The multiple alleles they are the different variations that a given gene can harbor. All genes have two alleles that define genetic traits of living organisms.
A species is said to possess genes with multiple alleles when they present more than two alternative forms. That is, when in a population a "trait" or characteristic is encoded by a gene that has more than two alleles (for diploid organisms such as humans, for example).
The number of alleles that a gene that codes for a given trait can have can be highly variable, since minimal variations in the genetic sequence of an allele give rise to a new “mutant” form, which may or may not provide a different phenotype..
In genetics, the different alleles of the same gene that present multiple allelisms are known as allelic series and members of the same allelic series can present variable degrees of dominance with respect to the other members of the series.
One of the branches of genetics in charge of the study of genes with multiple alleles is the well-known population genetics, very useful for the analysis of the genetic composition of species, whether they are animals, plants or microorganisms..
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The concept of multiple alleles is something applicable in a purely populational way, since an individual, seen from the genetic point of view, has a number of alleles for a gene that is equivalent to its chromosomal load.
In other words, diploid organisms (2n, with two sets of chromosomes) like mammals, for example, only possess two alternative forms of each gene, since they inherit a homologous chromosome from each of their two parental individuals during sexual reproduction..
Plants, which are the classic example of organisms with more than 2 sets of homologous chromosomes (polyploids) possess, individually speaking, as many alleles for a gene as its ploidy number, that is, four alleles for tetraploids (4n) , six for hexaploids (6n) and so on.
By understanding this, it can then be ensured that a gene has multiple alleles when it has more than the number of alleles equivalent to its chromosomal load in a population. Many authors are of the opinion that most of the genes in a population are represented by multiple alleles, which are the result of gene variations of different kinds..
In view of the fact that the concept is population-based, the inheritance of a gene with multiple alleles is not different from that of genes that only have two alternative forms, since, in a diploid individual, for example, through sexual reproduction only two forms of the same gene will be transmitted, one on each homologous chromosome.
The only real difference from genes with multiple alleles and from genes that exist in only two alternative forms is that, with the former, it is possible to achieve a vastly superior variety of genotypes and phenotypes for a particular trait..
The number of genotypes originating in a population that are due to the presence of genes with multiple alleles is a function of the number of alleles that exist for each given gene..
Thus, if there are 2, 3, 4 or 5 different alleles for the same gene in a population, 3, 6, 10 or 15 possible genotypes will be observed, correspondingly.
In the analysis of an allelic series for a given gene (the gene is defined according to the "wild" phenotype), the different alleles are written with the letter that characterizes the gene and a "superscript" that describes the phenotype or genotype. modified that this encodes.
In summary, genes with multiple alleles in a population follow the segregation principles proposed by Mendel, so their inheritance is not different from genes with only two alleles..
Different examples of characters encoded by multiple alleles in natural populations can be found in the literature. Among the most cited are the determination of blood type in humans, coat color in rabbits, eye color in fruit flies, and plumage patterns in ducks..
The locus to which the ABO gene belongs determines blood type in humans. It has been described that for this locus human populations have three possible alleles that code for the three different antigens that determine blood type.
The three alleles of the ABO locus are known as:
- ITO, encoding antigen A,
- IB, encoding antigen B,
- i, that does not code for any antigen.
The dominance relationship between these three alleles is ITO> i; IB> i; ITO= IB (codominance). Both the allele TO like allele B are dominant over the allele i, but these are codominant with each other; so, a person who has the AB blood type, has an allele TO and one allele B.
Since the allele i is recessive, people with one blood type (phenotype) OR have two alleles i.
The hair color of rabbits is determined by an allelic series of the locus C. The alleles in this series are: C, cch, ch Y c, that determine a homogeneous dark coloration, light gray (chinchilla), albino with dark extremities and completely albino, respectively.
The dominance of these alleles is, in the order of most dominant to recessive, as it was written: C> cch> ch> c, so there may be 10 different genotypes that originate only four particular phenotypes.
The locus that determines the plumage pattern of mallards has multiple alleles. Allele M is the one that codes for the “wild” pattern, but there are two other alleles: the allele MR, which produces a pattern known as "restricted" and the allele m¸ which produces a pattern known as “dusky" (Dark).
The dominant allele is MR, followed by allele M and the recessive md, of which six possible combinations are obtained that give rise to six phenotypes.
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