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A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosome abnormality. Although polygenic disorders are the most common, the term is mostly used when discussing disorders with a single genetic cause, either in a gene or chromosome. The mutation responsible can occur spontaneously before embryonic development, or it can be inherited from two parents who are carriers of a faulty gene or from a parent with the disorder. When the genetic disorder is inherited from one or both parents, it is also classified as a hereditary disease. Some disorders are caused by a mutation on the X chromosome and have X-linked inheritance. Very few disorders are inherited on the Y chromosome or mitochondrial DNA.
The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific gene depends on the number of copies of each chromosome found in that species, also referred to as ploidy. In diploid species like humans, two full sets of chromosomes are present, meaning each individual has two alleles for any given gene. If both alleles are the same, the genotype is referred to as homozygous. If the alleles are different, the genotype is referred to as heterozygous.
Mendelian inheritance is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized by William Bateson. These principles were initially controversial. When Mendel's theories were integrated with the Boveri–Sutton chromosome theory of inheritance by Thomas Hunt Morgan in 1915, they became the core of classical genetics. Ronald Fisher combined these ideas with the theory of natural selection in his 1930 book The Genetical Theory of Natural Selection, putting evolution onto a mathematical footing and forming the basis for population genetics within the modern evolutionary synthesis.
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and the second is called recessive. This state of having two different variants of the same gene on each chromosome is originally caused by a mutation in one of the genes, either new or inherited. The terms autosomal dominant or autosomal recessive are used to describe gene variants on non-sex chromosomes (autosomes) and their associated traits, while those on sex chromosomes (allosomes) are termed X-linked dominant, X-linked recessive or Y-linked; these have an inheritance and presentation pattern that depends on the sex of both the parent and the child. Since there is only one copy of the Y chromosome, Y-linked traits cannot be dominant or recessive. Additionally, there are other forms of dominance, such as incomplete dominance, in which a gene variant has a partial effect compared to when it is present on both chromosomes, and co-dominance, in which different variants on each chromosome both show their associated traits.
A quantitative trait locus (QTL) is a locus that correlates with variation of a quantitative trait in the phenotype of a population of organisms. QTLs are mapped by identifying which molecular markers correlate with an observed trait. This is often an early step in identifying the actual genes that cause the trait variation.
Online Mendelian Inheritance in Man (OMIM) is a continuously updated catalog of human genes and genetic disorders and traits, with a particular focus on the gene-phenotype relationship. As of 28 June 2019, approximately 9,000 of the over 25,000 entries in OMIM represented phenotypes; the rest represented genes, many of which were related to known phenotypes.
Aniridia is the absence of the iris, a muscular structure that opens and closes the pupil to allow light into the eye. It is also responsible for eye color. Without it, the central eye appears all black. It can be congenital, in which both eyes are usually involved, or caused by a penetrant injury. Isolated aniridia is a congenital disorder that is not limited to a defect in iris development, but is a panocular condition with macular and optic nerve hypoplasia, cataract, and corneal changes. Vision may be severely compromised and the disorder is frequently associated with some ocular complications: nystagmus, amblyopia, buphthalmos, and cataract. Aniridia in some individuals occurs as part of a syndrome, such as WAGR syndrome, or Gillespie syndrome.
Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each parent contributes one of two possible alleles for a trait. If the genotypes of both parents in a genetic cross are known, Mendel's laws can be used to determine the distribution of phenotypes expected for the population of offspring. There are several situations in which the proportions of phenotypes observed in the progeny do not match the predicted values.
Human genetics is the study of inheritance as it occurs in human beings. Human genetics encompasses a variety of overlapping fields including: classical genetics, cytogenetics, molecular genetics, biochemical genetics, genomics, population genetics, developmental genetics, clinical genetics, and genetic counseling.
Periodic paralysis is a group of rare genetic diseases that lead to weakness or paralysis from common triggers such as cold, heat, high carbohydrate meals, not eating, stress or excitement and physical activity of any kind. The underlying mechanism of these diseases are malfunctions in the ion channels in skeletal muscle cell membranes that allow electrically charged ions to leak in or out of the muscle cell, causing the cell to depolarize and become unable to move.
Oculocutaneous albinism is a form of albinism involving the eyes, the skin, and the hair. Overall, an estimated 1 in 20,000 people worldwide are born with oculocutaneous albinism. OCA is caused by mutations in several genes that control the synthesis of melanin within the melanocytes. Seven types of oculocutaneous albinism have been described, all caused by a disruption of melanin synthesis and all autosomal recessive disorders. Oculocutaneous albinism is also found in non-human animals.
Autosomal recessive multiple epiphyseal dysplasia (ARMED), also called epiphyseal dysplasia, multiple, 4 (EDM4), multiple epiphyseal dysplasia with clubfoot or –with bilayered patellae, is an autosomal recessive congenital disorder affecting cartilage and bone development. The disorder has relatively mild signs and symptoms, including joint pain, scoliosis, and malformations of the hands, feet, and knees.
Cutis laxa or pachydermatocele is a group of rare connective tissue disorders in which the skin becomes inelastic and hangs loosely in folds.
Acromicric dysplasia is an extremely rare inherited disorder characterized by abnormally short hands and feet, growth retardation and delayed bone maturation leading to short stature. Most cases have occurred randomly for no apparent reason (sporadically). However, autosomal dominant inheritance has not been ruled out.
Thomas James Hudson, O.C., is a Canadian genome scientist noted for his leading role in the generation of physical maps of the human and mouse genomes and also his role in the International HapMap Project whose goal is to develop a haplotype map of the human genome.
The interleukin-23 receptor is a type I cytokine receptor. It is encoded in human by the IL23R gene. In complex with the interleukin-12 receptor β1 subunit (IL-12Rβ1), it is activated by the cytokine interleukin 23 (IL-23). The IL23R mRNA is 2.8 kilobases in length and includes 12 exons. The translated protein contains 629 amino acids; it is a type I penetrating protein and includes a signal peptide, an N-terminal fibronectin III-like domain and an intracellular part that contains three potential tyrosine phosphorylation domains. There are 24 IL23R splice variants in mitogen-activated lymphocytes. IL23R includes some single-nucleotide polymorphisms in the region encoding the domain that binds IL-23, which may lead to differences between people in Th17 activation. There is also a variant of IL-23R that consists of just the extracellular part and is known as soluble IL-23R. This form can compete with the membrane-bound form to bind IL-23, modulating the Th17 immune response and regulation of inflammation and immune function.
In genetics, a morbid map is a chart or diagram of diseases and the chromosomal location of genes the diseases are associated with. A morbid map exists as an appendix of the Online Mendelian Inheritance in Man (OMIM) knowledgebase, listing chromosomes and the genes mapped to specific sites on those chromosomes, and this format most clearly reveals the relationship between gene and phenotype.
Pontocerebellar hypoplasia (PCH) is a heterogeneous group of rare neurodegenerative disorders caused by genetic mutations and characterised by progressive atrophy of various parts of the brain such as the cerebellum or brainstem. Where known, these disorders are inherited in an autosomal recessive fashion. There is no known cure for PCH.
Oligogenic inheritance describes a trait that is influenced by a few genes. Oligogenic inheritance represents an intermediate between monogenic inheritance in which a trait is determined by a single causative gene, and polygenic inheritance, in which a trait is influenced by many genes and often environmental factors.
Mendelian traits behave according to the model of monogenic or simple gene inheritance in which one gene corresponds to one trait. Discrete traits with simple Mendelian inheritance patterns are relatively rare in nature, and many of the clearest examples in humans cause disorders. Discrete traits found in humans are common examples for teaching genetics.
References
- 1 2 Marla J. F. O'Neill (11 January 2022). "INFLAMMATORY BOWEL DISEASE 22; IBD22". Online Mendelian Inheritance in Man.
- ↑ "Entrez Gene: Inflammatory bowel disease-22" . Retrieved 2016-05-31.
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