What is an allele

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

In this article, we will know what an allele is and the types that exist, and how this concept relates to genetics, based on the basis that an allele is each of the variants or variations that a gene has.

What is an allele?

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

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

In humans, each person, under normal circumstances, carries two alleles for each gene (not the father’s and another’s).

Counterpart chromosomes

Humans, like most mammals, are diploids. That means we have two sets of chromosomes, each from the father and mother, respectively. Each gene has two alleles located in the same locus or place on the chromosome.

An allele involves some dominance when one gene rivals another gene by occupying the final position on chromosomes during separation, which develops during cell meiosis. Thus, the “winner” (dominant) allele is the one that will end up being genetically transmitted.

The counterpart chromosomes are a pair of chromosomes (one from the mother and one from the father) paired within a cell during meiosis (which occurs in sexual reproduction). Thus, the counterpart chromosomes have the same DNA sequence arrangement 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 counterpart chromosomes; is each of the variants or variations that a gene has.

Examples

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

Homozygotic vs. heterozygote

We say that an individual is homozygotic for a certain character by a gene if the two alleles he has inherited corresponding to that gene are equal.

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

Instead, a person is heterozygotic for a gene when they have a different allele on each counterpart chromosome—for example, Aa.

Allele types

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

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

However, if they differ in function, alleles may or may not have known differences in sequences, but are evaluated for how they affect the body.

Let’s look at the two types of alleles that exist, based on their 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 impulsivity (behaviour):

Dominant alleles

If the allele is dominant, it will be expressed in the child only with one of the procreative copies, that is, if the parent possesses it, the child’s chromosome will always express it (only one allele will be needed).

They appear in the phenotype of heterozygous or hybrid individuals for a certain character, in addition to homozygous.

Recessive alleles

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

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

References:

  • Griffiths, A. J. F., Gelbart, W.M., Miller, J. H., and Lewontin, R.C. (2000). Modern Genetics. Inter-American/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 behavioural, cognitive and clinical neuroscience. Barcelona: Ariel.
  • Turnpenny, Ellard. (Emery). Medical Genetics Elements, 13th Ed., Elsevier Editorial.

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