MULTIPLE ALLELES: CONCEPT, INHERITANCE AND EXAMPLES - BIOLOGY - 2024

The multiple alleles are different variations that can accommodate a given gene. 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 alleles of a gene (Source: Thomas Splettstoesser via Wikimedia Commons) An allele is defined as one of the specific forms of a gene that codes for a possible phenotype; it can be mutant or wild, depending on whether it undergoes some type of modification or remains unaltered, giving an altered or "normal" phenotype, respectively.

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 varying degrees of dominance with respect to the other members of the series.

One of the branches of genetics responsible for 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.

Multiple allele concept

The concept of multiple alleles is somewhat 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 have 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.

Multiple allele inheritance

Given 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 originated 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.

Examples

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, fur color in rabbits, eye color in fruit flies, and plumage patterns in ducks.

ABO blood groups in humans

The locus to which the ABO gene belongs determines blood type in humans. For this locus, human populations have been described as having three possible alleles that code for the three different antigens that determine blood type.

The three alleles of the ABO locus are known as:

- IA, which codes for antigen A, - IB, which codes for antigen B, - i, which does not code for any antigen.

The dominance relationship between these three alleles is IA> i; IB> i; IA = IB (codominance). Both allele A and allele B are dominant over allele i, but these are codominant among themselves; So a person who has blood type AB has one A allele and one B allele.

Since the i allele is recessive, people with one blood type (phenotype) have two i alleles.

Coat color in rabbits

Rabbit hair color is determined by an allelic series from the C locus. The alleles of this series are: C, c ch, ch and c, which determine a homogeneous dark coloration, light gray (chinchilla), albino with dark extremities and completely albino, respectively.

Chinchilla colored rabbit (Source: Bodlina ~ commonswiki via Wikimedia Commons)

The dominance of these alleles is, in the order from most dominant to recessive, as it was written: C> c ch> ch> c, so there may be 10 different genotypes that originate only four particular phenotypes.

Duck plumage coloration patterns

The locus that determines the plumage pattern of mallard ducks has multiple alleles. The M allele is the one that codes for the “wild” pattern, but there are two other alleles: the MR allele, which produces a pattern known as “restricted” and the m ¸ allele, which produces a pattern known as “dusky” (dark)..

The dominant allele is the MR, followed by the M allele and the recessive md, from which six possible combinations are obtained that give rise to six phenotypes.

References

1. Bernasconi, Andrea "Multiple Alleles." Genetics. Retrieved December 10, 2019 from Encyclopedia.com: www.encyclopedia.com
2. Gardner, EJ, Simmons, MJ, Snustad, PD, & Santana Calderón, A. (2000). Principles of genetics.
3. Griffiths, AJ, Wessler, SR, Lewontin, RC, Gelbart, WM, Suzuki, DT, & Miller, JH (2005). An introduction to genetic analysis. Macmillan.
4. Pierce, BA (2012). Genetics: A conceptual approach. Macmillan.
5. Srb, AM, Owen, RD, & Edgar, RS (1965). General genetics (No. 04; QH431, S69 1965.). San Francisco: WH Freeman.