An allele is a different form of the same gene that is located in the same position on different chromosomes, which are the structures that contain our genetic material. During reproduction, each parent contributes one allele to the offspring’s chromosome, which is why humans are considered a diploid species.
Have you ever wondered why there is such a wide range of diversity in people’s appearances? Whether it’s their height, eye color, hair color, hand size, or face shape, every individual in the world (except identical twins) has a unique combination of physical features. That’s more than 7 billion original combinations. The question is, what determines this vast array of appearances? This question may be even more intriguing for those of you who have blue eyes in a family of brown eyes!
While many of you may have heard of genes and understand their influence on your health, the true control system behind your physical appearance lies in the alleles!
What Are Alleles?
An allele is a different form of the same gene that is located in the same position on different chromosomes, which are the structures that contain our genetic material. During reproduction, each parent contributes one allele to the offspring’s chromosome, which is why humans are considered a diploid species.
The main function of alleles is to determine our physical appearance (phenotype). The combinations of alleles from each parent, out 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 have a measurable effect on the organism’s phenotype and life, while non-coding genes are considered “filler” and have no effect. In fact, approximately 98% of the genetic information in the human genome is considered “junk”.
If there are two identical alleles at the same genetic locus, the gene’s genotype is homozygous and will typically result in a clear phenotypic response in the body. However, if the two alleles are different, the genotype is called heterozygous, which can be more complex.
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 (e.g., brown eyes – a dominant trait). In a homozygous situation where both alleles are dominant, the dominant trait will be present. In a homozygous situation where both alleles are recessive, the recessive trait will be expressed phenotypically (e.g., blue eyes – a recessive trait). This is a simplified explanation of how dominant and recessive alleles interact (in a Mendelian inheritance context), but there is often much more happening beneath the surface, as will be explained below.
Polygenic Inheritance, Mendelian Inheritance And Pleiotropy
Some pairings of alleles between chromosomes are simple in terms of their phenotypic response, while others may be more complex, with multiple genes and alleles affecting a specific physical trait. When multiple genes influence a given trait, it is known as polygenic inheritance. In this context, more than one location on different genes will determine the expression of a trait. These genes are called polygenes. This complex network of influence is common for our phenotypic expression and may explain the wide range of human characteristics. Polygenic inheritance determines traits such as hair color, body weight, height, and body shape, among others.
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 modern genetics. In Mendelian inheritance, basic crosses such as AA x BB, Aa x Bb, and aa x bb determine whether a trait appears or not. This type of inheritance determines various traits such as the presence of a widow’s peak, a cleft chin, facial freckles, or detached earlobes. The problem with Mendelian inheritance is that if there is a mutation in one of the genes for these traits, it can cause several diseases. Examples of 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 is not very common, similar to Mendelian inheritance, but it has similar risks. If there is a mutation in a pleiotropic gene, it can affect multiple traits and may cause various complications or diseases. Some of the conditions associated with pleiotropic genes include autism, schizophrenia, and albinism.
Why Are Alleles Important?
Aside from being responsible for our phenotypic expression and providing the genetic blueprint for our existence, researchers are highly interested in understanding alleles and their interactions, particularly within the human genome.
Alleles play a crucial role in predicting genetic disorders, whether in adults or unborn fetuses. By examining allele configurations and the intricacies of the genetic code, it is possible to determine the likelihood of certain diseases or conditions before birth. Now that researchers know the specific locations of certain alleles on chromosomes, disease prevention or early treatment can be pursued based on an individual’s genetic makeup.
Research on alleles has greatly benefited the use of DNA in law enforcement and criminal investigations. Identifying unique patterns in the genetic code is an effective way to match DNA samples, whether the goal is to determine paternity or solve a murder case!
A Final Note
While “genetic testing” and “gene therapy” may be familiar terms, it is important to understand the foundation of our basic genetic information. Without the ability to match or mismatch alleles in countless unique combinations, we would not have the incredible diversity of humanity that exists on this planet!