What Is An Allele?

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Alleles are variant forms of the same genes, which are found in the same location (locus) on different chromosomes, the structures that contain our genetic material. When humans reproduce, each parent contributes one allele to the chromosome of the offspring, which is why we are considered a diploid species.

Have you ever wondered how there can be so much incredible diversity in the way people look? Whether it is your height, eye color, hair color, the size of your hands, the shape of your face… every single person in the world (except identical twins, of course), have a unique composition of physical features. That is more than 7 billion original combinations. The question is, what determines this wide array of appearances? This might be an even more interesting question for those of you blue-eyed children in a family of brown eyes!

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While many of you have likely heard of genes and understand their impact on your health, the real control system behind your physical appearance lies in the hands of your alleles!

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What Are Alleles?

Alleles are variant forms of the same genes, which are found in the same location (locus) on different chromosomes, the structures that contain our genetic material. When humans reproduce, each parent contributes one allele to the chromosome of the offspring, which is why we are considered a diploid species.

The most important function of alleles is determining our physical appearance (phenotype). The combinations of alleles from each parent of the estimated 20,000-25,000 coding genes can either be homozygous or heterozygous. Note: There are coding genes and non-coding genes (commonly known as junk DNA) in the genetic code. Coding genes will have a measurable effect on the phenotype and life of the organism, while non-coding genes are considered “filler” and have no effect. In fact, in the human genome, roughly 98% of the genetic information is considered “junk”.

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If there are two of the same alleles at the same genetic locus, the genotype of that gene is considered homozygous, and will usually generate a very clear phenotypic response in the body. However, if the two alleles are not the same, the genotype is called heterozygous, which is where things can become a bit more complicated.

Homozygous and Heterozygous vector(gritsalak karalak)S
Homozygous vs heterozygous (Photo Credit : gritsalak karalak/Shutterstock)

In a heterozygous situation, one allele may be dominant, while the other is recessive; in this case, the dominant allele will be expressed (i.e., brown eyes – a dominant trait). In a homozygous situation where both alleles are dominant, the dominant trait will appear. In a homozygous situation, where both of the alleles are recessive, then the recessive trait will be phenotypically expressed (i.e., blue eyes – a recessive trait). This is a very simple explanation of how dominant and recessive alleles interact (in a Mendelian inheritance context), but there is often much more that is occurring beneath the surface, as will be explained below.

Also Read: What Is Mendel’s Law Of Dominance?

Polygenic Inheritance, Mendelian Inheritance And Pleiotropy

Some allele pairings between chromosome are quite simple in terms of their phenotypic response, but others may be more complex, with multiple genes and alleles affecting a particular physical trait. When multiple genes affect a given trait, it is called polygenic inheritance. In this context, more than one locus on various genes will control the appearance of a trait. Such genes are called polygenes. This sort of complex network of influence is very common for our phenotypic expression, which may explain the vast range of human features. Polygenic inheritance decides things like hair color, body weight, height and body shape, among many others.

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When a single gene controls a single trait, it is referred to as Mendelian inheritance, named after Gregor Mendel, who is considered the father of the modern study of genetics. In Mendelian inheritance, basic crosses like AA x BB, Aa x Bb, and aa x bb – those you may remember from high school science classes – determine whether a trait appears or not. This form of inheritance determines many different traits, such as whether or not you have a widow’s peak, a cleft chin, facial freckles or detached earlobes. The problem with Mendelian inheritance is that if there is a single mutation on one of the genes for these traits, it can cause a number of diseases. For example, diseases linked to mutations in Mendelian genes include Tay-Sachs disease, cystic fibrosis and sickle cell anemia, among others.

When one gene controls the manifestation of multiple traits, it is referred to as pleiotropy. This isn’t particularly common, and neither is Mendelian inheritance, but it does have similar risks as the latter. If there is a mutation on a pleiotropic gene, it can affect a number of traits, and may cause numerous complications or diseases. Some of the conditions linked to pleiotropic genes include autism, schizophrenia and albinism.

Also Read: What Are Monogenic And Polygenic Traits?

Why Do Alleles Matter?

Aside from being the site of our phenotypic expression to the world – essentially providing the genetic blueprint for our existence – researchers are extremely interested in learning as much as they can about alleles and their interactions, particularly within the human genome.

Alleles already play a key role in predicting genetic disorders, whether you are an adult or an unborn fetus. It is possible to determine the likelihood of certain diseases or conditions before you are even born by looking at your allele configurations and the intricacies of your genetic code. Now that researchers understand where certain alleles are located on particular chromosomes, disease prevention or preliminary treatment can be pursued if you have something “in your genes”.

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The use of DNA in law enforcement and criminal investigations has also been greatly benefited by research into alleles. Identifying unique patterns in the genetic code is an excellent way of matching up DNA samples, whether you are trying to identify the father of a child or crack a murder case!

A Final Word

While “genetic testing” and “gene therapy” may be terms that the world is familiar with, it’s important to understand what underlies our basic genetic information. Without alleles to match or mismatch in thousands of unique combinations, we wouldn’t be blessed with the incredible range of humanity that we can find on this planet!

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References (click to expand)
  1. What are Dominant and Recessive? - Learn Genetics Utah. The University of Utah
  2. Alleles - SPH - Boston University. Boston University
  3. Alleles of a gene - sites.stat.washington.edu
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About the Author

John Staughton is a traveling writer, editor, publisher and photographer who earned his English and Integrative Biology degrees from the University of Illinois. He is the co-founder of a literary journal, Sheriff Nottingham, and the Content Director for Stain’d Arts, an arts nonprofit based in Denver. On a perpetual journey towards the idea of home, he uses words to educate, inspire, uplift and evolve.