Multiple Alleles – Definition, Characteristics, Examples, Importance

What is Multiple Alleles?

  • In the realm of genetics, the blueprint of life lies within our genes—sequences of DNA that govern the characteristics and traits of living organisms. Genes don’t exist in isolation; rather, they can come in various forms known as alleles. Alleles are different variants of a gene located at the same position on homologous chromosomes. While the conventional understanding often involves two alleles—one inherited from each parent—there exists a more complex scenario called “multiple alleles,” where genes exhibit more than two allelic forms.
  • A fundamental concept to grasp is that of haploid and diploid cells. A haploid cell contains only one allele for each gene, while diploid cells can carry either of the two alleles present on their paired homologous chromosomes. When a gene has more than two allelic variations controlling a particular trait, it is referred to as multiple allelism.
  • Imagine a gene with a bustling locus, a spot on a chromosome where it resides. Here, different versions of the gene, known as alleles, cluster together. These alleles represent distinct expressions of the same gene, influencing a shared trait. Remarkably, a single gene can harbor numerous alleles at the same chromosomal site.
  • Within the intricate tapestry of alleles, one often encounters the wild-type allele, which typically holds dominance over mutant alleles. The wild type serves as the benchmark—the accepted norm to which other alleles are compared and categorized. Mutant or variant alleles can wield various degrees of influence on phenotype, some showcasing dominance, others remaining recessive, and some even exerting intermediate effects.
  • Zooming out to the population level, we see a multitude of alleles coexisting, fostering genetic diversity. However, an individual organism can only possess two alleles for a given gene, one inherited from each parent. This genetic dance is in line with Gregor Mendel’s initial hypothesis of a two-allele system. Nonetheless, at the population level, multiple alleles create a rich tapestry of genetic variation.
  • Heterozygosity and homozygosity are key terms in this narrative. Offspring with different alleles of a gene are termed heterozygous, while those carrying the same alleles are homozygous for that gene. In humans and other diploid organisms, individuals can only possess two alleles per gene, but populations can exhibit multiple allele traits, enhancing variability within a species.
  • In the classical framework of Mendelian inheritance, genes occupied specific chromosomal locations with just two distinct alternatives—a dominant and a recessive allele. However, reality can be more intricate. In certain populations, more than two alternatives, or alleles, coexist at a single locus. This occurrence is aptly labeled “multiple allelism.” Here, diverse forms of the same gene blossom within the population, with three or more variants interacting to mold traits.
  • For organisms with two gene copies, or diploid organisms, the stage is set for multiple alleles to take center stage. Each individual can express a pair of alleles simultaneously, either with both being identical (homozygous) or of different types (heterozygous). The story extends to haploid organisms and cells, where despite possessing just one gene copy, the broader population can house an array of alleles.
  • Novel alleles come to life through spontaneous mutations in both haploid and diploid organisms. These mutations might yield subtle or drastic changes in DNA sequences. Such changes ripple through the genetic code, ultimately influencing the sequence of amino acids in proteins. These proteins, the end products of gene expression, define the various phenotypes stemming from multiple alleles. While a specific phenotype can be the result of various mutations, the continuous emergence of mutations contributes to the fascinating mosaic of genetic diversity.
  • In this intricate dance of genetics, multiple alleles merge in diverse ways, yielding a spectrum of phenotypes. The phenotypic outcomes are shaped by the proteins encoded by the different alleles. These alleles, each encoding a similar protein type, introduce significant variability in protein function. The complex interplay of these proteins, combined with environmental effects, determines an organism’s overall fitness. Natural selection operates on this diversity, favoring those organisms whose allele combinations grant them advantages in surviving and reproducing.
  • As genetic lines evolve through new mutations and novel combinations, the intricate choreography of multiple alleles underpins the dynamic process of species origin and diversification. In the grand theater of genetics, the concept of multiple alleles stands as a cornerstone of diversity and adaptation.

Definition of Multiple Alleles

Multiple alleles refer to the situation where a gene exists in more than two alternative forms (alleles) within a population, contributing to genetic diversity and a range of possible traits.

Multiple Alleles Characteristics

The intricate world of genetics unveils itself through the concept of multiple alleles, each revealing unique aspects of gene expression and trait inheritance. The characteristics of multiple alleles offer a deeper understanding of the intricacies of genetic diversity and its impact on phenotypic traits.

  1. Shared Locus on Chromosome: Multiple alleles of a gene congregate at the same locus on a chromosome, forming a cluster of genetic possibilities. This arrangement paves the way for a rich array of allelic forms to influence a specific trait.
  2. Shared Influence on Traits: Multiple alleles, as alternative forms of the same gene, exert their influence over the same trait. This phenomenon highlights the versatility of genetic interactions and the intricate ways in which genes shape an organism’s characteristics.
  3. Dominance of Wild-Type Allele: In many instances, the wild-type allele takes center stage by exhibiting dominance over mutant alleles. This dynamic showcases the established norm and its prominence in shaping the trait under consideration.
  4. Standard vs. Variants: The wild type serves as the standard reference point, against which all other alleles are considered variants. This classification underlines the diversity that arises from genetic variations within a population.
  5. Diverse Phenotypic Effects: Mutant alleles, whether dominant, recessive, or intermediate, manifest diverse phenotypic effects. The spectrum of effects demonstrates the intricate interplay between alleles and how they contribute to trait expression.
  6. Population-Level Existence: Multiple alleles flourish at the population level, encapsulating the breadth of genetic diversity within a species. This genetic richness allows a species to adapt and respond to changing environmental conditions.
  7. Homozygous and Heterozygous States: An individual’s genetic makeup can manifest in two primary states: homozygous (carrying two identical alleles) or heterozygous (possessing two different alleles). This duality highlights the genetic complexity underlying each individual.
  8. Generation of Mutant Alleles: Spontaneous mutations, arising from alterations in the wild-type allele, give rise to a series of mutant alleles. This ongoing process contributes to the dynamic genetic landscape within a population.
  9. Phenotypic Diversity: The presence of multiple alleles fuels the creation of a diverse array of phenotypic traits associated with a particular characteristic. This diversity enables a species to adapt and thrive in various environments.

In essence, the characteristics of multiple alleles weave a tapestry of genetic intricacy, offering a glimpse into the multifaceted nature of trait inheritance and evolution. This genetic dance of variants and expressions showcases the remarkable adaptability and diversity that define life on our planet.

Examples of Multiple Alleles

1. Coat color of cats

  • In the captivating world of genetics, coat color in cats stands as a vivid example of the complexities of inheritance, with its rich palette of hues stemming from the interplay of multiple alleles.
  • Similar to the color-determining gene (c) in rabbits, cats boast a diverse array of coat colors governed by multiple alleles. This genetic landscape encompasses a spectrum of possibilities, from deep black and vibrant orange to nuanced shades of brown and elegant white.
  • In the realm of cat coat color genetics, these alleles possess the power to mold an individual’s appearance. Through a delicate dance of inheritance and expression, various alleles determine the production of pigment-producing proteins, ultimately shaping the coat color.
  • The captivating journey of cat coat color alleles parallels that of rabbits. Just as the c gene in rabbits boasts four alleles—c+ (wild-type), cch (chinchilla-type), ch (Himalayan-type), and c (albino-type)—so too do the coat color genes in cats exhibit multiple variants. This diversity is a product of evolution, with mutations leading to the development of these distinct alleles.
  • In the enchanting feline realm, the effects of these alleles are most evident when they manifest in homozygous conditions. The intricate dance of dominance relationships among these alleles reveals itself through heterozygous combinations. The wild-type allele (c+) emerges as a potent force, exercising complete dominance over all other alleles. The chinchilla allele (cch) showcases partial dominance in relation to both Himalayan (ch) and albino (c) alleles. Meanwhile, the Himalayan allele (ch) asserts its complete dominance over the albino allele (c).
  • Consider the captivating assortment of coat colors and patterns that grace the feline world. From tortoiseshell patterns comprising patches of black and orange to the interplay of black, grey, white, and patches of white fur in piebald spotting, the mosaic of possibilities epitomizes the concept of multiple allelism.
  • As breeders and genetics enthusiasts delve into the realm of coat color inheritance, a fascinating tapestry of genotypes and phenotypes unfurls. By deciphering the coat coloration and patterns, the underlying genetic makeup of a cat becomes discernible. Armed with knowledge of the phenotypes of parental cats, predictions about the potential coloration of kittens become feasible, even if the intricacies of these calculations can be complex.
  • In the world of feline genetics, the coat color story is a testament to the multilayered marvels of multiple alleles. Just as each unique coat tells a story, so too does the genetic code that underpins this diversity, offering a glimpse into the intricate dance of inheritance and variation that defines the world of living organisms.
Four different alleles exist for the rabbit coat color (C) gene.
Four different alleles exist for the rabbit coat color (C) gene. | Image Source:

2. Multiple alleles in fruit flies

  • In the realm of genetics, the common fruit fly, scientifically known as Drosophila melanogaster, has emerged as a remarkable model organism. Its compact genome, high reproductive rate, and the ease of analyzing large populations make it a laboratory favorite for exploring the intricacies of genetics. A significant milestone was reached in 2000 when scientists completed the complex genome mapping of this tiny creature.
  • Despite its diminutive size, the fruit fly’s genome packs a punch. With approximately 165 million base pairs, it pales in comparison to the human genome. Remarkably, fruit flies boast a mere four chromosomes, a stark contrast to humans’ 23. Yet, within this modest genetic framework, around 17,000 genes orchestrate the various aspects of a fruit fly’s existence. Each of these genes contributes to distinct traits and characteristics, all subject to the continuous dance of mutation, generating new alleles that drive genetic diversity.
  • The captivating tale of multiple alleles comes to life through the lens of the fruit fly’s wing length. Normally, the wings of Drosophila melanogaster extend quite long. However, intriguing mutations arose at a specific locus in different flies, leading to the development of vestigial (reduced) wings in some and antlered (less developed) wings in others.
  • An enchanting genetic experiment unfolds when a fruit fly with vestigial wings is crossed with one possessing antlered wings. The offspring, termed F1 hybrids, display wing lengths that fall between the two parental extremes. This interplay of wing lengths reflects the absence of dominance among the mutated genes at play. The resulting hybrid, often referred to as the vestigial antlered compound, encapsulates the genetic synergy of two mutations at the same locus. The principles of Mendelian segregation and recombination are beautifully evident in this genetic dance.
  • Beyond the realm of vestigial and antlered wings, a plethora of phenotypes emerge. Nicked wings, strap wings, or even the absence of wings altogether paint a vibrant canvas of genetic variation. These phenotypes are the handiwork of multiple alleles nestled within the diverse population of fruit flies.
  • In essence, the genetic tapestry of fruit flies embodies the intricate world of multiple alleles. With each gene harboring the potential for numerous allele variations, the fruit fly serves as a captivating example of how slight variations in genetic code can give rise to a stunning array of phenotypic outcomes. This tiny creature, with its four chromosomes and vast genetic potential, invites us to explore the intricate genetic dance that shapes life’s diversity.

3. Multiple Alleles in the ABO Blood Group System

The ABO blood group system, which determines our blood type, was discovered by Karl Landsteiner in the early 1900s. This pivotal discovery, which categorized human blood into four main types, was honored with a Nobel Prize in 1930. The determination of these blood types is based on the presence or absence of specific antigens and antibodies in the blood.


Genetics of the ABO Blood Group System

The ABO blood group system is determined by a single gene that exists in three different alleles: IA, IB, and i. These alleles are responsible for the synthesis of the different antigens found in human blood:

  1. IA allele produces the A antigen.
  2. IB allele synthesizes the B antigen.
  3. i allele leads to the synthesis of the O (or H) antigen.

From these three alleles, we can derive six possible genotypes:

  • IAIA and IAi produce the A blood group.
  • IBIB and IBi yield the B blood group.
  • IAIB results in the AB blood group.
  • ii produces the O blood group.

IA and IB alleles are codominant to each other, but both are dominant over the i allele.

Biosynthesis of ABO Blood Group Antigens

The synthesis of ABO blood group antigens begins with the H substance, a precursor made of galactose, N-acetylglucosamine, and fucose. The H substance is transformed into the different blood group antigens via various glycotransferase enzymes encoded by the ABO genes:

  1. H/O Antigen Synthesis: Almost everyone has the H antigen on their glycolipids. The addition of Fucose to the precursor by the α 1,2 fucosyltransferase enzyme results in the formation of the H antigen. If the i allele is present and no functional transferase enzyme is produced, only the H antigen remains, leading to the O blood type.
  2. A Antigen Synthesis: The IA allele encodes the acetylgalactosaminyltransferase enzyme, which adds N-galactosamine to the H antigen, converting it into the A antigen.
  3. B Antigen Synthesis: Conversely, the IB allele encodes the α 1,3 galactosyltransferase enzyme. This enzyme adds galactose to the H antigen, forming the B antigen.

ABO Blood Group and Blood Transfusions

When discussing blood transfusions, understanding the antigen-antibody relationships in the ABO system is crucial. For instance, individuals with blood type A produce the B antibody, making it unsafe for them to receive blood from type B individuals. A transfusion between incompatible blood types can lead to dangerous clumping of red blood cells.

  • Type A: Can donate to A and AB types.
  • Type B: Can donate to B and AB types.
  • Type AB: Known as universal recipients, can only donate to AB but can receive from all types.
  • Type O: Recognized as universal donors, can donate to all types but can only receive from O.
biosynthesis of A, B, and O antigens
biosynthesis of A, B, and O antigens | Image Source: intechopen

The Rare Bombay Blood Type

Distinct from the ABO system, the Bombay phenotype arises when an individual has a rare mutation in the dominant H gene, causing an inability to produce the H antigen. Though these individuals might appear to have O blood type due to the absence of A and B antigens, they produce anti-O antibodies, setting them apart from true O blood types.

Pedigree showing Bombay blood type
Pedigree showing Bombay blood type | Image Source: intechopen
Blood group phenotypeGenotype/allelesAntigen presentAntibody producedSpecial properties
AIAIA, IAiA antigenB antibody
BIBIB, IBiB antigenA antibody
ABIAIBA and B antigenNo antibodiesUniversal recipient
OiiBoth A and B antibodyUniversal donor
ABO blood type system.

4. Multiple alleles in plants

  • In the world of plants, genetic diversity often unfolds through the intricate dance of multiple alleles, giving rise to a breathtaking array of colors and patterns. A closer look at the genetic mechanisms underlying flower color in snapdragons and seed coat color in beans unveils the captivating interplay of multiple alleles.
  • The striking beauty of flower colors in snapdragons emerges from a complex web of alleles. In this genetic ballet, four key alleles take center stage: W (white), R (red), P (pink), and I (intermediate). Dominance relationships and codominance interactions weave a rich tapestry of flower colors and patterns.
  • When a plant harbors two copies of the dominant W allele, the result is white flowers. This allele takes precedence over all other alleles, painting the petals with a pristine hue. The R and P alleles, however, engage in a harmonious codominance, leading to a remarkable visual spectacle. Individuals carrying one copy of each allele display flowers that showcase a delicate dance between red and pink, adorning the petals with a mesmerizing splatter of colors.
  • The genetic story takes yet another intriguing turn with the I allele. All alleles other than I are recessive in comparison. In the presence of two I alleles, the outcome is flowers with intermediate colors, bridging the gap between the dominant and recessive hues. As a result, the snapdragon population boasts a remarkable diversity of flower colors and patterns, a testament to the multifaceted nature of multiple allele systems.
  • Venturing into the realm of bean seed coat colors, a similar genetic ballet unfolds, albeit with a slightly different cast of characters. Three central alleles play pivotal roles: W (white), Y (yellow), and R (red). Dominance relationships and codominance interactions once again orchestrate the vibrant spectrum of seed coat hues.
  • Embracing the genetic spotlight, the dominant W allele imbues seed coats with a resplendent white sheen. In a splendid demonstration of codominance, individuals possessing one copy each of the Y and R alleles enjoy seed coats adorned with a captivating medley of yellow and red flecks. The result is a visual feast, a testament to the intricate harmony of genetic interactions.
  • Through the prism of these examples, we witness how multiple alleles weave a story of diversity within plant populations. The delicate interplay of dominant and recessive alleles, coupled with the captivating dance of codominance, conjures a world of colors and patterns that enchant the eye and capture the imagination. Indeed, the genetic landscape of plants is a canvas painted with the hues of genetic variation, where each allele adds a brushstroke to the masterpiece of life.

5. Multiple alleles in bacteria

  • In the microscopic realm of bacteria, genetic diversity thrives through the presence of multiple alleles, each painting a unique portrait of virulence, classification, and treatment strategies. Bacteria, with their multitude of genes, harbor a fascinating tapestry of variation, a complexity that extends to their diverse alleles.
  • Among the myriad genes that populate bacterial genomes, several stand out with multiple alleles, each associated with distinct facets of virulence. These alleles, often linked to specific types of virulence, play a pivotal role in classifying bacterial subspecies. One notable example is the utilization of housekeeping genes for Multi Locus Sequence Typing (MLST), a technique that enables the differentiation of bacterial subspecies based on their alleles.
  • In the quest to understand and combat bacterial infections, identifying not just the gene in question, but the specific allele involved, takes center stage. The available sequencing-based methods, though effective, present challenges. These methods involve the intricate task of mapping reads to known allele references, a process that can be time-consuming.
  • Beyond unraveling the identity of the infectious agent, understanding the pathogenic impact and potential outbreak dynamics necessitates delving deeper. The story of bacterial virulence often unfolds on a sub-species level, governed by a repertoire of specific genes and alleles. This intricate dance of genetic diversity mandates tailored treatment strategies for infections emanating from the same bacterial species.
  • Consider antibiotic resistance, an eloquent demonstration of how minor gene variations birth a mosaic of antibiotic resistance profiles within a single taxonomic group. These subtle genetic differences wield profound implications, reshaping the landscape of bacterial response to treatments.
  • As the genetic saga continues, different alleles of a single gene emerge as architects of distinct strategies. From adhesion and invasion tactics to crafting responses against the host organism’s immune system, and even the production of toxins, these allelic variations leave an indelible mark on bacterial behavior.
  • However, the significance of identifying alleles extends beyond understanding virulence. It plays a vital role in precision classification, painting a finer portrait of bacterial diversity. Just as an artist employs various brushes to capture subtle nuances, the identification of alleles offers a nuanced view into the bacterial world.
  • In this intricate journey, we witness how multiple alleles shape the bacterial realm. They underscore the need for precise identification, classification, and treatment strategies. Each allele, a unique chapter in the bacterial story, brings us closer to untangling the complexities of these microscopic organisms that wield an outsized impact on our world.

What is Pleiotropy?

Pleiotropy is like a genetic multitasker. Instead of just one gene affecting one thing, multiple genes work together to influence different traits in an individual. Imagine it like a team of genes collaborating to create a bunch of effects on how someone looks or behaves.

For example, in some people with albinism (lack of pigment in skin and hair), they might also have a different trait, like cross-eyes. This shows how different genes can team up to create more than one effect.


Even eye color, which seems simple, is actually influenced by several genes working together. Think of it like genes joining forces to create a rainbow of eye colors.

So, pleiotropy is like a genetic teamwork where multiple genes come together to create a mix of traits, making each person unique in their own way.

  • In the intricate landscape of genetics, the concept of pleiotropy emerges as a fascinating phenomenon, where the effects of multiple genes intertwine to sculpt a single phenotype. Unlike the singular focus of multiple allelism, which involves various forms of a single gene, pleiotropy unfolds as a genetic tapestry woven by multiple genes orchestrating diverse effects on the phenotype.
  • At its core, pleiotropy represents the embodiment of genetic complexity. This phenomenon reveals itself when a constellation of genes collaborates to influence not just one, but multiple characteristics of an individual’s observable traits. A striking juxtaposition emerges when comparing pleiotropy to multiple allelism: while the latter delves into the variants of a single gene, the former encapsulates the intricate interplay of multiple genes influencing a multitude of traits.
  • Consider the intriguing case of albino individuals. In addition to the well-known lack of pigment production in their skin and hair, certain albino individuals display a distinct trait: cross-eyes. This revelation uncovers the essence of pleiotropy. Notably, the presence of multiple genes influences both the absence of pigment and the occurrence of cross-eyes. The interconnectedness of these traits provides a glimpse into the intricate dance of genetics.
  • However, it’s important to note that not all albino individuals exhibit cross-eyes. This observation, rather than detracting from the concept of pleiotropy, highlights its nuanced nature. In such cases, the lack of a link between the traits underscores the multifaceted interactions within the genetic web.
  • Another captivating example resides within the realm of eye color. An individual’s eye color, a seemingly simple trait, is actually under the influence of more than one gene. These genes, such as OCA2 and HERC2, collaborate to shape the captivating spectrum of eye colors that grace humanity.
  • In essence, pleiotropy shines a light on the intricate and intertwined nature of genetics. It serves as a reminder that the genetic orchestra playing within each living being’s cells is not a solo performance but a symphony of genes harmonizing to compose the multifarious melodies of phenotype. With each gene contributing to various aspects of an individual’s traits, pleiotropy invites us to unravel the intricate mysteries of genetics and the profound ways in which genes sculpt the living canvas of life.

What is Expressivity?

  • Expressivity delves into the extent or intensity of a trait associated with a specific genetic makeup. Imagine it like the volume knob for genetic traits – it determines how strong or mild a trait appears in an individual.
  • When it comes to genetics, things can get interesting with multiple alleles. These are different versions of a gene that can lead to varying expressivity. In simpler terms, even though two people might have the same genetic code, their traits might show up differently in terms of strength or severity.
  • Think of it as a combination of factors. Genetic expression can be influenced by variations in how genes are turned on or off, environmental factors, or even genetic modifiers that tweak the outcome. It’s like a mix of ingredients in a recipe that creates a unique flavor.
  • Consider the gene responsible for neurofibromatosis type 1 (NF1), a condition that leads to benign tumors growing on nerve tissue. Interestingly, even among people with the same genetic makeup causing this condition, the number and seriousness of tumors can vary widely. This happens because the expressivity of the NF1 gene isn’t consistent – it varies in how strongly it affects the condition’s severity.
  • This genetic variability is not limited to NF1. Multiple alleles, like pieces of a puzzle, contribute to the wide range of traits we see. These alleles bring forth diverse characteristics, often showcasing pleiotropy, incomplete penetrance, and, of course, variable expressivity.
  • Ultimately, understanding how genetics shape human health relies on deciphering these intricate interactions between genes and traits. By peeling back the layers of expressivity, we unlock the secret language that genes use to paint the canvas of our individuality.

Test of allelism

Genes are responsible for determining various traits in organisms. Sometimes, genes can have different versions called alleles. When a gene’s DNA sequence changes, a mutant allele is formed. However, these mutant alleles might not always result in noticeable changes in appearance. If a trait is controlled by multiple genes and a mutation occurs in one of them, it might affect the trait’s expression. Figuring out which gene has mutated is challenging, especially if the mutation is recessive.

To solve this puzzle, scientists conduct a test called the “test of allelism.” This test helps determine if two mutations are different versions of the same gene (alleles) or if they are mutations in different genes (non-allelic).

Let’s look at an example using recessive eye color mutations in fruit flies:

Example: Recessive Eye Color Mutations in Fruit Flies

In fruit flies (Drosophila melanogaster), there are three recessive eye color mutations: scarlet, cinnabar, and cinnabar-2. These mutations all result in bright red eye color, making it hard to differentiate them based on appearance alone. To figure out if these mutations are alleles of the same gene or different genes, scientists perform the test of allelism.

Performing the Test:

  1. Cross two homozygous (purebred) strains of flies, each with a different mutation.
  2. Analyze the eye color of the hybrid offspring (F1 generation).

Results and Interpretation:

  1. scarlet × cinnabar: If the hybrid offspring have the wild-type eye color (dark red), it means scarlet and cinnabar mutations are non-allelic, i.e., they affect different genes.
  2. cinnabar × cinnabar-2: If the hybrid offspring have the mutant eye color (bright red), it means cinnabar and cinnabar-2 mutations are allelic, i.e., they are different versions of the same gene.
  3. scarlet × cinnabar-2: If the hybrid offspring have the wild-type eye color (dark red), it means scarlet and cinnabar-2 mutations are non-allelic.

This test helps scientists understand whether mutations are in the same gene or different genes, based on the observed hybrid phenotypes.

In conclusion, the test of allelism is a powerful tool to unravel the genetic basis of traits. By crossing different mutations and analyzing the hybrid offspring’s traits, scientists can determine if mutations are alleles of the same gene or mutations in different genes. This information is crucial for understanding how genes control traits in organisms.

Structural and functional basis of allelism

When we talk about the relationship between different versions of a gene (alleles), we can understand it from two main viewpoints: structure and function. These viewpoints are known as structural allelism and functional allelism.

Structural Allelism: Imagine mutations that affect the same trait, like eye color. In structural allelism, we look at the position of these mutations on the gene’s DNA sequence. If two or more mutations happen at the exact same spot on the DNA, they’re called structurally allelic. However, if the mutations occur at different spots, they’re structurally non-allelic.

Scientists perform a test called a recombination test. This test checks whether these mutations can exchange parts through a process called crossing over. If they can and still produce the normal version of the gene, they’re structurally non-allelic. If they can’t exchange parts and the normal gene can’t be formed, they’re structurally allelic.

Functional Allelism: Functional allelism focuses on whether these mutations affect the same gene or different ones. To find out, scientists do a complementation test. This test checks if the mutations can “help” each other create the normal gene product (like a protein). If two mutations are in the same gene and they can’t work together to create the normal product, they’re functionally allelic. If they’re in different genes and can work together, they’re functionally non-allelic.

Differences Between Structural and Functional Allelism:

  • Where it happens: Structural allelism is about the DNA sequence, while functional allelism is about the end product (like a protein).
  • Test used: For structural allelism, scientists use a recombination test. For functional allelism, they use a complementation test.
  • Chromosome breakage: Structural allelism involves breaking and exchanging parts of chromosomes. Functional allelism doesn’t need chromosome breakage.

In simple terms, structural allelism looks at where mutations sit on the gene’s DNA, and functional allelism looks at whether these mutations can work together to create the normal gene product. When mutations are the same both structurally and functionally, they’re called homoalleles. When they’re functionally the same but structurally different, they’re heteroalleles.

This way, scientists can understand how mutations affect genes and traits, and whether they’re variations of the same gene or completely different ones.

Allelic relationship of different mutations.
Image Source: intechopen

Multiple Alleles vs. Polygenic Traits

When it comes to unraveling the genetic mysteries that shape our world, two key players take the stage: multiple alleles and polygenic traits. Let’s delve into their differences and similarities to illuminate the intricate dance of genetics.

Multiple Alleles: Imagine a gene with a wardrobe of options – that’s the essence of multiple alleles. This genetic phenomenon involves two or more variations of the same gene, each putting a unique twist on a trait. Picture these alleles as different colors of paint that can create a spectrum of shades.

  • Forms: These alleles reside on the same DNA strand, much like notes on a musical score.
  • Environmental Influence: Multiple alleles tend to stand firm against the sway of environmental factors. Their expression remains steady, like a lighthouse amidst changing tides.
  • Trait Determination: Co-dominance or complete dominance orchestrates how these alleles influence traits. It’s like a genetic tug-of-war where both sides share equal influence.
  • Genetic Exchange: Homologous crossing over doesn’t enter the scene for multiple alleles. They remain in their genetic lanes, unchanged.

Polygenic Traits: Polygenic traits, on the other hand, form a genetic orchestra led by multiple genes. Imagine each gene playing a different musical note that harmonizes to shape a symphony of traits.

  • Forms: Unlike multiple alleles, these genes reside on multiple DNA strands, as if each musician is in a different section of the orchestra.
  • Environmental Influence: Polygenic traits have a softer side – they sway to the rhythm of environmental factors. External cues influence their expression, like sunlight changing the mood of a painting.
  • Trait Determination: Co-dominance or incomplete dominance dictates the outcome here. The genes aren’t fighting for dominance; they’re blending together to create a nuanced masterpiece.
  • Genetic Exchange: Polygenic traits engage in homologous crossing over between alleles of each gene. It’s like swapping musical notes to create new harmonies.

Traits Involved:

  • Multiple alleles are the artisans of qualitative traits – they decide if a cat’s coat is black or white, or if blood is type A, B, AB, or O.
  • Polygenic traits hold the brush for quantitative traits – think about the spectrum of kernel colors in wheat or the heights of individuals.


  • Multiple alleles are present in a population, like a palette of colors available to artists.
  • Polygenic traits are found within an individual, crafting their unique genetic composition.

In essence, multiple alleles and polygenic traits are like two different orchestras playing unique melodies. One carries a variety of notes within a single gene, while the other harmonizes multiple genes to create a symphony of traits. Together, they illuminate the diversity and complexity that genetics brings to life’s canvas.

AspectMultiple AllelesPolygenic Traits
DefinitionTwo or more alternate forms of the same geneControlled by a group of non-allelic genes
Location on DNAPresent on the same DNA strandFound on multiple DNA strands
Environmental InfluenceUnaffected by environmental factorsAffected by environmental factors
Trait DeterminationCo-dominance or complete dominanceCo-dominance or incomplete dominance
Genetic ExchangeNo homologous crossing overHomologous crossing over between alleles of each gene
Types of TraitsResponsible for qualitative traitsResponsible for quantitative traits
DistributionMultiple alleles found in the populationPolygenic traits found in individuals
ExamplesABO blood grouping, white gene locus in drosophila, cat coat color, HLA geneKernel color in wheat

Pseudoalleles vs. multiple alleles

When it comes to genes, there are some interesting terms to understand: pseudoalleles and multiple alleles. Let’s break down the differences between these two concepts.

Pseudoalleles: Imagine two genes that are like close friends – they’re buddies on the same chromosome and have similar effects. These genes are called pseudoalleles. Because they’re so close, they’re usually inherited together and might seem like they’re just one gene working together. Pseudoalleles are not the same as pseudogenes, which are like nonfunctional copies of real genes. Pseudoalleles happen when two genes with similar effects are cozily located side by side on the chromosome.

Multiple Alleles: Now, let’s talk about multiple alleles. Picture a gene with a bunch of outfits. These outfits are versions of the gene, called alleles. When a gene has multiple alleles, it’s like having lots of clothing options. Each allele might make a little difference in how something works or looks. But they all come from the same gene family.

How to Tell Them Apart: Here are some clues to help tell the difference between pseudoalleles and multiple alleles:

  1. Location: Pseudoalleles are genes that are close together on the chromosome and act together. Multiple alleles are different versions of a single gene, like various outfits.
  2. Crossing Over: When scientists try to separate genes through crossing over (a gene swap), pseudoalleles stay put. Multiple alleles can switch around.
  3. Effects: Pseudoalleles often look like one gene because they work so closely. Multiple alleles can make small changes but still belong to the same gene.

An Example: In fruit flies, there are red eye color mutants called white eye (w) and apricot (apr). These two are pseudoalleles because they’re close buddies on the chromosome. They act together and sometimes create surprising eye colors.

Origins: Pseudoalleles are born from gene duplication. It’s like making a copy of a gene, and the two copies become neighbors. They might look and act similar at first, but over time, they might change and become a bit different.

So, remember, pseudoalleles are like gene buddies that are neighbors, while multiple alleles are different versions of the same gene. Each one adds a little flair to the gene party.

TraitPseudoallelePseudogeneMultiple Allele
DefinitionPseudoalleles refer to two closely linked genes with similar effects located near each other on a chromosome.Pseudogenes are non-functional copies of functional genes.Multiple alleles exist when there are more than two alleles in a group of individuals for a specific trait.
OriginOriginates from gene duplication.Arises through gene duplication or reverse transcription of mRNA.Arises from mutations.
Crossing overPossible, as pseudoalleles are closely linked.Possible, as pseudogenes can also undergo recombination.Not possible, as multiple alleles exist at the same locus.
Final productResults in functional gene products.Does not result in functional products; they are non-functional.Leads to functional gene products.
Locus involvedInvolves more than one locus, as pseudoalleles are closely linked genes.Can involve more than one locus, as pseudogenes can be found on different loci.Involves only one locus, where different alleles exist.
Trait affectedAffects the same trait, with pseudoalleles having similar effects.Affects the same trait, as pseudogenes are copies of functional genes.Affects the same trait, but with different variations of the gene.
ExampleRed eye color mutants like white and apricot in Drosophila are an example of pseudoalleles.PTENP1 pseudogene is an example of a non-functional pseudogene originating from a tumor-suppressing gene.The ABO blood group system in humans is an example of multiple alleles where different alleles determine blood type.

Importance of Multiple Alleles

The concept of multiple alleles holds significant importance in genetics and biology due to its implications for genetic diversity, inheritance patterns, and the complexity of trait expression. Here are some key reasons why multiple alleles are important:

  1. Genetic Diversity: Multiple alleles contribute to the genetic diversity within populations. The presence of various alleles for a single gene increases the potential range of phenotypes, allowing organisms to adapt to different environmental conditions.
  2. Evolutionary Adaptation: Multiple alleles provide the raw material for natural selection to act upon. This genetic variation allows populations to evolve and adapt to changing environments over time.
  3. Inheritance Patterns: Multiple alleles can lead to a variety of inheritance patterns, including complete dominance, co-dominance, and incomplete dominance. Understanding these patterns is crucial for predicting the outcomes of genetic crosses and breeding programs.
  4. Phenotypic Variation: Different combinations of alleles can result in a wide range of phenotypic variations, even within a single species. This variation is essential for the survival and success of a population in different ecological niches.
  5. Medical Research: Multiple alleles play a role in various human diseases and conditions. Studying the effects of different alleles can provide insights into the genetic basis of diseases, potential treatments, and personalized medicine.
  6. Blood Transfusions: In the ABO blood group system, multiple alleles determine blood types and compatibility for blood transfusions. Understanding these alleles helps ensure safe and successful blood transfusions.
  7. Biodiversity Conservation: In species conservation efforts, considering the genetic diversity within populations is essential to maintaining healthy and resilient populations that can withstand environmental challenges.
  8. Breeding Programs: In agriculture, multiple alleles are important for developing new plant and animal varieties with desirable traits. Breeders can select for specific alleles to enhance productivity, disease resistance, and other beneficial characteristics.
  9. Population Genetics: The study of multiple alleles within populations provides insights into gene flow, genetic drift, and how populations evolve over time.
  10. Education and Understanding: Understanding multiple alleles is a fundamental concept in genetics education. It helps students grasp the complexity of inheritance, gene expression, and the interactions between alleles.

In essence, multiple alleles contribute to the rich tapestry of genetic diversity and phenotypic variation that makes each individual and species unique. Their role in shaping the natural world and our understanding of genetics is essential for advancements in biology, medicine, and evolutionary science.

Multiple Alleles Infograph
Multiple Alleles Infograph

Mindmap on Multiple Alleles

Mindmap on Multiple Alleles
Mindmap on Multiple Alleles


What are multiple alleles?

Multiple alleles are different forms or variants of a single gene that occupy the same genetic locus on a chromosome.

How do multiple alleles differ from polygenic traits?

Multiple alleles refer to different versions of a single gene, while polygenic traits are influenced by multiple genes, each with various alleles.

Can you give an example of multiple alleles in humans?

The ABO blood group system is a classic example of multiple alleles in humans, involving three alleles: IA, IB, and i.

How do multiple alleles influence blood types?

In the ABO blood group system, IA and IB alleles produce A and B antigens, while i allele results in O antigen. Different combinations determine the blood type.

Do all individuals carry multiple alleles for every gene?

No, multiple alleles are specific to certain genes and traits. Not all genes have multiple alleles.

Can multiple alleles lead to intermediate phenotypes?

Yes, multiple alleles can result in intermediate phenotypes when neither allele is completely dominant, leading to a blending or combination of traits.

Can multiple alleles exhibit co-dominance?

Yes, multiple alleles can exhibit co-dominance, where both alleles are expressed equally in a heterozygous individual, such as the AB blood type.

What is meant by variable expressivity in multiple alleles?

Variable expressivity refers to the range of phenotypic effects produced by a single genotype due to factors like gene expression variations, environmental influences, or genetic modifiers.

How do multiple alleles contribute to genetic diversity?

Multiple alleles increase genetic diversity by offering various combinations of alleles that lead to diverse phenotypes within a population.

Are multiple alleles responsible for only qualitative traits?

No, multiple alleles can be responsible for both qualitative and quantitative traits. Qualitative traits have distinct categories, while quantitative traits exhibit a range of continuous variation.


  1. Vasisth, P., M. Limbalkar, O., & Sharma, M. (2023). Genetics of Multiple Alleles: Concept and Function. IntechOpen. doi: 10.5772/intechopen.1001464

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