What Is An Allele? – Definition And Examples

What is an allele

Alleles are crucial to fundamental genes. But you must know what is an allele? In this article, I will provide simple definitions and examples of an allele.

Definition of an Allele

Our genetic heritage comprises 22 pairs of chromosomes called autosomes. And a couple of sex chromosomes called gonosomes (XX in women, XY in men). Each chromosome of a pair of autosomes contains the same information on several gene pairs. An allele designates one of the two genes forming a team.

Not all alleles express. Let’s take the example of blood groups and look at the trait described in different situations.

If one of the two homologous chromosomes carries the A allele. And the other has the O allele, the blood group expressed will be A. The A allele is dominant over the O allele, which is recessive. Likewise, allele B is dominant over allele O.


What is an Allele?: Example and note

During campaigns against genetic diseases, especially on television, everyone has heard the word “gene.”What is it? Where are the genes located? Is the genetic program expressed in the same way in all cells?


  1. The distribution of genetic information on chromosomes

The units of genetic information that determine hereditary traits are small portions of chromosomes called genes. Each gene occupies a specific place on the chromosomes.

Example: The gene responsible for the expression of blood groups A, B, O. They occupies the end of the long arms of chromosome 9. And that albinism the end of the short components.

Note: The red areas do not represent genes but only sites. That react differently to the staining used to make chromosomes visible and differentiate them from one another.

In general, in a cell, a gene exists in two copies. And it occupies the same position on each of the two chromosomes of a pair.

Note: The chromatids of each chromosome necessarily carry the same genetic information. Since each chromatid is the exact copy of the other. It obtained during the duplication that separates two cell divisions.

Some genes, if they are defective, no longer allow the regular expression of a trait. They are then responsible for hereditary diseases.

Example:Duchenne muscular dystrophy is a severe hereditary disease, which affects the muscles. Affected children have difficulty walking from the age of 3. This disease is due to a defect in a gene carried by the X chromosome’s short arm. As a result of this defect, the genetic information necessary to manufacture a substance naturally present in muscle cells. It is no longer current, and the patient’s muscle cells can no longer synthesize it. It causes progressive paralysis of the muscles.


  1. Alleles: variable expressions of the same gene

The same gene can have several different forms called alleles. Cells have, for the same gene, either two identical alleles or two different alleles.

Example: An individual is of blood group A. When type A “markers”are made and presented on their red blood cells’ surface. For these markers to make, red blood cells. They use the genetic information carried at the end of chromosomes of pair number 9.

The two homologous chromosomes of pair number 9 can either carry the same information. Which is: the two chromosomes of the pair then carry the A allele. Either bring a different story and therefore different alleles such as A and O. In this case, only the A allele carried by one of the pair’s chromosomes. And makes it possible to provide the genetic information. Which is capable of causing the A marker to produces by the blood cells red. The O allele (the “zero”allele) does not provide information and therefore does not allow a character to express.

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When the alleles are different, most often one is expressed but not the other. Whoever speaks is said to be dominant.

Example: The A allele dominates the O allele.

But in some cases, the two alleles can be expressed together: they say to be codominant.

Example: This is the case of alleles A and B. When they are both presents on homologous chromosomes of pair number 9, are expressed together. And determine the blood group AB.


  1. The individual expresses an only part of the genetic program

Cloning experiments have shown that genetic information is ultimately present and identified in all cells’ nucleus in the body. Successive cell divisions transmit the genetic information present in the egg cell. Each cell of the embryo, the fetus, baby, and adult, therefore, contains the same genetic information.

However, during embryonic development, cells gradually specialize in forming different organs.

Example: Some cells become skin cells, other nerve cells constituting the future brain, still other blood cells.

These cell types will need different genes to provide the information required to produce other substances related to their specialization.

Example: A pancreatic cell responsible for producing insulin. Which is the substance that regulates the level of glucose in the blood. It will need the gene carrying the genetic information necessary to manufacture this insulin. On the other hand, it will not need the gene, making it possible to manufacture hemoglobin. It’s a pigment of red blood cells allowing oxygen transport.

The specialization of cells makes it possible to inactivate genes that are not directly useful to the specialized cell’s organ. And this even though all the genes of the species are present in any cell of the cell. ‘organization.

The essential

If the two alleles of the gene are identical, these alleles’ hereditary character will necessarily express.

– If the two alleles are different, most often one of the alleles. They called dominant, will take over and alone express a corresponding hereditary character. The other allele will not be described.

More rarely, the two different alleles are expressed together at the individual’s hereditary character. They are said to be codominant.

Each cell possesses the entire genetic program of the individual but expresses only a part of it.


The power of genetics: What is an allele?

What will my child look like? It is the question that all parents ask themselves as soon as they have a positive pregnancy test. Impossible to know in advance the physical characteristics that a child will inherit from his father or mother. It will necessarily resemble both in something the genetic information of each individual determined. Which from the genes contained in the chromosomes. It is through them that the physical characteristics are transmitted. However, living conditions, as well as the environment, can modify specific features. For example, the skin is darker or darker, depending on whether you expose to the sun.


The great lottery of heredity

It all starts at the time of fertilization. The sperm merges with the egg to create an egg. The first cell of a new human being is born. This one receives half of the father’s inheritance and half of the mother’s legacy. There 23 chromosomes of one and 23 chromosomes of the other. That does not mean that the child will look half like his father and half like his mother. The laws of heredity are much more complicated. Each gene is present in two copies (the alleles). Not all alleles express. Some dominate the others and impose their characteristic as regards the hereditary characters. A multitude of combinations is therefore possible.

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The color of the eyes and the hair, how is it transmitted?

The color of the eyes depends mainly on the alleles. If the child receives the same two blue alleles, he will have blue eyes. But if he gets two distinct alleles (blue and brown), there is a good chance that he has brown eyes. Because the brown allele is dominant over the blue allele (recessive). For a recessive allele to be visible, the person must have two.

But all is not so simple. It is possible (the probability is remote) that a child inherits the color blue while his parents have brown eyes. In this case, the parents each have two different alleles (blue and brown): they said a heterozygous. Thus, they can pass a blue allele to their child.


What about the hair? 

It is the same mechanism as for the eyes. The color and density of hair depend on the genetic information contained in the alleles. The brown allele dominates the blond and the red. So, to have a blond baby, two blond (recessive) alleles must be expressed. Beware, early childhood blondness, which fades over the years, is not a genetic trait. When it comes to hair shape, curly and thick hair is a dominant allele. While thin and smooth comes from a recessive allele.


The color of the skin, a genetic heritage

Several genes are involved in the color of the skin. These will determine, in particular, the production and distribution of a brown pigment called melanin. The greater the amount of melanin contained in the cells, the darker the color of the skin. A couple who have black leather will have children of the same skin color. But in a mixed couple, the children will have a variable mix. By a lottery effect, some may thus have very dark skin and other lighter skin.


What about genetic mutations?

A mutation is an error in the transmission of genes. Albinism is, for example, an inherited disease that corresponds to a defect in melanin synthesis. People with albinism have white skin and very light eyes. We also speak of genetic mutation to explain certain spectacular phenomena. Without white ancestors in their family, two black parents have a little blonde girl with blue eyes. Or vice versa.


The blood group and forms the ears: it is also hereditary!

The earlobe. Its transmission is also genetic. The earlobe exists in two forms. Either detached from the face or free (dominant genetic character) or attached to it (recessive genetic character). 

The blood group. It is transmitted hereditarily, according to the laws of classical genetics. It depends on the expression of a gene, says ABO. A baby inherits a gene from each of its parents. Which gives the following possibilities: AA, BO, AB, OO, BB, AO. The A and B genes are dominant (or “codominant”for A and B), the O gene is recessive.

Other blood groups exist in the Rhesus system, the Kell system. Their transmission to children is also genetic, according to complex processes. 


FAQ of What is an Allele?

  1. What is the recessive allele?

Refers to a character or an allele expressed only in the homozygous state. That is to say when it is present in two copies in the genome.

  1. Why are the A and B alleles dominant?

A dominant gene prevails over a recessive gene in the manifestation of the trait. The three alleles can determine four different blood groups. A and B give the genotype AB. Which means at the phenotype level, the blood group AB since A and B are dominant.

  1. What is a recessive trait?

Definition of the term Recessive trait: Characteristic disseminated by a gene and manifested barely in circumstances. That is the place the gene is existing on both chromosomes.

  1. How are genes transmitted?

The sexual transmission of genes from father and mother occurs by combining their chromosomes and meiosis cell division. Therefore, each of us created from a cell made up of a combination of the chromosomes. Of our father and mother.


Conclusion of What is an Allele? 

Chromosomes carry genes. Units of genetic information that each allow the expression of a hereditary trait. Two versions or alleles can correspond to a gene. It occupies the same position on each of the two chromosomes of a pair.

One of the two homologous chromosomes carries allele A and the other allele B. In this case, the blood group expressed will be AB. Alleles A and B are both dominant (they are codominant) over the O allele, which is recessive.

For a person to be of group O, both alleles must be O. That is to say that the father and mother are of group O.

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