What is an allele? Summary of this concept of genetics

We know that genes largely determine who we are physically, mentally, socially and personally. However, genes do not determine “everything” on their own, but the environment plays an essential modulating role.

In this article we will know what an allele isAs well as the types that exist and how this concept relates to genetics, assuming that an allele is each of the variants or variations of a gene.

    What is an allele?

    Etymologically, the term allele (also called allelomorph) comes from the Greek and literally means “from one to another”. One allele is each of the alternative forms can have the same gene. Let us remember that a gene is the hereditary unit which controls and determines each character of living beings.

    Many genes have multiple alleles, and allele expression determines the same trait or trait, such as eye or hair color. The term allele can be better understood from the word “allelomorph”, meaning “in allelic forms”; that is, it is something that manifests itself in various ways within a population of individuals.

    In humans, each person, under normal circumstances, carries two alleles for each gene (Not from the father and another from the mother).

      homologated chromosomes

      Humans, like most mammals, are diploid. This means that we have two sets of chromosomes, each from the father and the mother, respectively. In addition, each gene has two alleles which are located at the same locus or location on the chromosome.

      An allele implies a certain dominance when a gene competes with another gene for occupying the final position on the chromosomes during separation, which develops during cellular meiosis. Thus, the “dominant” (dominant) allele is the one that will eventually be transmitted genetically.

      Homologous chromosomes are a pair of chromosomes (one from the mother and one from the father) that mate in a cell during meiosis (which occurs during sexual reproduction). like that, homologous chromosomes have the same arrangement as the DNA sequence throughout, but different alleles.

      In short, an allele is each of a genes that occupies the same place on homologous chromosomes; is each of the variants or variations of a gene.


      Alleles differ in their sequence and in that they can be expressed in specific changes in the function of this gene. In other words, the alleles they produce variations of certain inherited characteristics, Like the color of the eyes (one allele would be blue and another brown, but there would be more), or the blood group (being the allele A +, B +, AB +, …)

      Homozygous Vs. heterozygous

      We say that an individual is homozygous for a certain character by a gene, in the case where the two alleles he has inherited correspond to this gene Be equal.

      In this case, each of the alleles is found on each of the two homologous chromosomes that the person has. For example AA (dominant) or aa (recessive) (we will see later what is meant by dominant and recessive).

      In contrast, a person is heterozygous for a gene when they have a different allele on each homologous chromosome. For example Aa.

      Allele type

      As we have seen, an allele is an alternate form of a gene, and alleles are differentiated by their sequence or function. Genetically determined characteristics, Depends on the minimal action of a pair of homologous genes (the alleles).

      But how do they vary in the order of alleles? The answer is that they have differences in their DNA as deletions, substitutions or insertions.

      On the other hand, if they differ in function, the alleles may or may not have known differences in sequence, but they are assessed by how they affect the organism.

      Let’s look at the two types of alleles that exist, according to its expression in the phenotype. Let’s be clear that the phenotype encompasses those characteristics or traits that are determined by genes and the environment, such as facial features (nose size) or impulsiveness (behavior):

      1. Dominant alleles

      In the event that the allele is dominant, it must be expressed in children only with one of the reproductive copiesIn other words, if the father or the mother has it, the child’s chromosome will always express it (only one allele will be needed).

      In other words, they appear in the phenotype of heterozygous or hybrid individuals for a given trait, in addition to the homozygous.

      2. Recessive alleles

      however, an allele will be recessive if two copies of the same gene are needed (I.e., two alleles) to be expressed on the procreated (child) chromosome.

      These are the masked alleles of the phenotype of a heterozygous individual and appear only in the homozygous, being homozygous for the recessive genes.

      Bibliographical references:

      • Griffiths, AJF, Gelbart, WM, Miller, JH and Lewontin, RC (2000). Modern genetics. Inter-American / McGraw-Hill.
      • Sir, Carey, Bamshad. (2011). Medical genetics. Editorial Elsevier Mosby, 4th ed.
      • Rosenweig, MR; Breedlove, SM; Watson, NV (2005). Psychobiology: an introduction to behavioral, cognitive and clinical neuroscience. Barcelona: Ariel.
      • Turnpenny, Ellard. (Emery). Elements of Medical Genetics, 13th ed., Editorial Elsevier.

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