An allele is any one of a number of viable DNA codings of the same gene (sometimes the term refers to a non-gene sequence) occupying a given locus (position) on a chromosome. Two alleles together comprise a gene. An example is the gene for blossom color in many species of flower - a single gene controls the color of the petals, but there may be several different versions of the gene. One version might result in red petals, while another might result in white petals. This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... The word locus (plural loci) is Latin for place: In biology and evolutionary computation, a locus is the position of a gene (or other significant sequence) on a chromosome. ... Figure 1: Chromosome. ... Wildflowers Flower (Latin flos, floris; French fleur), a term popularly used for the bloom or blossom of a plant, is the reproductive structure of those plants classified as angiosperms (flowering plants; Division Magnoliophyta). ...

Note that with the advent of the study of genetic markers, the term allele is often now used to refer to DNA codings in junk DNA. For example, the term allele frequency tables are often presented for genetic markers, such as the DYS markers. A genetic marker is a specific discovered single nucleotide polymorphism or SNP (or simply mutation) of certain section of DNA of a specific genome. ... 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 been identified. ... Allele frequency is a term of population genetics that is used in characterizing the genetic diversity of a species population, or equivalently the richness of its gene pool. ... 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. ...

Some organisms are diploid - that is, they have paired homologous chromosomes in their somatic cells, and thus contain two copies of each gene. An organism in which both copies of the gene are identical - that is, have the same allele - is said to be homozygous for that gene. An organism which has two different alleles of the gene is said to be heterozygous. Phenotypes associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A dominant phenotype will be expressed when only one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type. 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. ... Two or more structures are said to be homologous if they are alike because of shared ancestry. ... A somatic cell is a type of cell in an organism, such as the human body. ... A homozygotes cells are diploid or polyploid and have the same alleles at a locus (position) on homologous chromosomes. ... Heterozygote cells are diploid or polyploid and have different alleles at a locus (position) on homologous chromosomes. ... The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ... In genetics, the term dominant gene refers to the an allele that causes a phenotype that is seen in a heterozygous genotype. ... 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 snapdragons - flowers with codominant "red" and "white" alleles for petal color - the resulting offspring would have 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 result in a blood type of "AB". It has been suggested that this article or section be merged with dominance relationship. ... In modern genetics, this refers to incomplete dominance. ... The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ... 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. ... A blood type is a description of an individuals characteristics of red blood cells due to substances (carbohydrates and proteins) on the cell membrane. ...

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. ... A mutant (also known to early geneticists a monster) is an individual, organism, or new genetic character arising or resulting from an instance of mutation, which is a sudden structural change within the DNA of a gene or chromosome of an organism resulting in the creation of a new character...

Equations

There are two simple equations to look at the frequency of an allele (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 The Hardy–Weinberg principle (HWP) (also Hardy–Weinberg equilibrium (HWE), or Hardy–Weinberg law) states...

Equation 1: p² + 2pq + q² = 1

Equation 2: p + q = 1

Where p is the frequency of the dominant gene and, q is the frequency of the recessive gene. p² is the number of species that are homozygous dominant for a trait, pq is the number that are heterozygotes and q² is the number that are homozygous recessive. Natural selection can act on the aforementioned components to Equation 1, and obviously affect the frequency of genes seen in Equation 2. Natural selection is the process by which variants displaying favorable or deleterious traits end up producing more or fewer progeny relative to other individuals of the same population. ...