What is an allele? Summary of this genetics concept

What are alleles and how are they expressed in DNA? We examine this concept of genetics.

We know that genes largely determine who we are physically, mentally, socially and personally. However, genes alone do not determine "everything"; rather, the environment plays an essential modulating role.

In this article we will learn what an allele isThe first is that an allele is each of the variants or variations of a gene, and the second is how this concept is related to genetics.

What is an allele?

Etymologically, the term allele (also called allelomorph), comes from the Greek, and literally means "from one to the other". An allele is each of the alternative forms that the same gene can have.. Recall that a gene is the hereditary unit that controls and determines each 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 "allelomorph", which means "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 one from the mother). (one from the father and one from the mother).

Homologous chromosomes

Humans, like most mammals, are diploid. That means that we possess 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 occupation of the final position on the chromosomes during the separation that takes place during cell meiosis. Thus, the "winning" (dominant) allele is the one that will end up being transmitted genetically.

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

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

Examples

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

Homozygous vs. heterozygous

We say that an individual is homozygous for a given character for a gene, in the case that the two alleles inherited corresponding to that gene are the same.

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 dominant and recessive mean).

On the other hand, a person is heterozygous for a gene when he/she has a different allele on each homologous chromosome. For example Aa.

Types of allele

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

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

On the other hand, if they differ in function, alleles may or may not have known differences in sequences, but they are evaluated by the way 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 encompasses those characteristics or traits that are determined by the genes and the environment, such as facial features (nose size) or impulsivity (behavior):

1. dominant alleles

In case the allele is dominant, will be expressed in the offspring only with one of the procreative copies.that is to say, if the father or the mother possesses it, the chromosome of the offspring will always express it (only one allele will be necessary).

That is, they appear in the phenotype of heterozygous or hybrid individuals for a given character, as well as in the homozygote.

2. Recessive alleles

On the other hand, an allele will be recessive in the case where two copies of the same gene (i.e. two alleles) are required. (i.e. two alleles) are required for it to be expressed on the procreated chromosome (of the offspring).

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

Bibliographical 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. Elsevier Mosby Publishing House, 4th Ed.
• Rosenweig, M.R.; Breedlove, S.M; Watson, N.V.. (2005). Psychobiology: an introduction to behavioral, cognitive, and clinical neuroscience. Barcelona: Ariel.
• Turnpenny, Ellard. (Emery). Elements of Medical Genetics, 13th Ed., Elsevier Publishing.