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21-614: Pharmacogene Variation Consortium (abbreviated as PhamVar ) is an international group of experts that maintains a systematic nomenclature system for allelic variations of genes that affect the metabolism of drugs. The database is focused on cytochrome P450 enzymes, but is being expanded into other classes of enzymes. The original nomenclature was maintained by the Human CYP Allele Nomenclature Database. However PhamVar took over this function in 2017. Allele An allele , or allelomorph ,

42-476: A DNA polymerase when it encounters and bypasses a damaged base in the template strand. A DNA damage is an abnormal chemical structure in DNA, such as a strand break or an oxidized base, whereas a mutation, by contrast, is a change in the sequence of standard base pairs. Errors of repair occur when repair processes inaccurately replace a damaged DNA sequence. The DNA repair process microhomology-mediated end joining

63-431: A great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences. Wild type alleles are often denoted by a superscript plus sign ( i.e. , p for an allele p ). A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles ( polymorphism ) present, or

84-506: A process termed transgenerational epigenetic inheritance . The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence . A specific class of epiallele, the metastable epialleles , has been discovered in mice and in humans which is characterized by stochastic (probabilistic) establishment of epigenetic state that can be mitotically inherited. The term "idiomorph", from Greek 'morphos' (form) and 'idio' (singular, unique),

105-749: A single-gene trait. Recessive genetic disorders include albinism , cystic fibrosis , galactosemia , phenylketonuria (PKU), and Tay–Sachs disease . Other disorders are also due to recessive alleles, but because the gene locus is located on the X chromosome, so that males have only one copy (that is, they are hemizygous ), they are more frequent in males than in females. Examples include red–green color blindness and fragile X syndrome . Other disorders, such as Huntington's disease , occur when an individual inherits only one dominant allele. While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species,

126-424: Is Gregor Mendel 's discovery that the white and purple flower colors in pea plants were the result of a single gene with two alleles. Nearly all multicellular organisms have two sets of chromosomes at some point in their biological life cycle ; that is, they are diploid . For a given locus, if the two chromosomes contain the same allele, they, and the organism, are homozygous with respect to that allele. If

147-490: Is a short form of "allelomorph" ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders ) in the 1900s, which was used in the early days of genetics to describe variant forms of a gene detected in different phenotypes and identified to cause the differences between them. It derives from the Greek prefix ἀλληλο-, allelo- , meaning "mutual", "reciprocal", or "each other", which itself

168-545: Is a variant of the sequence of nucleotides at a particular location, or locus , on a DNA molecule. Alleles can differ at a single position through single nucleotide polymorphisms (SNP), but they can also have insertions and deletions of up to several thousand base pairs . Most alleles observed result in little or no change in the function of the gene product it codes for. However, sometimes different alleles can result in different observable phenotypic traits , such as different pigmentation . A notable example of this

189-423: Is not related to mutations. Chemicals that induce mutations are called mutagens . Most mutagens are also considered to be carcinogens . Mutations are distinctly different from epigenetic alterations, although they share some common features. Both arise as a chromosomal alteration that can be replicated and passed on to subsequent cell generations. Both, when occurring within a gene, may silence expression of

210-480: Is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus. Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.) The frequency of alleles in a diploid population can be used to predict

231-407: Is particularly error-prone. Although not all mutations have a noticeable phenotypic effect, the common usage of the word "mutant" is generally a pejorative term, only used for genetically or phenotypically noticeable mutations. Previously, people used the word " sport " (related to spurt ) to refer to abnormal specimens. The scientific usage is broader, referring to any organism differing from

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252-452: Is related to the Greek adjective ἄλλος, allos (cognate with Latin alius ), meaning "other". In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive , according to which of the two homozygous phenotypes the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed

273-439: Is the one that leads to the "dominant" phenotype, and the other allele is said to be "recessive". The degree and pattern of dominance varies among loci. This type of interaction was first formally-described by Gregor Mendel . However, many traits defy this simple categorization and the phenotypes are modelled by co-dominance and polygenic inheritance . The term " wild type " allele is sometimes used to describe an allele that

294-403: Is thought to contribute to the typical phenotypic character as seen in "wild" populations of organisms, such as fruit flies ( Drosophila melanogaster ). Such a "wild type" allele was historically regarded as leading to a dominant (overpowering – always expressed), common, and normal phenotype, in contrast to " mutant " alleles that lead to recessive, rare, and frequently deleterious phenotypes. It

315-630: The wild type . The word finds its origin in the Latin term mūtant- (stem of mūtāns), which means "to change". Mutants should not be confused with organisms born with developmental abnormalities , which are caused by errors during morphogenesis . In a developmental abnormality, the DNA of the organism is unchanged and the abnormality cannot be passed on to progeny. Conjoined twins are the result of developmental abnormalities. Chemicals that cause developmental abnormalities are called teratogens ; these may also cause mutations, but their effect on development

336-502: The alleles are different, they, and the organism, are heterozygous with respect to those alleles. Popular definitions of 'allele' typically refer only to different alleles within genes. For example, the ABO blood grouping is controlled by the ABO gene , which has six common alleles (variants). In population genetics , nearly every living human's phenotype for the ABO gene is some combination of just these six alleles. The word "allele"

357-484: The alternative allele. If the first allele is dominant to the second then the fraction of the population that will show the dominant phenotype is p + 2 pq , and the fraction with the recessive phenotype is q . With three alleles: In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) with a number of alleles (a) is given by the expression: A number of genetic disorders are caused when an individual inherits two recessive alleles for

378-403: The frequencies of the corresponding genotypes (see Hardy–Weinberg principle ). For a simple model, with two alleles; where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, p is the fraction of the population homozygous for the first allele, 2 pq is the fraction of heterozygotes, and q is the fraction homozygous for

399-791: The proportion of heterozygotes in the population. A null allele is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive. For example, at the gene locus for the ABO blood type carbohydrate antigens in humans, classical genetics recognizes three alleles, I , I , and i, which determine compatibility of blood transfusions . Any individual has one of six possible genotypes (I I , I i, I I , I i, I I , and ii) which produce one of four possible phenotypes : "Type A" (produced by I I homozygous and I i heterozygous genotypes), "Type B" (produced by I I homozygous and I i heterozygous genotypes), "Type AB" produced by I I heterozygous genotype, and "Type O" produced by ii homozygous genotype. (It

420-489: Was formerly thought that most individuals were homozygous for the "wild type" allele at most gene loci, and that any alternative "mutant" allele was found in homozygous form in a small minority of "affected" individuals, often as genetic diseases , and more frequently in heterozygous form in " carriers " for the mutant allele. It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that

441-473: Was introduced in 1990 in place of "allele" to denote sequences at the same locus in different strains that have no sequence similarity and probably do not share a common phylogenetic relationship. It is used mainly in the genetic research of mycology . Mutant Mutants arise by mutations occurring in pre-existing genomes as a result of errors of DNA replication or errors of DNA repair . Errors of replication often involve translesion synthesis by

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