Allele

For the hard rock band, see Allele (band).

An allele is any one of a number of viable DNA codings occupying a given locus (position) on a chromosome. Usually alleles are DNA sequences that code for a gene, but sometimes the term is used to refer to a non-gene 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.

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 (or alleles) of the gene. One version might result in red petals, while another might result in white petals. The resulting color of an individual flower will depend on which two alleles it possesses for the gene and how the two interact.

Introduction

Organisms that are diploid such as 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 said to be homozygous for that gene. An organism which has two different alleles of the gene is said to be 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.

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".

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.

(Note that with the advent of neutral genetic markers, the term 'allele' is now often used to refer to DNA sequence variants in non-functinal, or junk DNA. For example, allele frequency tables are often presented for genetic markers, such as the DYS markers.)

Equations

There are two simple equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle). The second is a consequence of the first, obtained by squaring both sides and applying the binomial theorem to the left-hand side:

Equation 1: $p+q=1$ ,

Equation 2: $p^2+2pq+q^2=1$

where p is the frequency of one allele and q is the frequency of the other allele. 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 alleles seen in Equation 2. It should be noted that the second equation can be derived from the first (or vice versa) since $p^2+2pq+q^2=1$ implies $(p+q)^2=1$ and p and q are positive numbers.

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Introduction

Equations

See also