The 16 Most Common Genetic Diseases

A genetic disorder is a health condition caused by abnormalities in an individual’s genetic material (the DNA, or the genome). There are four different types of genetic disorders. They are those that feature the following changes [1]:

We know that some ethnic groups are predisposed to certain genetic disorders.

People originating from the Mediterranean areas of Europe, for example are more likely to have a form of anemia that is genetic — the thalassemias. [2]

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 the person encounters. In sickle cell anemia, the mutation that causes the red blood cells to change shape also helps minimize infection with the malarial parasite, making the condition a remarkable defensive mechanism as well as one with serious consequences. [3]

It has also long been established that a woman is born with all the eggs she will ever have in her ovaries — they develop within a fetus, in fact — and the longer she waits to have children, the greater the risk of a genetic disorder in her child. 

Thanks to modern science, scientists can also test for a great variety of 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.

Tests for these common genetic diseases are only a small portion of genetic tests, as more than 1000 genetic tests are currently in use, and more are being developed. When a woman undergoes genetic testing during pregnancy, she is subjected to a type of medical test that identifies changes in chromosomes, genes, or proteins. The results of a genetic test can confirm or rule out a suspected genetic condition or help determine a person’s chance of developing or passing on a genetic disorder. [3]

Cystic Fibrosis

Cystic fibrosis (CF) is among the most common inherited diseases in Caucasian people and it is caused by the lack of a protein that controls the balance of chloride in the body. The effects seen in cystic fibrosis patients include breathing difficulties and recurrent lung infections.

Cystic fibrosis also involves problems with the digestive tract and challenges with reproductive health. The symptoms of cystic fibrosis can vary from relatively mild to severe. Both parents must be carriers of the gene that causes CF for a child to be affected by cystic fibrosis — each child then has a 25 percent (1 in 4) chance of having cystic fibrosis. Cystic fibrosis is most common in Caucasians, and particularly in Ashkenazi Jews, but is found in people of all ethnic groups.[4]

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 to 1000 newborn babies. It can be detected during pre-natal testing with great reliability.

There is a pattern of features very common in people with Down syndrome, some of which are usually immediately apparent at birth — these include specific facial characteristics (a flattened face, short nose bridge, smaller ears, and a shorter neck), decreased muscle tone, heart and digestive system defects, small hands and feet, and developmental delays. Children with Down Syndrome are variably affected, the affects ranging from mild to moderate to severe. People of any ethnic group can have Down syndrome and it is most often associated with increased age of the mother. [5]

Fragile X syndrome

Fragile X syndrome is associated with the most common type of inherited developmental delays and intellectual disability. The delays and cognitive difficulties someone with fragile X syndrome has can range from very mild to severe and are sometimes associated with autism. About 1 in 1500 males and 1 in 2500 females have fragile X syndrome.

Normally, men have one X chromosome and women have two. In Fragile X syndrome, part of the X chromosome can break apart; hence the name of the condition. The area on the X chromosome that causes the fragility can be repeated on that chromosome — and the more the number of repeated areas, the greater the fragility and the more serious the syndrome. [6]

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 and conditions. These clotting problems can result in too much bleeding (hemorrhage) and the formation of abnormal blood clots throughout the body, most commonly in the veins. The most common such inherited clotting disease is the Factor V Leiden abnormality and it can particularly present a problem in pregnancy, leading to pre-eclampsia, small-for-gestational-age babies, stillbirths and problems with the placenta. [7, 8]

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 blood clotting disorder — and 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 [9]:

• 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

Familial combined hyperlipidemia and familial hypercholesterolemia are inherited disorders that result in an increase in blood lipids and cholesterol levels. These disorders predispose those people who have them to obesity, glucose intolerance and diabetes, strokes and heart disease. Treatment includes lifestyle and dietary changes. [10]

Huntington's disease

Huntington's disease 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, altered speech and cognitive functions, and difficulty in swallowing. 

Treatment for Huntington's disease 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 their 30’s and 40’s.  There is also an early-onset form of the disease, which begins in childhood.

Huntington's disease is an autosomal dominant disorder, which means that if one parent has the condition, the child will have a 50 percent chance of having the disease  as well — and if you inherited the gene, you will develop the disease and can pass it on to your own children. [4]

The muscular dystrophies

Many of the muscular dystrophies —  which are characterized by worsening muscle weakness — are inherited disorders.  The most common forms of musular dystrophy are [11]:

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 intellectual disability, and muscle weakness, beginning in the legs and then spreading to the upper body.

People with Duchenne muscular dystrophy can also suffer from 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 will need 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 they are slower to appear and slower to worsen. Those symptoms include fatigue, possible intellectual disability, 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 will often require a wheelchair by the age of 25 to 30.

Sickle Cell Anemia

Sickle cell anemia (SCA) is an inherited disease in which the red blood cells, normally shaped like discs, instead form a crescent or sickled shape, which is where the name comes from.

The changes seen in sickle cell anemia reduce the red blood cells' ability to carry oxygen to all the tissues of the body — which can, in turn, result in painful sickle cell crises that 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, an increased heart rate, delayed growth and puberty, fever and leg ulcers. Treatments for sickle cell anemia include pain medication, hydroxyurea to decrease the number of pain episodes, 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. [12]

READ Personalised Medicine: Genetic Screening For Cancer Patients

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. [13]

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 for PKU 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.