What is an Allele?

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 1. Introduction

Why do some people have blue eyes while others have brown? Why are some plants tall while others are short? The answer lies in alleles, the different forms of a gene that determine variations in inherited traits.

Alleles are the core of genetics and heredity, explaining why offspring resemble their parents yet show unique differences. Understanding alleles helps us grasp how traits are inherited, diseases occur, and evolution progresses.

Let’s dive deep into what alleles are, how they work, and why they’re fundamental to life and diversity on Earth.

2. Definition of Allele

An allele is an alternative form of a gene found at the same position (called a locus) on a chromosome. Each gene in an organism can have two or more alleles, which may produce different versions of a trait.

For example:

  • The gene for eye color has different alleles — one for brown eyes, one for blue eyes, etc.

  • In plants, the gene for flower color may have alleles for red, white, or pink petals.

In diploid organisms (like humans), each individual inherits two alleles for each gene — one from each parent.

                                          Fig. A symbolic photo of an allele in a DNA sequence

3. Origin and Discovery of Alleles

The concept of alleles comes from Gregor Mendel, the Father of Genetics, who experimented with pea plants in the 1860s.

Mendel observed that traits such as plant height or seed color were determined by pairs of factors (now known as alleles). One factor could mask the effect of another — a phenomenon he called dominance.

His experiments laid the foundation for Mendelian inheritance, explaining how alleles are passed from one generation to the next.

4. Structure and Location of Alleles

Alleles are located on chromosomes, which are thread-like structures in the nucleus of cells.
Each chromosome contains many genes, and each gene can exist in multiple allelic forms.

For instance, humans have 23 pairs of chromosomes. The same gene located on the same position in a pair of chromosomes can carry different alleles — one from the mother and one from the father.

5. Types of Alleles

Alleles can be classified based on how they influence traits.

1. Dominant Allele

A dominant allele expresses its effect even if only one copy is present.

  • Example: The allele for brown eyes (B) is dominant over the allele for blue eyes (b).

  • Genotypes BB and Bb both result in brown eyes.

2. Recessive Allele

A recessive allele shows its effect only when both alleles are the same.

  • Example: The blue eye trait appears only in bb individuals.

3. Co-dominant Alleles

Both alleles express themselves equally in the phenotype.

  • Example: In the AB blood group, both A and B alleles are expressed.

4. Incomplete Dominance (Intermediate Inheritance)

The resulting trait is a blend of the two alleles.

  • Example: Crossing red and white flowers produces pink offspring in snapdragons.

5. Multiple Alleles

When a gene has more than two possible alleles.

  • Example: The human ABO blood group system has three alleles — IA, IB, and i.

6. Alleles and Genotypes

A genotype is the genetic makeup of an organism concerning a particular gene.
It is represented by a combination of alleles:

  • Homozygous Dominant (AA) – Both alleles are dominant.

  • Homozygous Recessive (aa) – Both alleles are recessive.

  • Heterozygous (Aa) – One dominant and one recessive allele.

The phenotype, or physical expression of the trait, depends on the combination of alleles in the genotype.

7. Role of Alleles in Heredity

Alleles play a critical role in determining how traits are inherited across generations.
During sexual reproduction, alleles segregate and recombine to create genetic diversity.

Mendel’s Law of Segregation and Law of Independent Assortment describe how alleles behave during gamete formation and fertilization.

For example:

  • A tall plant with genotype Tt can produce both T and t gametes.

  • When crossed with another Tt plant, offspring can have TT, Tt, or tt genotypes — showing tall and dwarf variations.

8. Alleles and Mutation

New alleles arise through mutations, which are changes in the DNA sequence.

  • Some mutations are beneficial, helping species adapt to their environment.

  • Others can be harmful, leading to genetic disorders like cystic fibrosis or sickle cell anemia.

Over time, mutations create genetic variation, which fuels evolution and the survival of species under changing environmental conditions.

9. Alleles and Genetic Disorders

Certain diseases are caused by abnormal or mutated alleles.
Examples include:

  • Cystic Fibrosis: Caused by a recessive defective allele in the CFTR gene.

  • Huntington’s Disease: Caused by a dominant mutant allele.

  • Sickle Cell Anemia: A co-dominant condition where carriers show partial symptoms but gain malaria resistance.

Understanding alleles helps geneticists predict disease inheritance, improve diagnostic testing, and develop gene therapies.

10. Importance of Alleles in Evolution and Biodiversity

Allelic variation is the foundation of biological diversity.
Through natural selection, beneficial alleles increase in frequency, allowing species to adapt and evolve.

For example:

  • In peppered moths, alleles for dark and light coloration shifted in frequency during the Industrial Revolution, demonstrating evolution in action.

Thus, alleles are not just molecular variations — they are the drivers of life’s diversity.

11. Application of Allele Study in Modern Science

  1. Genetic Engineering: Scientists insert or modify alleles to produce desired traits in crops and animals.

  2. Genomic Selection: Plant breeders use allele-based DNA markers to predict yield and resistance traits.

  3. Personalized Medicine: Doctors analyze patient alleles to tailor treatments for genetic diseases.

  4. Forensic Science: Allelic DNA profiles help identify individuals in criminal investigations.

12. Conclusion

Alleles are the fundamental units of heredity that make every organism unique. They determine physical traits, control genetic diseases, and drive evolution. From Mendel’s pea plants to modern-day CRISPR gene editing, the study of alleles continues to transform our understanding of life, genetics, and biotechnology.

By appreciating how alleles work, students and scientists can uncover deeper insights into the mystery of inheritance and the genetic blueprint of life itself.

Keywords: allele definition, what is an allele, types of alleles, dominant and recessive alleles, Mendelian genetics, allele and genotype, genetic variation, mutation and allele, gene vs allele, examples of alleles

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