Allele

One names allele a given alternative of a Gène within a species. All the alleles of the same gene occupy same the Locus (site) on same a Chromosome.

Example: in the case of the gene determining the Blood group, located on the human chromosome 9, one of the alleles codes the group has, another for the group B, and a third allele determines the group O.

General information

The gene consists of a sequence of nucleotide S, fragments unit of DNA. This sequence can be modified in a way natural and random on the level of one or several Nucléotide S, one speaks about change. A change introduces a variation compared to the starting sequence: the modified sequence is an allele of starting gene.

In a cell diploïde, there are two alleles for each gene: an allele transmitted by each relative. The alleles transmitted by the parents can be identical or not.

At the human being, each gene is present in double specimen, one coming from the father, the other of the mother.

In a population, one can have several alleles of a gene, representing several alternative shapes of same gene.

If the alleles brought by each relative are identical in their nucleotidic sequence, the individual is Homozygote for this gene. If they are different, the individual is Hétérozygote. In this last case, two possibilities are possible as for the Phénotype resulting from the form of gene. If one of the two alleles is expressed and the other remains “dumb”, it is said that first is dominating and the other recessive . The dominant alleles are symbolized by a capital letter, and them recessive by a small letter.

Types of alleles

The various types of alleles are the following:
  • of the Co-dominant alleles is alleles which are expressed at the same time. At the individuals Hétérozygote S, in the event of Co-predominance, the two alleles are expressed. In theory, the phenotype is thus the joint expression of the two alleles. For as much, the phenotype can not reflect the contribution of the two alleles so in addition the expression of another gene " contredit" the expression of one of the two alleles. To take a simple example, the color rouan of the dress at the bovines results from a mixture of red hairs and hairs white due to the hétérozygotie of the alleles coding each one a color of hairs;
  • a lethal allele is the mutant shape of a gene, fatal with the state Homozygote;
  • a null allele is an allele which provides a nonfunctional product genic;
  • a recessive allele is the state allelic of a gene where the homozygotie is necessary for the expression of the adapted phenotype, contrary to a allele dominating ;
  • the multiple alleles are observed when it there with the existence, in a population, of more than two alleles on same a Locus.

In a population, a gene can thus exist under several alternatives, the locus and the function remaining constant. It is said that a gene is polyallelic if it is represented by more than two alleles, if these alleles are present in the population with a higher frequency or equalizes to 1%, one speaks about polymorphic gene . See allelic Frequency.

Related articles

Random links:Benjamin Vautier | Diepoldsau | Elme Marie Caro | Jacques Minassian | Prudential arbitration | Albert II of Brunswick-Göttingen