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Genetic Disorders: Types and Kinds

Genetic Disorders: Types and Kinds

What is a Genetic Disorder?

Genetic disorders occur when abnormalities emerge in the unique genome of an individual. It is commonly thought that most genetic disorders are inherited when defective genes from one or both parents are passed to offspring. However, many genetic disorders are simply novel changes (mutations) affecting a person’s DNA. In these cases, genetic defects leading to a specific genetic disorder will be inherited only if it happens in the germ line.

Single gene disorders resulting from one mutated gene constitute nearly 4000 diseases currently known to exist. Capable of being passed on generationally, single gene disorders involve dominant and recessive gene types, with genetic disorders depending on “recessive” genes lying dormant for three or four generations before they inexplicably emerge again.

The earliest evidence of human genetic disorders exist in descriptions and pictures of ancient Egyptian royalty, specifically King Tutankhamun who is thought to have died from sickle cell anemia, a genetic disorder affecting red blood cells. Additional research conducted by radiologists, archaeologists and geneticists have determined that King Tut’s mummy showed signs of numerous congenital flaws probably resulting from inbreeding. In fact, King Tut may have suffered one of more of the following genetic diseases: Marfan Syndrome, Klinefelter Syndrome, Wilson-Turner Syndrome, androgen insensitivity syndrome or temporal lobe epilepsy.

Autosomal Dominant Genetic Disorders

A single mutated copy of a gene is all that is needed for someone to inherit an autosomal dominant genetic condition. Autosomal dominant disorders typically involve one affected parent, with a 50 percent chance that his or her offspring will inherit the disorder. However, some AD genetic disorders may never be expressed due to reduced penetrance. Examples of autosomal dominant genetic diseases include Huntington’s disease, Marfan Syndrome, neurofibromatosis type 1 and 2 and hereditary nonpolyposis colorectal cancer.

Autosomal Recessive Genetic Disorders

Two copies of a gene must contain an anomaly (mutation) for someone to have an autosomal recessive genetic disorder. Those with AR conditions usually have parents who each carry one copy of the mutation but have not developed symptoms of the disease themselves. Moreover, each child they have together has a 25 percent chance of inheriting the disorder. Examples of autosomal recessive genetic disorders include cystic fibrosis, Tay-Sachs disease, sickle-cell anemia and spinal muscular atrophy. Phenotypes (like having dry or wet earwax, also stem from autosomal recessive genes.

X-Linked Dominant Genetic Disorders

X chromosomes with genetic mutations cause X-linked dominant disorders that are rare and seen mostly in males. Rett syndrome, Aicardi syndrome and incontinentia pigmenti Type 2 are fatal to boys at birth and are therefore only diagnosed in girls. Passing of an X-linked genetic disorder depends on the gender of the parent. For example, a man with an X-link disease will have sons who do not inherit the disease because they will inherit their father’s Y chromosome. Alternately, any daughters this man has will inherit the condition. Women with X-linked genetic disorders have a 50/50 chance of passing the disorder to both male and female offspring.

X-Linked Recessive Genetic Disorders

Also caused by genetic mutations on the X chromosome, X-linked recessive disorders include hemophilia, Lesch-Nyhan syndrome, Duchenne muscular dystrophy, male pattern baldness and green-red color blindness. Men are much more susceptible to X-linked genetic disorders, although X-linked recessive diseases may manifest in women due to a female developing Turner syndrome or skewed X-inactivation.

Common Genetic Disorders

Cystic Fibrosis

When chloride levels in the body are imbalanced due to insufficient amounts of protein, recurring lung infections, gastrointestinal problems and breathing difficulties produce symptoms of a genetic disorder called cystic fibrosis. Both parents must have the CF gene before it can be inherited by offspring, who each have a 25 percent chance of developing cystic fibrosis symptoms.

Huntington’s disease

A disease of the central nervous system, Huntington’s causes progressive degradation of neurons that eventually induces behavioral changes, uncontrollable movements, walking difficulties, loss of cognitive abilities and inability to swallow. Adult-onset Huntington’s typically emerges in adults older than 30 but less than 50 years of age. Because it is an autosomal disorder, only one parent has to carry the HD gene to pass it on to offspring who have a 50 percent chance of eventually suffering symptoms of the disorder.

Down Syndrome

A chromosomal abnormality that affects about one out of every 1000 newborns, Down Syndrome is a genetic disorder resulting from defects in chromosome 21. Often called trisomy 21 because the defect involves the person having three chromosomes instead of two, Down Syndrome is indicated by a set of classic physical features (slanted eyes, flattened nose, overly large tongue), decreased muscle tone and IQ scores between 50 and 70. Although Down Syndrome is considered a genetic disorder, it is frequently correlated with mothers who are over 40 years old or those who have entered perimenopause.

Fragile X Syndrome

Delayed development of language/speech, hyperactivity, mild to moderate cognitive problems and attention deficit disorder are signs that a person has inherited Fragile X Syndrome, a genetic disorder caused by mutations in the FMR1 gene. About one in three individuals diagnosed with Fragile X exhibit features of an autism spectrum disorder that inhibit their ability to interact normally with others. Physical characteristics of those with Fragile X include a narrow, extended face, larger than average ears, prominent forehead and jaw and flat feet.

Hemophilia

Hemophilia “A” is the most common human genetic disorder and is associated with blood clotting difficulties, specifically the lack of clotting factor VIII in blood cells. Signs of hemophilia include excessive bleeding of the nose and gums, abnormal bleeding into the joints and bleeding in the gastrointestinal system. Skin rashes, deep bruising and blood in the stool and/or urine may also indicate hemophilia when all other diseases have been negated. An X-chromosome gene is responsible for hemophilia, which means that a father cannot pass the disorder to his sons. Interestingly, most mothers who possess the defective gene are carriers who rarely exhibit symptoms of hemophilia themselves.

Hyperlipidemia/Hypercholsterolemia

Increased cholesterol and blood lipids is an inherited disorder that predispose affected individuals to diabetes, obesity, cardiovascular disease, stroke and glucose intolerance.

Becker Muscular Dystrophy

Symptoms of Becker Muscular Dystrophy are slow to appear and slow to worsen, typically affecting more boys than girls and confining those with the disease to wheelchairs by age 30. The onset of Becker Muscular Dystrophy begins with chronic fatigue and muscle weakness that starts in the legs and eventually debilitates the upper body. Some people with Becker’s MD may suffer from mental retardation as well.

Duchenne Muscular Dystrophy

Signs of Duchenne MD are similar to Becker’s MD but appear earlier, often by age five or six, and may include heart problems, back or chest deformities and breathing difficulties. By age 12 or 13, those with Duchenne Muscular Dystrophy must use a wheelchair since they are no longer able to walk.

Sickle Cell Anemia

The classic sign of sickle cell anemia involves red blood cells that are shaped like a crescent or sickle instead of a disc. This shape inhibits the ability of red blood cells to transport oxygen throughout the body, which causes pain in the chest and abdomen, overwhelming fatigue, frequent fevers, increased heart rate, possible leg ulcers and developmental delays. Sickle cell anemia predominantly affects African-Americans, Middle Easterners and people from the Mediterranean and the Americas.

A more comprehensive list of genetic disorders can be found here

Is Bipolar Disorder Genetic?

Scientists began suspecting many psychiatric disorders tend to run in families over 30 years ago after conducting longitudinal studies with twins who were raised in separate environments. In addition to schizophrenia, major depression, panic disorder and ADHD, bipolar disorder appears to have a strong genetic component that involves variations in several genes responsible for coding the cellular operation that regulates calcium flow to neurons in the brain.

Referred to as the CACNA1C, this gene had previously been associated with depression and schizophrenia and was known to substantially influence brain activity involved in cognition, emotion, memory and attention, all functions that are affected by mental illness.

Researchers have also discovered variations in specific areas of chromosomes three and 10 in people with bipolar disorder, schizophrenia and depression. However, it is the variated region in chromosome three that presented the strongest link to bipolar disorder in genetic studies.

Further studies have shown that although someone may carry genetic markers for bipolar disorder, the illness may never surface due to the presence or lack of certain environmental triggers. For example, psychosocial stressors such as physical or sexual abuse, extreme poverty or abandonment may “set off” the gene and initiate symptoms of bipolar disorder. Neuroscientists think that developing brains (from birth until age 18) in those with the bipolar gene are much more sensitive to stressors than the brains of those without the gene. Other “triggers” may involve diet, abnormal sleep/wake cycles and serious viral illnesses occurring in childhood.

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