We know that genes largely determine who we are on a physical, mental, social and personal level. However, genes do not determine “everything” by themselves, but the environment plays an essential modulating role.

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

What is an allele?

Etymologically, the term allele (also called allelomorphic), comes from the Greek, and literally means “from one to another. An allele is each of the alternative forms that the same gene can have . Let’s remember that a gene is the hereditary unit that controls and determines every character in living beings.

Many genes have several alleles, and the expression of the alleles determines the same character or trait, such as eye or hair color. The term allele can best be understood from the word “allelomorphic”, which means “in allele forms”; that 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 one from the mother).

Homologous chromosomes

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

An allele implies a certain dominance when a gene competes against another gene for the final position on the chromosomes during separation, which develops during cell meiosis. Thus, the “winning” (dominant) allele is the one that will end up being genetically transmitted.

Homologous chromosomes are a pair of chromosomes (one from the mother and one from the father) that pair up inside a cell during meiosis (which occurs in sexual reproduction). Thus, the homologous chromosomes have the same arrangement of the DNA sequence from one end to the other, but different alleles.

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


The alleles differ in their sequence and in that they can be expressed in specific modifications of the function of that gene. In other words, the alleles produce variations in certain inherited characteristics , such as eye colour (one allele would be blue, and another brown, but there would be more), or blood group (the allele being A+, B+, AB+,…)

Homozygous vs. heterozygous

We say that an individual is homozygous for a given character because of a gene, if the two alleles he has inherited corresponding to that gene are the same .

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

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

Allele types

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

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

In contrast, if they differ in function, the alleles may or may not have known differences in sequences, but are evaluated by how they affect the organism.

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

1. Dominant alleles

In case the allele is dominant, will be expressed in the child with only one of the procreative copies , i.e. if the parent has it, the child’s chromosome will always express it (only one allele will be necessary).

That is, they appear in the phenotype of the heterozygous individuals or hybrids for a given characteristic, in addition to the homozygous.

2. Recessive alleles

In contrast, an allele will be recessive in the case that two copies of the same gene are needed (i.e. two alleles) to be expressed on the procreated (child’s) chromosome.

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

Bibliographic references:

  • Griffiths, A. J. F., Gelbart, W. M., Miller, J. H., and Lewontin, R. C. (2000). Modern Genetics. Interamericana/McGraw-Hill.
  • Jorde, Carey, Bamshad. (2011). Medical genetics. Editorial Elsevier Mosby, 4th Edition
  • Rosenweig, M.R.; Breedlove, S.M; Watson, N.V. (2005). Psychobiology: an introduction to behavioural, cognitive and clinical neuroscience. Barcelona: Ariel.
  • Turnpenny, Ellard. (Emery). Elements of Medical Genetics, 13th Ed.