Genetic Disorders in Humans

Tap to Read ➤ Debopriya Bose

There are thousands of genetic disorders in humans. Some are common whereas quite a few are rare. Whatever be their incidence, what is most vexing about these disorders is that scientists are still trying to find cures for these disorders.

While some headway has been made in the direction, a lot more research is required. Here is a comprehensive view of genetics and various genetic disorders that are often noticed in human beings.

Within their coiled strands, chromosomes hide a world of information about how a person should look, how tall he should be, what should be his skin color and also the diseases that he may be prone to (which are referred to as genetic disorders in human beings).

They have this information coded in the form of nucleotide sequences that form DNA (deoxyribonucleic acid) molecules. A single molecule of DNA is coiled to form a single chromosome. These chromosomes are present in pairs. Human beings have 23 pairs of chromosomes or 46 chromosomes in all. Of them, 1 pair forms the sex chromosomes.

The rest 22 pairs are the autosomes. Sex chromosomes are of two types - X and Y. A man has one X and one Y chromosome (XY) while a woman has two X chromosomes (XX). It is just the difference of one sex chromosome that decides an individual's gender.

The chemical information of every organism is specifically coded in the nucleotide sequences of genes - the unit of heredity. Each gene codes for an enzyme that plays an important role in various biochemical reactions.

Your browser doesn't support HTML5 video. In other words, a gene contains a specific sequence of nucleotide bases that are responsible for the organization of amino acids in the correct order to form an enzyme. Any disruption in this sequence, causes genetic disorders in humans.

Now you can understand the importance of genetics and the reason why so much of research is being done in the field of human genetics.

Understanding genes will not only help us unravel the chemical blueprint of human beings, it will also enable us to deal with genetic disorders for most of which, till date, there is no cure.

Causes of Genetic Disorders Genetic diseases in humans are caused due to abnormalities in genes or chromosomes. Such defects can be caused by the following mechanisms:

• Mutations: These are sudden inheritable changes in the nucleotide sequence of a gene. • Aneuploidy: Aneuploidy is caused when there are abnormal number of chromosomes in an organism. This could be due to loss of a chromosome (monosomy) or presence of extra copy of a chromosome (trisomy, tetrasomy, etc.)

• Deletions: Loss of a part of chromosome as in the case of Jacobsen syndrome. • Duplications: Duplication of a portion of chromosome that results in extra amount of genetic material.

• Inversions: Inversion of the nucleotide sequence because a portion of chromosome has broken off, got inverted and reattached at the original location of the chromosome.

Translocations: When a portion of chromosome has got transferred on to some other chromosome. Sometimes translocation can take place between two chromosomes, in which case they interchange chromosome segments. However, in some cases a portion of a chromosome may simply get attached to another chromosome.

Types of Genetic Disorders

• Autosomal Dominant Genetic Disorders: These disorders are caused when an individual has inherited the defective gene from a single parent. This defective gene belongs to an autosome. Such an inheritance is also known as autosomal dominant pattern of inheritance.

• Autosomal Recessive Genetic Disorders: Such disorders manifest only when an individual has got two defective alleles of the same gene, one from each parent. These genetic disorders are inherited via the autosomal recessive pattern of inheritance.

• Sex-Linked Disorders: These are disorders related to sex chromosomes or genes in them. • Multi-factorial Genetic Disorders: Such disorders are the result of genetic as well as environmental factors.

List of Common Human Genetic Disorders

Achondroplasia It is an autosomal dominant genetic disorder which is the most common genetic cause of dwarfism. Individuals suffering from achondroplasia vary from 4 feet to 4 feet 4 inches in height. They have disproportionately short limbs. However, there is no intellectual disability.

In this disorder, the cartilage, specially in the long bones, fail to convert into bones. It is caused due to mutation in the FGFR3 gene (located on chromosome 4. which codes for a protein that regulates transformation of cartilage to bone).

Although, an individual may inherit the disorder from an affected parent, the disorder is usually the result of a mutation in the sperm or egg of a healthy parent. Achondroplasia can be detected before birth with the help of prenatal ultrasound.

There is no treatment for this disorder. However, limb extending surgeries can be done, although this is a controversial issue.

Achromatopsia It is an autosomal congenital recessive disorder which is characterized by visual acuity loss, colorblindness, light sensitivity and nystagmus. It is also known as rod monochromatism. Statistics show that 1 person in 33000 in the U. S. is affected by achromatopsia.

The symptoms are first noticed in children at the age of six months when they exhibit nystagmus and photophobic activities. Achromatopsia is of two forms. The more severe form is known as compete achromatopsia.

Those who exhibit milder symptoms are known to suffer from partial achromatopsia. Using optical and visual aids are useful in improving vision of those suffering from achromatopspia.

Acid Maltase Deficiency

It is an autosomal recessive disorder, in which the defect is in the gene for the acid maltase enzyme, which leads to accumulation of glycogen in the lysosomes (cellular organelles that contain enzymes to break down waste materials or cellular debris). Glycogen build up weakens the muscles and nerve cells throughout the body of the patient.

This may affect respiratory muscles resulting in respiratory failure. It is also known as the Pompe Disease. Although, in childhood and adolescence the symptoms show slow progress and are less severe, infantile forms may cause death within the first year, if not treated on time.

Albinism

Albinism is a congenital disorder in which there is little or completely no production of melanin in hair, skin and iris of the eyes. Hence albinos (people suffering from albinism) have light-colored skin, hair and eyes.

It is caused due to inheritance of recessive alleles from parents. This disorder can't be cured. However, the symptoms can be alleviated with the help of surgical treatment, vision aids and using devices that provide protection from sun.

Low/no production of the pigment, melanin is described as type 1 albinism. A defect in the "P" gene causes type 2 albinism.

Alzheimer's Disease

Alzheimer's disease is the most common form of dementia which is characterized by gradual memory loss, irritability, mood swings, confusion and language breakdown.

Although, scientists are not unequivocal about the cause of this disease, the most widely accepted reason is the amyloid cascade hypothesis, that suggests excess production of a small protein fragment called ABeta (Aβ).

Also known as Senile Dementia of the Alzheimer Type (SDAT) or simply Alzheimer's, this is a degenerative disease and scientists are yet to find its cure. However, balanced diet, mental exercises and stimulation are often suggested for prevention and management of the disease.

Angelman syndrome

It is a neurological disorder that was first described by a British pediatrician, Dr. Harry Angelman, in 1965. This disorder is marked by intellectual and developmental delays, severe speech impairment and problems in movement and balance, recurrent seizures and small heads.

Children with Angelman syndrome typically have a happy demeanor. They are hyperactive with short attention span and show jerky hand movements. These children appear normal at birth.

This genetic disorder in human is a classical case of genetic imprinting, in which the disorder is caused due to deletion or activation of the maternally inherited chromosome 15. Its sister syndrome is the Prader-Willi syndrome in which there is a similar loss or inactivation of the paternally inherited chromosome 15.

Bardet-Biedl Syndrome

It is a pleiotropic recessive genetic disorder that is characterized by obesity, polydactyly, deterioration of rod and cone cells, mental retardation and defect in the gonads and kidney disease.

It is difficult to diagnose Bardet-Biedl Syndrome, specially in the young. As no cure is yet known for the disorder, treatment is concentrated on specific organs and systems.

Barth Syndrome

A rare but serious sex-linked genetic disorder, the Barth syndrome is caused due to mutations or alterations in the BTHS gene. The gene is located on the long arm of X chromosome.

This disorder primarily affects the heart. Besides heart defects, Barth syndrome results in poor skeletal musculature, short stature, mitochondrial abnormalities and deficiency of white blood cells. There is no cure for this disorder. Treatment focuses on managing the symptoms and preventing infections.

Bipolar Disorder

Also known as manic-depressive disorder or bipolar affective disorder, individuals suffering from bipolar disorder suffer from highly elevated moods, referred to as mania or episodes of severe depression.

Research shows that both genetic as well as environmental factors are responsible for this disorder. Medicines as well as psychotherapy is found to be useful in dealing with the severe mood swings associated with the disorder.

Bloom Syndrome

Bloom syndrome is an autosomal recessive genetic disorder, which is characterized by a high frequency of breaks and rearrangements in the chromosomes of an effected person.

Symptoms include short stature, butterfly shaped facial rash, high-pitched voice, increased susceptibility to cancer, leukemia, respiratory illnesses and infections.

Some may even show mental retardation. Like other genetic disorders in human, there is no treatment for Bloom's syndrome. All treatment is preventive in nature. This disease is more common in Ashkenazi Jews with a frequency of 1/100 individuals suffering from this disorder.

Color Blindness

Color blindness refers to the inability of differentiating among certain colors. This can be genetically inherited and can also be caused due to a damage to the eye, nerve or brain. As far as genetics is concerned, color blindness is most commonly the result of mutations in the X chromosome.

However, research has shown that mutation in 19 different chromosomes can cause color blindness. There is no treatment to cure color blindness. However, certain types of tinted filters and contact lenses may enable an individual to differentiate colors.

Cri-du-Chat Syndrome

Cri du Chat syndrome is caused due to deletion of short (p) arm of chromosome 5. Most cases of this disorder are not inherited. In such cases, they are caused due to spontaneous deletion of a segment of chromosome 5 during formation of egg or sperm or during early stages of fetal development.

The syndrome gets the name from the characteristic high-pitched cry of an infant that resembles the cry of a cat. Other symptoms are intellectual disabilities, delayed development, microcephaly (small head), low birth weight, typical facial features and weak muscle tones during infancy. No specific treatment is available for this disorder.

Cystic Fibrosis

Cystic fibrosis is an inherited disease of the glands that secrete mucus and sweat. Cystic fibrosis causes the mucus to become thick and sticky that clogs various organs of the body, that results in other complications. It mostly affects lungs, liver, pancreas, sinuses, intestines and sex organs.

Cystic fibrosis also causes excess loss of salts through sweat that results in dehydration, tiredness, weakness and elevated heart rate. This is an autosomal recessive disorder in which the mutation is caused in the CFTR gene.

At present, there is no cure for cystic fibrosis. However, doctors treat the symptoms using antibiotic therapy along with other treatments that would clear the mucus that accumulates in different organs.

Down Syndrome

Also called Trisomy 21, it is a genetic disorder in human that is the result of extra copy of chromosome 21, that a child inherits from his/her parent. This extra genetic material causes delays in mental as well as physical development of a child.

Physical peculiarities caused due to this syndrome include a narrow chin, a prominently round face, protruding tongue, short limbs, the Simian crease and poor muscle tone. Almost 1 in every 800 to 1000 births may have genetic abnormality. The incidence of this disorder increases with maternal age at pregnancy.

Amniocentesis during pregnancy or birth can detect this abnormality. Karyotyping test of a child confirms this syndrome, if done after birth. No specific cure is available. However, treatment of the health problems and training and special education of such individuals is of great help.

Duchenne Muscular Dystrophy

It is an X linked recessive trait which is characterized by progressive degeneration of muscles that results in loss of ambulation and finally leading to death. It is one of the most prevalent muscular dystrophies that affect only males. Females are just the carriers and hence don't show the symptoms.

The disorder is caused due to mutation in the gene DMD (located on X chromosome) that codes for protein dystrophin which is an important component of muscle tissue.

Physical therapy is effective in lessening physical disabilities of children suffering from this disorder. Besides this, recent advancements in medicines are helping in extending lives of those affected. Stem cell research also shows promising developments in dealing with this form of muscular dystrophy.

Fragile X Syndrome

Also known as the Martin-Bell syndrome or marker X syndrome, the Fragile X syndrome is the most common cause of inherited form of mental retardation. It is the result of trinucleotide repeat disorder, in which, the trinucleotide gene sequence CGG in the X chromosome is repeated several times.

The result is intellectual disabilities, high levels of anxiety and hyperactivity like fidgeting, autistic behavior like hand flapping, avoiding eye contact, shyness, mental retardation and attention deficit disorder and other symptoms.

It is an X-linked dominant disorder that has no cure. Medicines, educational, behavioral and physical therapies are the only help available to individuals suffering from this disorder.

Galactosemia A rare metabolic genetic disorder in human, galactosemia impairs body's ability to break down galactose. Alternately, it is also known as Galactose-1-phosphate uridyl transferase deficiency (Galactosemia type I), Galactokinase deficiency (Galactosemia type II), Galactose-6-phosphate epimerase deficiency (Galactosemia type III).

Infants suffering from galactosemia show symptoms within a few days after birth or soon after they start nursing. The symptoms include yellowing of skin, eyes, diarrhea, vomiting, refusal to drink milk, malnourishment and also mental retardation.

If milk or milk products are given to infants suffering from galactosemia, then accumulation of galactose in their system damages brain, eyes, liver and kidneys.

Eliminating lactose and galactose from diet of an individual is the only way to treat classic galactosemia (galactosemia type I) which is the most common and most severe form of galactosemia. Although very different, galactosemia is often confused with lactose intolerance.

Hemophilia This is a recessive X-linked genetic disorder in which the bodies of individuals lose the ability to coagulate blood or blood clotting. As the mutation is caused in X chromosome and the condition is recessive, the females are carriers and males suffer from the symptoms of hemophilia.

However, under rare occasions, females may also suffer from hemophilia. There are two variations of the disorder. Hemophilia A which is more common than the other variation, Hemophilia B. Regular infusion of the coagulating factor that lacks in an individual, helps one control blood loss caused due to excessive bleeding.

Huntington's Disease It is an autosomal dominant genetic disorder in which there are foot abnormalities, and premature fusion of bones in the skull which lead to deformations of the facial features (widely spaced eyes, bulging forehead) and the skull.

In this syndrome, the great toes are short and wide and turn away from the rest of the toes. Some toes may be fused or have some other abnormalities. The mutation is caused in the FGFR2 gene which is located in chromosome 10. Treatment involves corrective surgery for deformed bones in face and foot.

Klinefelter Syndrome It is the most common sex-linked genetic disorder in which males have an extra X chromosome. Hence, this disorder is also known as 47, XXY or XXY syndrome. The most common symptom is infertility. Besides this, males with the XXY syndrome have impaired physical, language and social developments.

As these individuals produce less testosterone than other males, such teenagers may be less muscular and have less facial hair than their peers. The presence of the extra X chromosome can't be undone. However, testosterone replacement therapy, a variety of therapeutic options like behavioral, speech and occupational therapy and educational treatments are the options available for those suffering from Klinefelter's syndrome.

Krabbe Disease Krabbe disease is a rare degenerative disorder of the nervous system. It occurs due to mutation in the GALC gene which results in deficiency of enzyme galactosylceramidase. Deficiency of this enzyme affects the development of the myelin sheath of nerve cells.

Langer-Giedion Syndrome Langer-Giedion syndrome is a genetic disorder in human that is caused due to deletion or mutation of at least two genes on chromosome 8. This is not an inherited disorder. It is caused due to random events during formation of reproductive cells (sperms and eggs) in individuals.

This is a rare disorder that causes bone abnormalities and typical facial features. Individuals suffering from Langer-Giedion syndrome have multiple non-cancerous tumors in their bones that cause pain, restrict joint movement and exerts pressure on nerves, blood vessels, spinal cord and the tissues surrounding the tumors. Some intellectual disability may be associated with this disorder. External fixators can be used for facial and limbic reconstructions.

Lesch-Nyhan Syndrome It is an X-linked recessive disorder which causes deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT). Lack of HPRT leads to accumulation of uric acid in body, which leads to gout and kidney problems, poor muscle control and mental retardation of moderate degree.

A striking feature of this disorder is a child biting his lips and fingers. This self mutilating behavior appears in second year of a child's life. Other than these features, an individual suffering from this disorder shows facial grimacing, involuntary writhing and repetitive movement of limbs that is characteristic of Huntington's disease.

This disorder is alternatively also known as Nyhan's syndrome, Juvenile gout and Kelley-Seegmiller syndrome. Treatment for the Lesch-Nyhan syndrome is symptomatic.

Marfan Syndrome Marfan syndrome is an inherited genetic disorder of the connective tissue, in which mutation is caused in the FBN1 gene that codes for the protein fibrillin-1. Marfan syndrome can be mild or severe. There is great variability in the features of this disorder that are associated with skeleton, skin and joints.

However, confirmatory symptoms of the disorder are long limbs, dislocated lenses and dilation of the aortic root. Individuals suffering from Marfan syndrome or Marfan's syndrome usually have heart problems. They typically are tall and thin with slender, tapering fingers.

Once diagnosed with Marfan's syndrome, regular visit to the cardiologist is required. Treatment depends upon the organ system that is affected. Regular check ups, medicines and surgery may be required to treat the symptoms of this disorder that is alternately also known as leodosis.

Muscular Dystrophy Muscular dystrophy (MD) refers to a group of genetically inherited disorders of progressive degeneration of skeletal muscles. It also causes defects in muscle proteins and death of muscle cells and tissues. These disorders vary in severity and the extent and distribution of muscle weakness.

Although the skeletal muscles are primarily affected, muscular dystrophy may impair functions of other systems of the body as well. While in some cases the symptoms appear in infancy of childhood, in certain instances muscular degeneration sets in during adulthood.

Duchenne MD is the most common form of MD. Other common disorders are Becker MD, Facioscapulohumeral MD and Myotonic MD . There is no specific treatment to cure or reverse MD. However, therapeutic options like speech therapy, respiratory therapy, speech therapy and corrective orthopedic surgery and orthopedic appliances are used to treat disabilities due to MD.

Myotonic Dystrophy Myotonic dystrophy, also known as dystrophia myotonica (DM) is an autosomal recessive genetic disorder that is caused due to repetition of a trinucleotide sequence. It affects the muscles of the body and is a multi-system disorder. Other than progressive muscle wasting, there is formation of cataracts in the eye, cardiac conduction defects and hormonal imbalances.

There are two variations of this disorder - DM 1 and DM 2. DM 1 is more severe than DM 2. In DM 1 the trinucleotide sequence repeat is located on chromosome 9 whereas in case of DM 2 the trinucleotide sequence repeat occurs in chromosome 3.

Although this disorder can manifest at any stage of one's life, variability with respect to age on onset of the symptoms reduces within successive generation. Hence it is a good example of anticipation. There is no cure for this disorder. Nevertheless the affected organs can be treated to manage the symptoms.

Nail-patella syndrome (NPS) is inherited via autosomal dominant pattern. It is a disorder that affects the joints, bones, fingernails and kidneys. It is most commonly characterized by lack of nail and knee caps. Bone deformations manifest in elbow and abnormally shaped hip bone. Nail-Patella Syndrome Research shows that individuals suffering from the NPS are susceptible to developing glaucoma and scoliosis. Other names for NPS are hereditary onychoostedysplasia, iliac horn syndrome, Fong disease or Turner-Kiser syndrome.

Neurofibromatosis An autosomal dominant condition, neurofibromatosis (abbreviated NF) is a genetically inherited condition in which nerve tissue grow tumors, that may be benign or may cause medical complications by compressing nerves and tissues around them. Hence, in this disorder the bones, nervous system, the spine and the skin are affected

Tumors under the skin may appear as bumps and are associated with skin discoloration. Learning disabilities are also associated with this disorder. There are two types of this disorder. Neurofibromatosis type 1 is more common than neurofibromatosis type 2. While type 1 is caused due to mutation in chromosome 17, neurofibromatosis type 2 is the result of a mutation in chromosome 22.

Due to lack of any cure, treatment is aimed at managing symptoms and complications. In case, the tumor becomes cancerous (as happens with 10% of the cases), chemotherapy may be required. Surgery is resorted to, when the tumor compresses any organ or specific tissue of the body.

Noonan Syndrome It is an autosomal dominant genetic disorder that may be inherited or arise due to spontaneous mutation in genes KRAS, PTPN11, RAF1, and SOS1. Individuals suffer from developmental disabilities that result in heart malformations, short stature, characteristic facial features, impaired blood clotting and indentation of the chest. Speech, language and learning disabilities are also common.

Triple X Syndrome As the name suggests, trisomy refers to the genetic disorder which results in an extra copy of the X chromosome in females. This disorder is variably known as the XXX syndrome, triplo-X, trisomy X, and 47,XXX aneuploidy. This genetic disorder is not inherited.

It is caused due to non-disjunction during cell division, that results in an extra copy of chromosome in reproductive cells. In some cases, the extra copy of X chromosome may be caused during cell division in early embryonic development. Women with this disorder have three X chromosomes.

Osteogenesis Imperfecta This is an autosomal dominant disorder of the connective tissue in which bones break easily and sometimes due to no apparent reason. Hence, it is also known as brittle bone syndrome or Lobstein disorder. Genetic mutation impairs synthesis of collagen - a protein that makes bones strong.

Osteogenesis Imperfecta may also weaken muscles, cause brittle bones, curved spine and impaired hearing. Exercise, physical therapy, medicines and orthopedic devices are the only treatment available for people suffering from this disorder, as cure hasn't yet been found.

Patau Syndrome Also known as trisomy D or trisomy 13, Patau syndrome is caused due to non-disjunction of chromosome 13 during meiosis, due to which, an affected individual inherits an extra copy of the chromosome. Robertsonian translocation can be another cause of this disorder.

Like other genetic disorders in human that originate due to non-disjunction of chromosomes, the incidence of Patu syndrome increases with maternal age at pregnancy. The extra copy of chromosome results in kidney and heart defects, neurological problems, facial defects, polydactyly (having extra fingers) and deformed feet.

Features of this disorder are present from birth and may be confused with Edward's syndrome. Hence, genetic testing is important to confirm diagnosis. While certain infants may be able to survive only for a couple of days, depending upon the severity of the conditions, those who survive with milder symptoms undergo treatment, focusing the particular disability that each individual suffers from.

Phenylketonuria Phenylketonuria (abbreviated as PKU) is an autosomal recessive disorder that causes deficiency in the enzyme phenylalanine hydroxylase. The result of this deficiency is that instead of being metabolized to tyrosine, the amino acid phenylalanine gets converted into phenylketone (also known as phenylpyruvate). Phenylketonuria (abbreviated as PKU) is an autosomal recessive disorder that causes deficiency in the enzyme phenylalanine hydroxylase. The result of this deficiency is that instead of being metabolized to tyrosine, the amino acid phenylalanine gets converted into phenylketone (also known as phenylpyruvate).

If left untreated, excess phenylketone can impair development of the brain. This will manifest as mental retardation, seizures or brain damage. A diet low in phenylalanine or other means that lowers the amount of the compound helps in dealing with the disorder.

Porphyria Porphyria is an inherited genetic disorder in which, synthesis of any one of the 8 enzymes involved in the process of synthesis of heme, is disrupted. Heme is linked to a chemical called protoporphyrin. Disruption in the heme biosynthetic pathway results in accumulation of porphyrin or its precursors in the body, that cause neurological or dermal problems.

Acute porphyria affects the nervous system, whereas cutaneous porphyria or erythropoeitic porphyria primarily affects the skin. This disorder is inherited in autosomal dominant pattern. Taking heme externally through a vein and medicines can alleviate the symptoms of this disorder.

Retinoblastoma Retinoblastoma is a cancer of the retina, that affects children younger than 5 years. It can be genetic as well as non genetic. The genetic form, which is the cause in almost half of the cases of retinoblastoma, is the result of mutation in chromosome 13. Retinoblastoma usually affects one eye.

However, if not treated on time, cancer may spread out from the eye to other parts of the body. Retinoblastoma usually affects one eye. Characteristic physical feature is whiteness of the retina, which is referred to as "cat's eye reflex" or leukocoria. Other symptoms include, deterioration in vision, eye pain, redness and irritation in the eye. Retiblastoma is curable if treated at an early stage.

Rett syndrome is a neurological and developmental disorder that is inherited through X-linked dominant pattern. It occurs almost exclusively in females. For about a year of normal growth, girls with Rett syndrome show clinical features that include decreased rate of head growth, small hands and feet, disabilities related to learning Rett Syndrome communication, coordination and speech. Affected girls lose control over purposeful use of hands and show repetitive movements like wringing of the hands and clapping.

Sickle Cell Disease Sickle cell disease or sickle cell anemia is a blood disorder which is inherited via autosomal recessive pattern. Mutation in the HBB gene, causes sickling of the red blood cells that may lead to a number of complications. Sickling causes the red blood cells to break down prematurely causing anemia and conditions related to it like, fatigue and shortness of breath. Severe anemia may delay development in children.

Sickle cell disease makes one prone to infections and one may experience painful episodes when the sickled blood cells get stuck in small blood vessels. This deprives organs and tissues of enough oxygen and may cause damage to organs like lungs, kidney, spleen and brain.

Life expectancy due to this disease is reduced to 42 years for males and 48 years for females. Bone marrow transplant and introduction of certain compounds like cyanate, penicillin and folic acid is used to manage complications due to this genetic disorder.

Tay-Sachs Disease It is an autosomal recessive genetic disorder that progressively destroys nerve cells in brain and the spinal cord, leading to neurological and physical disabilities. The most common form is the infantile Tay-Sachs disease that presents itself at the age of 3 to 6 months. These infants lose motor skills like ability to turn, sit or crawl. As the disease progresses, there is mental disability, vision and hearing loss, occurrence of seizures and paralysis.

Such infants usually die within the initial years of childhood. Other forms of Tay-Sachs disease, that may affect adolescents and adults are rare and less severe. Sadly, there is absolutely no cure for the disease. The only help that a suffering child can be provided is, to make him as comfortable as possible till the disease has run its course.

Turner Syndrome Turner syndrome refers to a condition in which a woman lacks either one whole or a part of an X chromosome. The most characteristic feature of Turner syndrome is small stature that becomes evident when one is around 5 years old. Other symptoms include loss of ovarian functions, absence of menstrual cycle, swelling, broad chest, low hairline and webbed neck.

Such women are prone to heart diseases, hypothyroidism, diabetes and visual and auditory problems. Since, this is a chromosomal abnormality, there is no cure. However, estrogen replacement therapy and doses of growth hormones are successful in minimizing the symptoms.

Usher Syndrome Inherited in autosomal recessive pattern, Usher syndrome or Usher's syndrome is the result of mutation in chromosome 10, that results in deafness and progressive loss of vision. Vision loss is caused due to an eye disease called retinitis pigmentosa (RP), whereas hearing loss is associated with a defective inner ear.

There are three clinical varieties of this disorder, designated as I, II and III in decreasing order of severity. Currently, no cure for this disease is available. However, gene therapy is being progressively investigated to find a possible cure. Till then, educational programs to facilitate communication, use of hearing aids or cochlear implants are some options to minimize the symptoms of this disorder.

Von Hippel-Lindau Syndrome Von Hippel-Lindau syndrome is a rare autosomal dominant genetic disorder in human, that is characterized by the formation of tumors and fluid-filled sacs (cysts) in different parts of the body. The tumors are called hemangioblastomas that are typical of this disorder and consist of newly formed blood vessels and are typically noncancerous.

The tumors when formed in brain or spinal cord cause headaches, vomiting and loss of muscle coordination. People suffering from this disorder are prone to developing cysts in the kidney, pancreas and the male genital tracts. The Von Hippel-Lindau syndrome is of two types. Type I is associated with low risk of developing tumors as opposed to the type II variety in which this risk is quite high.

Waardenburg Syndrome Waardenburg Syndrome is an autosomal dominant genetic disorder that is characterized by varying degrees of deafness and changes in hair and skin pigmentation. One of the most commonly observed features is, eyes of different colors or brilliantly colored blue eyes.

Cleft lip and/or cleft palate is also associated with this syndrome. Deafness arising due to the genetic defect is treated as any irreversible deafness would be. Other disabilities - physical or neurological are treated symptomatically.

Wilson's Disease Wilson's disease is an autosomal recessive genetic disorder in which the mutation is caused in the ATP7B gene. This defect in the gene impairs body's ability to get rid of excess copper in the body. As a result, copper accumulates in tissues and this manifests as neurological and psychiatric problems. Liver diseases are also common.

With the build up of copper in liver, excess of this mineral is released into the bloodstream that harms other organs, commonly eyes, kidneys, heart and hormones. Neuropsychiatric problems include mild cognitive deterioration to start with. It is then followed by lack or coordination, involuntary repeated movement of hands, seizures and migraines. Medicines to facilitate removal of copper from body and low copper diet is suggested, for those who have this syndrome. However, a liver transplant may be occasionally required.

Xeroderma Pigmentosum Xeroderma pigmentosum is an autosomal recessive genetic disorder in which the DNA loses its ability to repair damage caused due to ultraviolet rays. Symptoms include, severe sunburn at the slightest exposure to sun, early development of freckles, irregular dark spots or premature aging of the skin.

Such individuals are prone to skin cancer and other skin malignancies. The main way to control worsening of skin condition is to avoid exposure to sun and to use sun protective creams, clothes and physical sunscreens. Medications and other forms of treatment like cryotherapy can be used (cryotherapy is suggested to treat existing keratoses).

Also known as tetrasomy X, 48, XXXX or quadruple X, the XXXX syndrome is a chromosomal aneuploidy, that is, it arises due to a defect in meiosis. This happens when homologous chromosomes fail to separate during meiosis when the reproductive cells (sperms and eggs) are formed. This results in the presence of four instead of two X chromosomes in females. XXXX Syndrome

The symptoms of this disorder range from mild to severe. Physically, these women have epicanthic folds, flat nose bridge, cleft or highly arched palate, teeth and enamel defects. Delay in development of speech, language and motor skills is also found. 50% of women may have delayed or no puberty at all. Some may have menstrual irregularities and lack secondary sexual characteristics.

Tetrasomy X females may have defects in vision, hearing, kidneys, circulatory system and nervous system. While many girls suffering from tetrasomy X lead a normal life, most of those who have problems are placed on estrogen treatment. Treatment is symptomatic and one may require occupational, speech and physical therapy as well. Prognosis is relatively good for those suffering from this disorder.

YY Syndrome As the name suggests, YY syndrome is the result of chromosomal aneuploidy due to which a male has an extra Y chromosome. Other than being noticeably taller than their peers, males suffering from this condition do not have any other unusual physical features. However, these individuals are at a risk of suffering from learning disabilities and delayed development of speech and language skills.

More Genetic Disorders in Human Beings • Aceruloplasminemia • Gilbert's Syndrome • Aicardi Syndrome • Hurler Syndrome • Alexander Disease • Hypophosphatasia • Alkaptonuria • Joubert Syndrome • Best's Disease • Leukodystrophy

• Canavan Syndrome • Menkes Syndrome • Carnitine Deficiencies • Mowat-Wilson Syndrome • Celiac Disease Mucopolysaccharidosis • Charcot-Marie-Tooth Disease • Muenke Syndrome • Coffin Lowry Syndrome • Niemann-Pick Disease • Cooley's Anemia • Pfeiffer Syndrome • Cowden Syndrome • Prader-Willi Syndrome

• Crouzon Syndrome •Rubinstein-Taybi Syndrome • Cystinosis • Shwachman Syndrome • Epidermolysis Bullosa • Smith-Magenis Syndrome • Fabry Disease • Stickler Syndrome • Fibrodysplasia Ossificans Progressiva • Variegate Porphyria • Gaucher's Disease • Wolf-Hirschhorn Syndrome

Dealing with Genetic Disorders As genetic disorders are a part of the genetic make-up of an individual, the only way to cure them would be to correct the corresponding genetic defect. This can't be cured by medicines. As of now it is mostly medicines that physicians resort to, to treat the symptoms of genetic disorders.

Depending upon the symptoms, therapeutic treatments are also used to manage the disabilities caused by such disorders. However, scientists all over the world are trying to find ways of finding cures for genetic disorders. For this, the first requirement was to know each and every gene present in the human genome and the sequence of the nucleotide bases that code for the amino acids that make up proteins and enzymes.

The Human Genome Project was started with the very same aim in 1990. Its goal was to identify and map all the 20,000 - 25,000 genes of the human genome and to determine the sequence of base pairs which make up DNA.

Other promising fields are genetic engineering, recombinant DNA technology, genetic modification/manipulation (GM) and gene splicing that involve direct manipulation with an organism's genes. It's essential to know about the rare genetic diseases and disorders, and about the inherited genetic diseases as well.

While some inherit genetic defects from their parents, there are others who suffer from one of the various genetic disorders, due to spontaneous mutation that takes place during embryonic development. Scientists are constantly trying to find cures for various genetic disorders that are found in people. Although commendable headway has been made, more research is required to enable us to deal with genetic diseases and disorders