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. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance Usually alleles are coding sequences, but sometimes the term is used to refer to a non-coding sequence. The coding region of a Gene is the portion of DNA or RNA that is transcribed into another RNA, such as a messenger RNA In Molecular biology, junk DNA is a provisional label for the portions of the DNA sequence of a Chromosome or a Genome for which no An individual's genotype for that gene is the set of alleles it happens to possess. The genotype is the genetic constitution of a cell an organism or an individual (i In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype. "Haplo" redirects here For the fictional character see The Death Gate Cycle. Alleles are prominently represented in a Punnett square. The Punnett square is a diagram that is used to predict the outcome of a particular cross or breeding experiment

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. A flower, also known as a bloom or Blossom, is the reproductive structure found in Flowering plants (plants of the division Magnoliophyta, also A petal (from Ancient Greek petalon "leaf" "thin plate" regarded as a highly modified leaf is one member or part of the corolla 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.

Introduction

Diploid organisms (e. g. humans) have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene. In Evolutionary biology, homology has come to mean any similarity between characters that is due to their shared ancestry. Somatic cells are any cells forming the body of an organism as opposed to Germline cells An organism in which the two copies of the gene are identical — that is, have the same allele — is called homozygous for that gene. Zygosity refers to the genetic condition of a Zygote. In genetics zygosity describes the similarity or dissimilarity of DNA between Homologous An organism which has two different alleles of the gene is called heterozygous. Zygosity refers to the genetic condition of a Zygote. In genetics zygosity describes the similarity or dissimilarity of DNA between Homologous Phenotypes (the expressed characteristics) associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A phenotype is any observable characteristic of an Organism, such as its morphology, Development, biochemical or physiological properties 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.

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. In Darwin's time biologists held to the theory of blending inheritance -- an offspring was an average of its parents 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 blood type (also called a blood group) is a classification of Blood based on the presence or absence of inherited Antigenic substances on the A person with one "A" blood type allele and one "B" blood type allele would have a blood type of "AB".

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. Wild type, sometimes written wildtype or wild-type, is the typical form of an organism strain gene or characteristic as it occurs in nature A mutant is an individual organism or new genetic character arising or resulting from an instance of Mutation, which is a base-pair sequence change within the DNA

(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. A genetic marker is a known DNA sequence. It can be described as a variation which may arise due to mutation or alteration in the genomic loci that can be observed In Molecular biology, junk DNA is a provisional label for the portions of the DNA sequence of a Chromosome or a Genome for which no For example, allele frequency tables are often presented for genetic markers, such as the DYS markers. Allele frequency is a measure of the relative frequency of an Allele at a genetic place(locus in a Population. DYS is short for D NA Y -chromosome S egment and is used to designate a segment of DNA on the Y chromosome where a sequence of ) Also there are many different types of alleles.

Equations

There are two equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle).

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.

Natural selection can act on p and q in Equation 1, and obviously affect the frequency of genotypes seen in Equation 2. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of

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

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. Albinism (from Latin albus, "white" see extended etymology) is a form of hypopigmentary Congenital disorder, Cystic fibrosis (also known as CF, mucoviscoidosis, or mucoviscidosis) is a hereditary disease affecting the exocrine (mucus glands of the lungs Galactosemia is a rare genetic metabolic disorder which affects an individual's ability to properly metabolize the sugar Galactose. Phenylketonuria ( PKU) is an Autosomal recessive Genetic disorder characterized by a deficiency in the enzyme Phenylalanine hydroxylase Tay-Sachs disease (abbreviated TSD, also known as GM2 Gangliosidosis, Hexosaminidase A deficiency or Sphingolipidosis) is a Genetic In these cases the two alleles are autosomal (not sex chromosomes). An autosome is a non-sex Chromosome. It is an ordinarily paired type of chromosome that is the same in both Sexes of a species. 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. 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

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. Huntington's disease, also called Huntington's chorea, chorea major, or HD, is a genetic neurological disorder characterized after

See also

• Evolution
• Genealogical DNA test
• Haplo-sufficiency
• Meiosis
• Mendelian error
• Mendelian inheritance
• Mitosis
• Polymorphism
• Punnett square

External links

• ALFRED: The ALlele FREquency Database

References

National Geographic Society, Alton Biggs, Lucy Daniel, Edward Ortleb, Peter Rillero, Dinah Zike. eVolution is the third Album by eLDee, it was due to be released in 2008 A genealogical DNA test examines the Nucleotides at specific locations on a person's 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 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 A Mendelian error in the genetic analysis of a species describes an Allele in an individual which could not have been received from either of its biological parents Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Polymorphism in biology occurs when two or more clearly different Phenotypes exist in the same population of a species — in other words the occurrence of more than one The Punnett square is a diagram that is used to predict the outcome of a particular cross or breeding experiment "Life Science". New York, Ohio, California, Illinois: Glencoe McGraw-Hill. 2002