What is an allele? Summary of this concept of genetics

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, rather 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 is related to genetics, based on the fact that an allele is each of the variants or variations that a gene has.

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What is an allele?

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

Many genes have multiple alleles, and the expression of the alleles determines the same character or trait, such as eye or hair color. The term allele can be better 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 another from the mother).

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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 some dominance when one gene competes against another gene for the occupation of the final position on the chromosomes during separation, which occurs 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). So, homologous chromosomes have the same DNA sequence arrangement from one extreme to the other, but different alleles.

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

examples

The alleles differ in their sequence and in that they can be expressed in specific modifications of the function of that gene. That is, the alleles produce variations in certain inherited characteristics, such as eye color (one allele would be blue, and another would be brown, 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 certain character for a gene, in the case that the two alleles that he has inherited corresponding to that 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 dominant and recessive mean).

Instead, a person is heterozygous for a gene when they have a different allele on each homologous chromosome. For example Ah.

allele types

As we have seen, an allele is an alternate form of a gene, and alleles differ in sequence or function. Characteristics that are genetically determined, depend 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.

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

We are going to see the two types of alleles that exist, according to its expression in the phenotype. Let us be clear that the phenotype encompasses those characteristics or traits that are determined by the genes and environment, such as facial features (nose size) or impulsivity (behavior):

1. dominant alleles

If the allele is dominant, will be expressed in the child with only one of the procreative copies, that is, if the father or the mother possesses it, the child's chromosome will always express it (only one allele will be necessary).

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

2. recessive alleles

Instead, An allele will be recessive if two copies of the same gene are needed. (ie, two alleles) to be expressed on the procreated (son's) chromosome.

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

Bibliographic references:

• Griffiths, A. J. F., Gelbart, W. M., Miller, J. H., and Lewontin, R. c. (2000). Modern Genetics. Interamerican/McGraw-Hill.
• Jorde, Carey, Bamshad. (2011). Medical genetics. Elsevier Mosby Publisher, 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 Editorial.