Allele

From Freepedia

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. 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.

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.

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.

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".

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.

Equations

There are two simple equations to look at the frequency of an allele (see Hardy-Weinberg principle):

Equation 1: <math>p^2+2pq+q^2=1</math>

Equation 2: <math>p+q=1</math>

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.