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Encyclopedia > Allele

An allele (pronounced /ˈæliːl/ (UK), /əˈliːl/ (US)) (from the Greek αλληλος allelos, meaning each other) is one member of a pair or series of different forms of a gene. Usually alleles are coding sequences, but sometimes the term is used to refer to a non-coding sequence. An individual's genotype for that gene is the set of alleles it happens to possess. In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype. Alleles are prominently represented in a Punnett square. Animation of a section of DNA rotating. ... For other uses, see Gene (disambiguation). ... The coding region of a gene is the portion of DNA that is transcribed into mRNA and translated into proteins. ... In molecular biology, junk DNA is a collective label for the portions of the DNA sequence of a chromosome or a genome for which no function has yet been identified. ... This article does not cite any references or sources. ... Diploid (meaning double in Greek) cells have two copies (homologs) of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. ... The Punnett square is a diagram designed by Reginald Punnett and used by biologists to determine the probability of an offspring having a particular genotype. ...


An example is the gene for blossom colour in many species of flower — a single gene controls the colour of the petals, but there may be several different versions (or alleles) of the gene. One version might result in red petals, while another might result in white petals. The resulting colour of an individual flower will depend on which two alleles it possesses for the gene and how the two interact. For other uses, see Flower (disambiguation). ... For the petals of chakras, see Petal (chakra). ...

Contents

Introduction

Diploid organisms (e.g. humans) have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene. An organism in which the two copies of the gene are identical — that is, have the same allele — is called homozygous for that gene. An organism which has two different alleles of the gene is called heterozygous. Phenotypes (the expressed characteristics) associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A dominant phenotype will be expressed when at least one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type. In biology, homology is any similarity between structures that is due to their shared ancestry. ... This article does not cite any references or sources. ... It has been suggested that this article be split into multiple articles accessible from a disambiguation page. ... It has been suggested that this article be split into multiple articles accessible from a disambiguation page. ... Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ... In genetics, dominant allele refers to a genetic feature that hides the recessive allele. ... In genetics, the term recessive gene refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in a homozygous genotype (an organism that has two copies of the same allele). ...


However, there are exceptions to the way heterozygotes express themselves in the phenotype. One exception is incomplete dominance (sometimes called blending inheritance) when alleles blend their traits in the phenotype. An example of this would be seen if, when crossing Antirrhinums — flowers with incompletely dominant "red" and "white" alleles for petal color — the resulting offspring had pink petals. Another exception is co-dominance, where both alleles are active and both traits are expressed at the same time; for example, both red and white petals in the same bloom or red and white flowers on the same plant. Codominance is also apparent in human blood types. A person with one "A" blood type allele and one "B" blood type allele would have a blood type of "AB". It has been suggested that this article or section be merged with dominance relationship. ... Gregor Johann Mendel In Darwins time, biologists held to the theory of blending inheritance -- an offspring was an average of its parents. ... For the Transformers character, see Snapdragon (Transformers). ... Co-dominance and the closely related incomplete dominance are terms in genetics that refer to the situation where an organism inherits a combined or blended phenotype instead of just the dominant trait, when two different alleles are present in the genotype. ... This article is about human blood types (or blood groups). ...


A wild type allele is an allele which is considered to be "normal" for the organism in question, as opposed to a mutant allele which is usually a relatively new modification. In biology, a wild type is one of the major genotypes of a species that occur in nature, in contrast to induced mutations or artificial cross-breeding. ... This article is about biological mutants. ...


(Note that with the advent of neutral genetic markers, the term 'allele' is now often used to refer to DNA sequence variants in non-functional, or junk DNA. For example, allele frequency tables are often presented for genetic markers, such as the DYS markers.) Also there are many different types of alleles. A genetic marker is a known DNA sequences (e. ... In molecular biology, junk DNA is a collective label for the portions of the DNA sequence of a chromosome or a genome for which no function has yet been identified. ... Allele frequency is a measure of the relative frequency of an allele on a genetic locus in a population. ... DYS is short for DNA Y-chromosome Segment, and is used to designate a segment of DNA on the Y chromosome where a sequence of nucleotides repeats. ...


Equations

There are two equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle). Hardy–Weinberg principle for two alleles: the horizontal axis shows the two allele frequencies p and q, the vertical axis shows the genotype frequencies and the three possible genotypes are represented by the different glyphs In population genetics, the Hardy–Weinberg principle is a relationship between the frequencies of alleles...


Equation 1: p + q = 1,


Equation 2: p2 + 2pq + q2 = 1


where p is the frequency of one allele and q is the frequency of the other allele. Under appropriate conditions, subject to numerous limitations regarding the applicability of the Hardy-Weinberg principle, p2 is the population fraction that is homozygous for the p allele, 2pq is the frequency of heterozygotes and q2 is the population fraction that is homozygous for the q allele. Hardy–Weinberg principle for two alleles: the horizontal axis shows the two allele frequencies p and q, the vertical axis shows the genotype frequencies and the three possible genotypes are represented by the different glyphs In population genetics, the Hardy–Weinberg principle is a relationship between the frequencies of alleles...


Natural selection can act on p and q in Equation 1, and obviously affect the frequency of genotypes seen in Equation 2. For other uses, see Natural selection (disambiguation). ...


Equation 2 is a consequence of Equation 1, obtained by squaring both sides and applying the binomial theorem to the left-hand side. Conversely, p2 + 2pq + q2 = 1 implies p + q = 1 since p and q are positive numbers. In mathematics, the binomial theorem is an important formula giving the expansion of powers of sums. ...


The following equation (commonly termed the Lee equation) can be used to calculate the number of possible genotypes in a diploid organism for a specific gene with a given number of alleles.


G = (a2 + a) / 2


where a is the number of different alleles for the gene being dealt with and G is the number of possible genotypes. For example, the human ABO blood group gene has three alleles; A (for blood group A), B (for blood group B) and O (for blood group O). As such, (using the equation) the number of possible genotypes a human may have with respect to the ABO gene are 6 (AA, AO, AB, BB, BO, OO). The equation does not specify the number of possible phenotypes, however. Such an equation would be quite impossible as the number of possible phenotypes varies amongst different genes and their alleles. For example, in a diploid heterozygote some traits may show complete dominance, incomplete dominance etc., depending of the gene involved.


Genetic disorders

Genetic disorders are normally caused if an individual carries two alleles associated with a recessive, single-gene trait. Genetic disorders such as these include Albinism, Cystic Fibrosis, Galactosemia, Phenylketonuria (PKU), and Tay-Sachs Disease. In these cases the two alleles are autosomal (not sex chromosomes). Other disorders are also recessive, but because they are located on the X chromosomes (of which men have only one copy), they are much more frequent in men than in women. One example of such a disorder is the Fragile X syndrome. Albino redirects here. ... Cystic fibrosis (also known as CF, mucoviscoidosis, or mucoviscidosis) is a hereditary disease that affects mainly the exocrine (mucus) glands of the lungs, liver, pancreas, and intestines, causing progressive disability due to multisystem failure. ... Galactosemia is a rare genetic metabolic disorder which affects an individuals ability to properly digest the sugar galactose. ... Phenylketonuria (PKU) is an autosomal recessive genetic disorder characterized by a deficiency in the enzyme phenylalanine hydroxylase (PAH). ... Tay-Sachs disease (abbreviated TSD, also known as GM2 gangliosidosis, Hexosaminidase A deficiency or Sphingolipidosis) is a genetic disorder, fatal in its most common variant known as Infantile Tay-Sachs disease. ... An autosome is a non-sex chromosome. ... Fragile X syndrome is a syndrome of X-linked mental retardation. ...


Some other disorders, such as Huntington's disease, are dominant and it is sufficient to carry only one allele associated with the disorder to be affected.


See also

This article is about evolution in biology. ... A genealogical DNA test examines the nucleotides at specific locations on a persons DNA for genetic genealogy purposes. ... Recessive alleles (any one of a number of viable DNA codings of the same gene) typically produce little or no product. ... For the figure of speech, see meiosis (figure of speech). ... In the genetic analysis of families, a Mendelian error describes an allele in a child which could not have been received from any of their biological parents. ... Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent organisms to their children; it underlies much of genetics. ... Mitosis divides genetic information during cell division. ... In biology, polymorphism can be defined as the occurrence in the same habitat of two or more forms of a trait in such frequencies that the rarer cannot be maintained by recurrent mutation alone. ... The Punnett square is a diagram designed by Reginald Punnett and used by biologists to determine the probability of an offspring having a particular genotype. ...

External links

  • ALFRED: The ALlele FREquency Database

References

National Geographic Society, Alton Biggs, Lucy Daniel, Edward Ortleb, Peter Rillero, Dinah Zike. "Life Science". New York, Ohio, California, Illinois: Glencoe McGraw-Hill. 2002


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