Genetics is the study of genes, and tries to explain what they are and how they work. Genes are how living organisms inherit features from their ancestors; for example, children usually look like their parents because they have inherited their parents' genes. Genetics tries to identify which features are inherited, and explain how these features are passed from generation to generation.
In genetics, a feature of a living thing is called a "trait". Some traits are part of an organism's physical appearance; such as a person's eye-color, height or weight. Other sorts of traits are not easily seen and include blood types or resistance to diseases. The way our genes and environment interact to produce a trait can be complicated. For example, the chances of somebody dying of cancer or heart disease seems to depend on both their genes and their lifestyle.
Some traits are inherited through our genes, so tall and thin people tend to have tall and thin children; such traits which result due to inheritance alone are called genotypes. Other traits come from interactions between our genes and the environment, so a child might inherit the tendency to be tall, but if they are poorly nourished, they will still be short; such traits which are manifested due to the combined action of inherited genes and environmental circumstances are called phenotypes.
Genes are made from a long molecule called DNA, which is copied and inherited across generations. DNA is made of simple units that line up in a particular order within this large molecule. The order of these units carries genetic information, similar to how the order of letters on a page carries information. The language used by DNA is called the genetic code, which lets organisms read the information in the genes. This information is the instructions for constructing and operating a living organism.
The information within a particular gene is not always exactly the same between one organism and another, so different copies of a gene do not always give exactly the same instructions. Each unique form of a single gene is called an allele. As an example, one allele for the gene for hair color could instruct the body to produce a lot of pigment, producing black hair, while a different allele of the same gene might give garbled instructions that fail to produce any pigment, giving white hair. Mutations are random changes in genes, and can create new alleles. Mutations can also produce new traits, such as when mutations to an allele for black hair produce a new allele for white hair. This appearance of new traits is important in evolution.
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Here are some examples of genetic disorders :
DiGeorge syndrome
22q11.2 deletion syndrome, which has several presentations including DiGeorge syndrome (DGS), DiGeorge anomaly,[2][3] velo-cardio-facial syndrome, Shprintzen syndrome, conotruncal anomaly face syndrome, Strong syndrome, congenital thymic aplasia, and thymic hypoplasia is a syndrome caused by the deletion of a small piece of chromosome 22. The deletion occurs near the middle of the chromosome at a location designated q11.2 i.e., on the long arm of one of the pair of chromosomes 22, on region 1, band 1 , sub-band 2. It has a prevalence estimated at 1:4000.[4] The syndrome was described in 1968 by the pediatric endocrinologist Angelo DiGeorge.[5][6]
The syndrome is caused by genetic deletions (loss of a small part of the genetic material) found on the long arm of one of the two 22nd chromosomes. Very rarely, patients with somewhat similar clinical features may have deletions on the short arm of chromosome 10.
The mechanism that causes all of the associated features of the syndrome is unknown. 22q11.2 deletion syndrome may involve migration defects of neural crest-derived tissues, particularly affecting development of the third and fourth branchial pouches (pharyngeal pouches). This affects the thymus gland; a mediastinal organ largely responsible for differentiation and induction of tolerance in T-cells, and the Parathyroid glands, responsible for regulation of blood calcium levels.[11]
There is no cure for 22q11.2 deletion syndrome. Certain individual features are treatable using standard treatments. The key is to identify each of the associated features and manage each using the best available treatments.
For example, in children it is important that the immune problems are identified early as special precautions are required regarding blood transfusion and immunisation with live vaccines. Thymus transplantationcan be used to address absence of the thymus in the rare, so-called "complete" DiGeorge syndrome.[12] Bacterial infections are treated with antibiotics. Cardiac surgery is often required for congenital heart abnormalities. Hypoparathyroidism causing hypocalcaemia often requires lifelong vitamin D and calcium supplements
Angelman syndrome
Angelman syndrome
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Angelman syndrome (AS) is a neuro-genetic disorder characterized by intellectual and developmental delay, sleep disturbance, seizures, jerky movements (especially hand-flapping), frequent laughter or smiling, and usually a happy demeanor.
AS is a classic example of genomic imprinting in that it is usually caused by deletion or inactivation of genes on the maternally inherited chromosome 15 while the paternal copy, which may be of normal sequence, is imprinted and therefore silenced. The sister syndrome, Prader-Willi syndrome, is caused by a similar loss of paternally inherited genes and maternal imprinting. AS is named after a British pediatrician, Dr. Harry Angelman, who first described the syndrome in 1965.[1] An older, alternative term for AS, happy puppet syndrome, is generally considered pejorative and stigmatizing so it is no longer the accepted term, though it is sometimes still used as an informal term of diagnosis. People with AS are sometimes known as "angels", both because of the syndrome's name and because of their youthful, happy appearance.
Dr. Harry Angelman, a pediatrician working in Warrington, England, first reported three children with this condition in 1965.[1] Angelman later described his choice of the title "Puppet Children" to describe these cases as being related to an oil painting he had seen while vacationing in Italy:
The history of medicine is full of interesting stories about the discovery of illnesses. The saga of Angelman's syndrome is one such story. It was purely by chance that nearly thirty years ago (e.g., circa 1964) three handicapped children were admitted at various times to my children's ward in England. They had a variety of disabilities and although at first sight they seemed to be suffering from different conditions I felt that there was a common cause for their illness. The diagnosis was purely a clinical one because in spite of technical investigations which today are more refined I was unable to establish scientific proof that the three children all had the same handicap. In view of this I hesitated to write about them in the medical journals. However, when on holiday in Italy I happened to see an oil painting in the Castelvecchio Museum in Verona called . . . a Boy with a Puppet. The boy's laughing face and the fact that my patients exhibited jerky movements gave me the idea of writing an article about the three children with a title of Puppet Children. It was not a name that pleased all parents but it served as a means of combining the three little patients into a single group. Later the name was changed to Angelman syndrome. This article was published in 1965 and after some initial interest lay almost forgotten until the early eighties.
—Angelman quoted by Charles Williams[2]
Case reports from the United States first began appearing in the medical literature in the early 1980s.[3][4] In 1987, it was first noted that around half of the children with AS have a small piece of chromosome 15 missing (chromosome 15q partial deletion).[5]
Coeliac disease
Coeliac disease (
/ˈsiːli.æk/; spelled celiac disease in North America[1] and often celiac sprue) is an autoimmune disorder of the small intestine that occurs ingenetically predisposed people of all ages from middle infancy onward. Symptoms include chronic diarrhoea, failure to thrive (in children), and fatigue, but these may be absent, and symptoms in other organ systems have been described.
A growing portion of diagnoses are being made in asymptomatic persons as a result of increased screening;[2] the condition is thought to affect between 1 in 1,750 and 1 in 105 people in the United States.[3] Coeliac disease is caused by a reaction to gliadin, a prolamin (gluten protein) found in wheat, and similar proteins found in the crops of the tribe Triticeae (which includes other common grains such as barley and rye). Upon exposure to gliadin, and specifically to three peptides found in prolamins, the enzyme tissue transglutaminase modifies the protein, and the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction. That leads to a truncating of the villi lining the small intestine (called villous atrophy). This interferes with the absorption of nutrients, because the intestinal villiare responsible for absorption. The only known effective treatment is a lifelong gluten-free diet.[4] While the disease is caused by a reaction to wheat proteins, it is not the same as wheat allergy.
This condition has several other names, including: cœliac disease (with œ ligature), c(o)eliac sprue, non-tropical sprue, endemic sprue, gluten enteropathy or gluten-sensitive enteropathy, and gluten intolerance. The term coeliac derives from the Greek κοιλιακός (koiliakós, "abdominal"), and was introduced in the 19th century in a translation of what is generally regarded as an ancient Greek description of the disease by Aretaeus of Cappadocia.[5][6]
Charcot–Marie–Tooth disease
Charcot–Marie–Tooth disease- (CMT), known also as Morbus Charcot-Marie-Tooth, Charcot-Marie-Tooth neuropathy, hereditary motor and sensory neuropathy(HMSN), hereditary sensorimotor neuropathy (HSMN), or peroneal muscular atrophy, is an inherited disorder of nerves (neuropathy) that takes different forms. It is predominantly characterized by loss of muscle tissue and touch sensation, in the feet, ankles and legs as it progresses over time, but also in the hands, wrists and arms in various types of the disease. Early and late onset forms occur with 'on and off' painful spasmodic muscular contractions that can be disabling when the disease activates. High arched 'Cavus Feet' are associated with the disorder. Sensory and positioning nerves in the hands and feet are often damaged, while pain nerves are left intact. Overuse of an affected hand or limb can activate symptoms ranging from numbness, to spasm, to very painful cramping. Currently incurable, this disease is one of the most common inherited neurological disorders, and was considered a type of Muscular Dystrophy for years. [1]
Estimates of incidence vary widely from 1 in 380,000 people affected [1] to 1 in 2,500 people affected [1]. This larger figure might equate to approximately 23,000 people in the UK and 125,000 people in the USA.
Signs and symptoms
Symptoms of CMT usually begin in late childhood or early adulthood. Some people don't experience symptoms until their early thirties or forties. Usually, the initial symptom is foot drop early in the course of the disease. This can also cause claw toe, where the toes are always curled. Wasting of muscle tissue of the lower parts of the legs may give rise to "stork leg" or "inverted bottle" appearance. Weakness in the hands and forearms occurs in many people later in life as the disease progresses.
Symptoms and progression of the disease can vary. Breathing can be affected in some; so can hearing, vision, as well as the neck and shoulder muscles. Scoliosis is common. Hip sockets can be malformed. Gastrointestinal problems can be part of CMT, as can chewing, swallowing, and speaking (as vocal cords atrophy). A tremor can develop as muscles waste. Pregnancy has been known to exacerbate CMT, as well as extreme emotional stress. Patients with CMT must avoid periods of prolonged immobility such as when recovering from a secondary injury as prolonged periods of limited mobility can drastically accelerate symptoms of CMT.[2]
Neuropathic pain is often a symptom of CMT, though, like other symptoms of CMT, its presence and severity varies from case to case. For some people, pain can be significant to severe and interfere with daily life activities. However, pain is not experienced by all people with CMT. When pain is present as a symptom of CMT, it is comparable to that seen in other peripheral neuropathies, as well as Postherpetic neuralgiaand Complex regional pain syndrome, among other diseases.[3]
Causes
Charcot–Marie–Tooth disease is caused by mutations that cause defects in neuronal proteins. Nerve signals are conducted by an axon with a myelin sheath wrapped around it. Most mutations in CMT affect the myelin sheath. Some affect the axon.
The most common cause of CMT (70-80% of the cases) is the duplication of a large region in chromosome 17p12 that includes the gene PMP22. Some mutations affect the gene MFN2, which codes for a mitochondrial protein. Cells contain separate sets of genes in their nucleus and in their mitochondria. In nerve cells, the mitochondria travel down the long axons. In some forms of CMT, mutated MFN2 causes the mitochondria to form large clusters, or clots, which are unable to travel down the axon towards the synapses. This prevents the synapses from functioning.[4]
CMT is divided into the primary demyelinating neuropathies (CMT1, CMT3, and CMT4) and the primary axonal neuropathies (CMT2), with frequent overlap. Another cell involved in CMT is the Schwann cell, which creates the myelin sheath, by wrapping its plasma membrane around the axon in a structure that is sometimes compared to a Swiss roll.[5]
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired. "Color blind" is a term of art; there is no actual blindness but there is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. Color blindness is a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a good gene on only one of the two X chromosomes is enough to yield the needed photopigments).[1]
The symptoms of color blindness also can be produced by physical or chemical damage to the eye, optic nerve, or the brain generally. These are not true color blindness; however, they represent conditions of limited actual blindness. Similarly, a person with achromatopsia, although unable to see colors, is not "color blind" per se but they suffer from a completely different disorder, of which an atypical color deficiency is only one manifestation.
The English chemist John Dalton published the first scientific paper on this subject in 1798, "Extraordinary facts relating to the vision of colours",[2] after the realization of his own color blindness. Because of Dalton's work, the condition was often called daltonism, although in English this term is now used for a single type of color blindness, called deuteranopia. Many other languages, however, continue to use words based on his name for general color blindness.
Color blindness is usually classed as a mild disability but there are situations where color blind individuals can have an advantage over those with normal color vision. Some studies conclude that color blind individuals are better at penetrating certain color camouflages; this may be an evolutionary explanation for the surprisingly high frequency of congenital red–green color blindness.[3]
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Angelman syndrome (AS) is a neuro-genetic disorder characterized by intellectual and developmental delay, sleep disturbance, seizures, jerky movements (especially hand-flapping), frequent laughter or smiling, and usually a happy demeanor.
AS is a classic example of genomic imprinting in that it is usually caused by deletion or inactivation of genes on the maternally inherited chromosome 15 while the paternal copy, which may be of normal sequence, is imprinted and therefore silenced. The sister syndrome, Prader-Willi syndrome, is caused by a similar loss of paternally inherited genes and maternal imprinting. AS is named after a British pediatrician, Dr. Harry Angelman, who first described the syndrome in 1965.[1] An older, alternative term for AS, happy puppet syndrome, is generally considered pejorative and stigmatizing so it is no longer the accepted term, though it is sometimes still used as an informal term of diagnosis. People with AS are sometimes known as "angels", both because of the syndrome's name and because of their youthful, happy appearance.
Dr. Harry Angelman, a pediatrician working in Warrington, England, first reported three children with this condition in 1965.[1] Angelman later described his choice of the title "Puppet Children" to describe these cases as being related to an oil painting he had seen while vacationing in Italy:
The history of medicine is full of interesting stories about the discovery of illnesses. The saga of Angelman's syndrome is one such story. It was purely by chance that nearly thirty years ago (e.g., circa 1964) three handicapped children were admitted at various times to my children's ward in England. They had a variety of disabilities and although at first sight they seemed to be suffering from different conditions I felt that there was a common cause for their illness. The diagnosis was purely a clinical one because in spite of technical investigations which today are more refined I was unable to establish scientific proof that the three children all had the same handicap. In view of this I hesitated to write about them in the medical journals. However, when on holiday in Italy I happened to see an oil painting in the Castelvecchio Museum in Verona called . . . a Boy with a Puppet. The boy's laughing face and the fact that my patients exhibited jerky movements gave me the idea of writing an article about the three children with a title of Puppet Children. It was not a name that pleased all parents but it served as a means of combining the three little patients into a single group. Later the name was changed to Angelman syndrome. This article was published in 1965 and after some initial interest lay almost forgotten until the early eighties.—Angelman quoted by Charles Williams[2]
Case reports from the United States first began appearing in the medical literature in the early 1980s.[3][4] In 1987, it was first noted that around half of the children with AS have a small piece of chromosome 15 missing (chromosome 15q partial deletion).[5]
Coeliac disease
Coeliac disease ( /ˈsiːli.æk/; spelled celiac disease in North America[1] and often celiac sprue) is an autoimmune disorder of the small intestine that occurs ingenetically predisposed people of all ages from middle infancy onward. Symptoms include chronic diarrhoea, failure to thrive (in children), and fatigue, but these may be absent, and symptoms in other organ systems have been described.
/ˈsiːli.æk/; spelled celiac disease in North America[1] and often celiac sprue) is an autoimmune disorder of the small intestine that occurs ingenetically predisposed people of all ages from middle infancy onward. Symptoms include chronic diarrhoea, failure to thrive (in children), and fatigue, but these may be absent, and symptoms in other organ systems have been described.
A growing portion of diagnoses are being made in asymptomatic persons as a result of increased screening;[2] the condition is thought to affect between 1 in 1,750 and 1 in 105 people in the United States.[3] Coeliac disease is caused by a reaction to gliadin, a prolamin (gluten protein) found in wheat, and similar proteins found in the crops of the tribe Triticeae (which includes other common grains such as barley and rye). Upon exposure to gliadin, and specifically to three peptides found in prolamins, the enzyme tissue transglutaminase modifies the protein, and the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction. That leads to a truncating of the villi lining the small intestine (called villous atrophy). This interferes with the absorption of nutrients, because the intestinal villiare responsible for absorption. The only known effective treatment is a lifelong gluten-free diet.[4] While the disease is caused by a reaction to wheat proteins, it is not the same as wheat allergy.
This condition has several other names, including: cœliac disease (with œ ligature), c(o)eliac sprue, non-tropical sprue, endemic sprue, gluten enteropathy or gluten-sensitive enteropathy, and gluten intolerance. The term coeliac derives from the Greek κοιλιακός (koiliakós, "abdominal"), and was introduced in the 19th century in a translation of what is generally regarded as an ancient Greek description of the disease by Aretaeus of Cappadocia.[5][6]
Charcot–Marie–Tooth disease
Charcot–Marie–Tooth disease- (CMT), known also as Morbus Charcot-Marie-Tooth, Charcot-Marie-Tooth neuropathy, hereditary motor and sensory neuropathy(HMSN), hereditary sensorimotor neuropathy (HSMN), or peroneal muscular atrophy, is an inherited disorder of nerves (neuropathy) that takes different forms. It is predominantly characterized by loss of muscle tissue and touch sensation, in the feet, ankles and legs as it progresses over time, but also in the hands, wrists and arms in various types of the disease. Early and late onset forms occur with 'on and off' painful spasmodic muscular contractions that can be disabling when the disease activates. High arched 'Cavus Feet' are associated with the disorder. Sensory and positioning nerves in the hands and feet are often damaged, while pain nerves are left intact. Overuse of an affected hand or limb can activate symptoms ranging from numbness, to spasm, to very painful cramping. Currently incurable, this disease is one of the most common inherited neurological disorders, and was considered a type of Muscular Dystrophy for years. [1]
Estimates of incidence vary widely from 1 in 380,000 people affected [1] to 1 in 2,500 people affected [1]. This larger figure might equate to approximately 23,000 people in the UK and 125,000 people in the USA.
Signs and symptoms
Symptoms of CMT usually begin in late childhood or early adulthood. Some people don't experience symptoms until their early thirties or forties. Usually, the initial symptom is foot drop early in the course of the disease. This can also cause claw toe, where the toes are always curled. Wasting of muscle tissue of the lower parts of the legs may give rise to "stork leg" or "inverted bottle" appearance. Weakness in the hands and forearms occurs in many people later in life as the disease progresses.
Symptoms and progression of the disease can vary. Breathing can be affected in some; so can hearing, vision, as well as the neck and shoulder muscles. Scoliosis is common. Hip sockets can be malformed. Gastrointestinal problems can be part of CMT, as can chewing, swallowing, and speaking (as vocal cords atrophy). A tremor can develop as muscles waste. Pregnancy has been known to exacerbate CMT, as well as extreme emotional stress. Patients with CMT must avoid periods of prolonged immobility such as when recovering from a secondary injury as prolonged periods of limited mobility can drastically accelerate symptoms of CMT.[2]
Neuropathic pain is often a symptom of CMT, though, like other symptoms of CMT, its presence and severity varies from case to case. For some people, pain can be significant to severe and interfere with daily life activities. However, pain is not experienced by all people with CMT. When pain is present as a symptom of CMT, it is comparable to that seen in other peripheral neuropathies, as well as Postherpetic neuralgiaand Complex regional pain syndrome, among other diseases.[3]
Causes
Charcot–Marie–Tooth disease is caused by mutations that cause defects in neuronal proteins. Nerve signals are conducted by an axon with a myelin sheath wrapped around it. Most mutations in CMT affect the myelin sheath. Some affect the axon.
The most common cause of CMT (70-80% of the cases) is the duplication of a large region in chromosome 17p12 that includes the gene PMP22. Some mutations affect the gene MFN2, which codes for a mitochondrial protein. Cells contain separate sets of genes in their nucleus and in their mitochondria. In nerve cells, the mitochondria travel down the long axons. In some forms of CMT, mutated MFN2 causes the mitochondria to form large clusters, or clots, which are unable to travel down the axon towards the synapses. This prevents the synapses from functioning.[4]
CMT is divided into the primary demyelinating neuropathies (CMT1, CMT3, and CMT4) and the primary axonal neuropathies (CMT2), with frequent overlap. Another cell involved in CMT is the Schwann cell, which creates the myelin sheath, by wrapping its plasma membrane around the axon in a structure that is sometimes compared to a Swiss roll.[5]
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired. "Color blind" is a term of art; there is no actual blindness but there is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. Color blindness is a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome (in females, a good gene on only one of the two X chromosomes is enough to yield the needed photopigments).[1]
The symptoms of color blindness also can be produced by physical or chemical damage to the eye, optic nerve, or the brain generally. These are not true color blindness; however, they represent conditions of limited actual blindness. Similarly, a person with achromatopsia, although unable to see colors, is not "color blind" per se but they suffer from a completely different disorder, of which an atypical color deficiency is only one manifestation.
The English chemist John Dalton published the first scientific paper on this subject in 1798, "Extraordinary facts relating to the vision of colours",[2] after the realization of his own color blindness. Because of Dalton's work, the condition was often called daltonism, although in English this term is now used for a single type of color blindness, called deuteranopia. Many other languages, however, continue to use words based on his name for general color blindness.
Color blindness is usually classed as a mild disability but there are situations where color blind individuals can have an advantage over those with normal color vision. Some studies conclude that color blind individuals are better at penetrating certain color camouflages; this may be an evolutionary explanation for the surprisingly high frequency of congenital red–green color blindness.[3]
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