A genetic disorder is a condition caused by abnormalities in genes or chromosomes. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance A chromosome is an organized structure of DNA and Protein that is found in cells. While some diseases, such as cancer, are due to genetic abnormalities acquired in a few cells during life, the term "genetic disease" most commonly refers to diseases present in all cells of the body and present since conception. Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled For soil improvement see Fertilization (soil. Some genetic disorders are caused by chromosomal abnormalities due to errors in meiosis, the process which produces reproductive cells such as sperm and eggs. A chromosome is an organized structure of DNA and Protein that is found in cells. In Biology or life science meiosis (pronounced my-oh-sis is a process of reductional division in which the number of chromosomes per cell is cut in half The term sperm is derived from the Greek word (σπέρμα sperma (meaning "seed" and refers to the male reproductive cells. An ovum (plural ova) is a Haploid Female reproductive cell or Gamete. Examples include Down syndrome (extra chromosome 21), Turner Syndrome (45X0) and Klinefelter's syndrome (a male with 2 X chromosomes). Down syndrome, Down's syndrome, or trisomy 21 is a Chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. Turner syndrome or Ullrich-Turner syndrome encompasses several conditions of which monosomy X is the most common Klinefelter's syndrome, 47XXY or XXY syndrome is a condition caused by a Chromosome Aneuploidy. Other genetic changes may occur during the production of germ cells by the parent. One example is the triplet expansion repeat mutations which can cause fragile X syndrome or Huntington's disease. Fragile X syndrome, or Martin-Bell syndrome, is a genetic Syndrome which results in a spectrum (from none to severe of characteristic physical intellectual emotional Huntington's disease, also called Huntington's chorea, chorea major, or HD, is a genetic neurological disorder characterized after Defective genes may also be inherited intact from the parents. This can often happen unexpectedly when two healthy carriers of a defective recessive gene reproduce, but can also happen when the defective gene is dominant. Currently about 4,000 genetic disorders are known, with more being discovered. Most disorders are quite rare and affect one person in every several thousands or millions. Cystic fibrosis is one of the most common genetic disorders; around 5% of the population of the United States carry at least one copy of the defective gene. Cystic fibrosis (also known as CF, mucoviscoidosis, or mucoviscidosis) is a hereditary disease affecting the exocrine (mucus glands of the lungs The United States of America —commonly referred to as the Some types of recessive gene disorder confer an advantage in the heterozygous state in certain environments. Zygosity refers to the genetic condition of a Zygote. In genetics zygosity describes the similarity or dissimilarity of DNA between Homologous ^[1]

Genetic diseases are typically diagnosed and treated by geneticists. A geneticist is a scientist who studies Genetics, the science of Heredity and variation of Organisms A geneticist can be employed as a researcher Genetic counselors assist the physicians and directly counsel patients. Genetic counseling is the process by which patients or relatives at risk of an inherited disorder are advised of the consequences and nature of the disorder the probability of developing A physician, medical practitioner or medical doctor who practices Medicine, and is concerned with maintaining or restoring human Health The study of genetic diseases is a scientific discipline whose theoretical underpinning is based on population genetics. Population genetics is the study of the Allele frequency distribution and change under the influence of the four evolutionary forces Natural selection, Genetic

Contents 1 Single gene disorders 1.1 Autosomal dominant 1.2 Autosomal recessive 1.3 X-linked dominant 1.4 X-linked recessive 1.5 Y-linked 1.6 Mitochondrial 2 Multifactorial and polygenic disorders 3 See also 4 References 5 External links

Single gene disorders

Where genetics are the result of a single mutated gene they can be passed on to subsequent generations in several ways. Genomic imprinting and uniparental disomy, however, may affect inheritance patterns. Genomic imprinting is a genetic phenomenon by which certain Genes are expressed in a Parent -of-origin-specific manner Uniparental disomy ( UPD) occurs when a person receives two copies of a Chromosome, or part of a chromosome from one parent and no copies from the other parent The divisions between recessive and dominant are not "hard and fast" although the divisions between autosomal and X-linked are (related to the position of the gene). For example, achondroplasia is typically considered a dominant disorder, but children with two genes for achondroplasia have a severe skeletal disorder that achondroplasics could be viewed as carriers of. Achondroplasia is a type of Autosomal dominant Genetic disorder that is a common cause of Dwarfism. Sickle-cell anemia is also considered a recessive condition, but carriers that have it by half along with the normal gene have increased immunity to malaria in early childhood, which could be described as a related dominant condition. Sickle-cell disease or sickle-cell anaemia (or anemia) is a Blood disorder characterized by Red blood cells that assume an abnormal rigid

Autosomal dominant

Main article: Autosomal dominant

Only one mutated copy of the gene will be necessary for a person to be affected by an autosomal dominant disorder. Each affected person usually has one affected parent. There is a 50% chance that a child will inherit the mutated gene. Conditions that are autosomal dominant have low penetrance, which means that, although only one mutated copy is needed, a relatively small proportion of those who inherit that mutation go on to develop the disease, often later in life. Penetrance is a term used in Genetics describing the proportion of individuals carrying a particular variation of a Gene (an Allele or genotype that also Examples of this type of disorder are Huntington's disease, Neurofibromatosis 1, Marfan Syndrome, Hereditary nonpolyposis colorectal cancer, and Hereditary multiple exostoses,which is a high penetrance autosomal dominant disorder. Huntington's disease, also called Huntington's chorea, chorea major, or HD, is a genetic neurological disorder characterized after Neurofibromatosis type I ( NF-1) also known as '''von Recklinghausen''' disease, is a rare inherited disease Marfan syndrome (or Marfan's syndrome is a genetic disorder of the Connective tissue. Hereditary nonpolyposis colorectal cancer (HNPCC also known as Lynch syndrome, is characterised by a risk of Colorectal cancer and other cancers of the endometrium Hereditary multiple exostoses ( HME) is a rare medical condition in which multiple bony spurs or lumps (also known as exostoses, or Osteochondromas develop

Autosomal recessive

Main article: Autosomal recessive

Two copies of the gene must be mutated for a person to be affected by an autosomal recessive disorder. An affected person usually has unaffected parents who each carry a single copy of the mutated gene (and are referred to as carriers). Two unaffected people who each carry one copy of the mutated gene have a 25% chance with each pregnancy of having a child affected by the disorder. Examples of this type of disorder are Cystic fibrosis, Sickle cell anemia(Also Partial Sickle Cell Anemia), Tay-Sachs disease, Spinal muscular atrophy, and Dry (otherwise known as "rice-brand") earwax^[2]

X-linked dominant

Main article: X-linked dominant

X-linked dominant disorders are caused by mutations in genes on the X chromosome. Cystic fibrosis (also known as CF, mucoviscoidosis, or mucoviscidosis) is a hereditary disease affecting the exocrine (mucus glands of the lungs Sickle-cell disease or sickle-cell anaemia (or anemia) is a Blood disorder characterized by Red blood cells that assume an abnormal rigid Tay-Sachs disease (abbreviated TSD, also known as GM2 Gangliosidosis, Hexosaminidase A deficiency or Sphingolipidosis) is a Genetic Spinal Muscular Atrophy ( SMA) is a term applied to a number of different disorders all having in common a genetic cause and the manifestation of weakness due to loss Earwax, also known by the Medical term cerumen, is a yellowish waxy substance secreted in the Ear canal of Humans and many other X-linked dominant is mode of inheritance in which a Gene on the X chromosome is Dominant. The X chromosome is one of the two sex-determining Chromosomes in many animal species including mammals (the other is the Y chromosome) Only a few disorders have this inheritance pattern. Males are more frequently affected than females, and the chance of passing on an X-linked dominant disorder differs between men and women. The sons of a man with an X-linked dominant disorder will not be affected, and his daughters will all inherit the condition. A woman with an X-linked dominant disorder has a 50% chance of having an affected daughter or son with each pregnancy. Some X-linked dominant conditions, such as Aicardi Syndrome, are fatal to boys, therefore only girls have them (and boys with Klinefelter Syndrome). Aicardi syndrome is a rare genetic malformation syndrome characterized by the partial or complete absence of a key structure in the brain called the corpus callosum Klinefelter's syndrome, 47XXY or XXY syndrome is a condition caused by a Chromosome Aneuploidy. Other examples of this type of disorder are Hypophosphatemia, Aicardi Syndrome, and Chokenflok Syndrome. Hypophosphatemia is an Electrolyte disturbance in which there is an abnormally low level of Phosphate in the Blood. Aicardi syndrome is a rare genetic malformation syndrome characterized by the partial or complete absence of a key structure in the brain called the corpus callosum

X-linked recessive

Main article: X-linked recessive

X-linked recessive disorders are also caused by mutations in genes on the X chromosome. X-linked recessive is a mode of inheritance in which a mutation in a Gene on the X chromosome causes the phenotype to be expressed (1 in males (who are Males are more frequently affected than females, and the chance of passing on the disorder differs between men and women. The sons of a man with an X-linked recessive disorder will not be affected, and his daughters will carry one copy of the mutated gene. With each pregnancy, a woman who carries an X-linked recessive disorder has a 50% chance of having sons who are affected and a 50% chance of having daughters who carry one copy of the mutated gene. Examples of this type of disorder Hemophilia A, Duchenne muscular dystrophy, Color blindness, Muscular dystrophy and Androgenetic alopecia. Haemophilia A (also spelled Hemophilia A or Hæmophilia A) is a Blood clotting disorder caused by a mutation of the Factor VIII gene Duchenne muscular dystrophy ( DMD) is a severe recessive x-linked form of Muscular dystrophy that is characterized by rapid progression of muscle degeneration eventually Color blindness, a Color vision deficiency is the inability to perceive differences between some of the Colors that others can distinguish Muscular dystrophy (MD refers to a group of genetic, Hereditary Muscle diseases that cause progressive muscle weakness Androgenic Alopecia (also known as androgenetic alopecia or alopecia androgenetica) is a common form of hair loss in both male and female

Y-linked

Main article: Y linkage

Y-linked disorders are caused by mutations on the Y chromosome. In mammals Y-linkage refers to when a phenotypic trait is determined by an allele (or gene on the Y chromosome. Only males can get them, and all of the sons of an affected father are affected. Since the Y chromosome is very small, Y-linked disorders only cause infertility, and may be circumvented with the help of some fertility treatments. Examples are Male Infertility. Female infertility|Male infertility Infertility primarily refers to the biological inability of a Man or a Woman to contribute to conception.

Mitochondrial

Main article: Mitochondrial disease

This type of inheritance, also known as maternal inheritance, applies to genes in mitochondrial DNA. Mitochondrial diseases are a group of disorders relating to the mitochondria, the Organelles that are the "powerhouses" of the eukaryotic cells Mitochondrial DNA ( mtDNA) is the DNA located in Organelles called mitochondria. Because only egg cells contribute mitochondria to the developing embryo, only females can pass on mitochondrial conditions to their children. Examples of this type of disorder are Human mitochondrial genetics, and Leber's Hereditary Optic Neuropathy. Mitochondrial genetics is the study of the Genetics of the DNA contained in Mitochondria. Leber’s hereditary optic neuropathy (LHON or Leber optic atrophy is a mitochondrially inherited (mother to all offspring degeneration of Retinal

Multifactorial and polygenic disorders

Genetic disorders may also be complex, multifactorial or polygenic, this means that they are likely associated with the effects of multiple genes in combination with lifestyle and environmental factors. Multifactoral disorders include heart disease and diabetes. Heart disease is an Umbrella term for a variety for different diseases affecting the Heart. Diabetes mellitus (ˌdaɪəˈbiːtiːz or /ˌdaɪəˈbiːtəs/ /məˈlaɪtəs/ or /ˈmɛlətəs/ often referred to simply as diabetes ( Ancient Greek: grc Although complex disorders often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of inheriting or passing on these disorders. Complex disorders are also difficult to study and treat because the specific factors that cause most of these disorders have not yet been identified.

On a pedigree, polygenic diseases do tend to “run in families”, but the inheritance does not fit simple patterns as with Mendelian diseases. But this does not mean that the genes cannot eventually be located and studied. There is also a strong environmental component to many of them (e. g. , blood pressure).

• autism
• heart disease
• hypertension
• diabetes
• obesity
• cancers
• cleft palate
• Mental retardation

See also

• Genetic epidemiology
• Inborn errors of metabolism
• List of genetic disorders
• Medical genetics

References

External links

• HOPES - A layperson's guide to Huntington's Disease
• CDC’s National Center on Birth Defects and Developmental Disabilities
• Genetic Disease Information from the Human Genome Project
• ColonCancerFamily.com FREE Newsletter: topics include Lynch Syndrome, HNPCC, Genetic Testing and more
• Genes and Disease from the Wellcome Trust
• OMIM - Online Mendelian Inheritance in Man, a catalog of human genes and genetic disorders
• Genetic Alliance

The Wellcome Trust was established in 1936 as an independent charity funding research to improve human and animal health

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