught early enough, high levels of phenylalanine accumulate and cause mental retardation, brain damage and seizures. Treatment consists of a phenylalanine restricted diet and the use of a cofactor—thetrahydrobiopterin (BH4) to reduce the amount of phenylalanine in the blood.

Alpha-1-antitrypsin deficiency leads to a decreased amount of alpha-1 antitrypsin in the lungs and in the blood—this results in lung diseases such as emphysema. The earliest symptoms are shortness of breath and wheezing. Other symptoms can include weight loss, frequent respiratory infections, fatigue, and a rapid heartbeat.

There are many other genetically based diseases.  There are no cures for these disorders, though there is the potential for gene therapy.  Many of these disorders can be treated and those with the disease can live a fuller life.

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