Most people learn early in biology that a gene has two alleles in an individual—one inherited from each parent. That is true for a diploid organism such as a human, but it can create a misleading impression: a gene in a population can have many different alleles, even though each individual usually carries only two of them at a time.
This is the idea behind multiple alleles. It helps explain why some genetic traits have more than two possible forms and why inheritance can be more varied than a simple dominant-versus-recessive pattern suggests.
What are multiple alleles?
An allele is a particular version of a gene. Different alleles arise when the DNA sequence at a gene differs in ways that can affect the gene’s function or regulation.
A gene may have just two known alleles in a population, but some genes have three, four, or many more. When a gene has more than two alleles in a population, those variants are called multiple alleles.
The key point is that multiple alleles describe variation within a population, not the number of alleles an individual carries.
Humans are diploid, meaning most of our cells contain two copies of each autosomal gene. Consequently, an individual can generally carry only two alleles of a particular autosomal gene at once. For example, if a population contains alleles A, B, C, and D, one person might carry A and C, while another carries B and D. The population has four alleles, but each person has two copies.
This distinction resolves a common misconception: multiple alleles do not mean that one person normally has three or more copies of a gene.
The ABO blood group is a classic example
The human ABO blood group provides one of the clearest examples of multiple alleles.
The ABO gene has three common alleles, conventionally written as Iᴬ, Iᴮ, and i. Because people have two copies of the gene, these three alleles can form six possible genotype combinations:
| Genotype | Blood type |
|---|---|
| IᴬIᴬ | A |
| Iᴬi | A |
| IᴮIᴮ | B |
| Iᴮi | B |
| IᴬIᴮ | AB |
| ii | O |
What makes the ABO system especially useful for understanding genetics is that it demonstrates both multiple alleles and codominance.
The Iᴬ and Iᴮ alleles are codominant to each other. When a person inherits Iᴬ from one parent and Iᴮ from the other, both are expressed, producing type AB blood.
The i allele is recessive to both Iᴬ and Iᴮ. A person therefore needs two copies of i to have type O blood.
So a single gene can have three alleles in the population while following different dominance relationships depending on which pair of alleles an individual inherits.
Multiple alleles are not the same as multiple genes
It is important to distinguish multiple alleles from polygenic inheritance.
With multiple alleles, there are several versions of one gene in the population. The ABO blood group is an example.
With polygenic inheritance, multiple genes contribute to a trait. Many human characteristics, including traits such as height and skin pigmentation, involve the combined effects of numerous genes as well as environmental influences.
The two ideas can sometimes be discussed together because both contribute to genetic variation, but they describe different mechanisms.
A useful way to remember the distinction is:
- Multiple alleles: many versions of one gene exist in a population.
- Polygenic inheritance: multiple genes contribute to one trait.
How multiple alleles arise
New alleles ultimately originate from changes in DNA. A mutation is a change in the DNA sequence. Some mutations have little or no observable effect, while others can alter how a gene works or how its product functions.
If a DNA change occurs in a cell lineage that can contribute genetic material to offspring, the resulting variant can potentially be inherited. Over generations, different variants of the same gene can persist in a population.
Population processes such as natural selection, genetic drift, mutation, and migration can influence how common particular alleles become.
The existence of multiple alleles therefore reflects genetic variation accumulated within a population over time. It does not mean that every allele is equally common or that every allele produces a distinct visible trait.
Dominance relationships can be more complicated than dominant and recessive
In elementary genetics, alleles are often described as either dominant or recessive. Those terms are useful in many situations, but they do not describe every relationship between alleles.
A dominant allele is one whose associated phenotype is expressed in a heterozygote under the relevant conditions. A recessive phenotype generally requires two copies of a recessive allele in a simple complete-dominance system.
With multiple alleles, however, the relationships can vary.
An allele may be dominant to one allele but recessive to another. Two alleles may be codominant, as Iᴬ and Iᴮ are in the ABO system. In other genes, different variants can produce a range of molecular or physiological effects rather than fitting neatly into a single dominant/recessive category.
This is why knowing that a gene has multiple alleles is not, by itself, enough to predict a person’s phenotype. You also need to know the relevant relationships among those alleles and how the gene influences the trait.
Multiple alleles increase the possible genetic combinations
Suppose a gene has three alleles: A, B, and C. A diploid individual can inherit two copies from the set, producing these possible combinations:
AA, AB, AC, BB, BC, and CC.
That gives six possible genotypes when the order of the two alleles does not matter.
If a gene has four alleles, there are ten possible two-allele genotypes. In general, for a diploid organism with n different alleles in a population, the number of possible unordered genotypes is:
n(n + 1) / 2
This illustrates why even a single gene can contribute substantial genetic diversity when many alleles exist.
The number of possible phenotypes, however, does not necessarily equal the number of genotypes. Different genotypes can produce the same phenotype, as happens with IᴬIᴬ and Iᴬi, which both produce type A blood.
Why the population perspective matters
Genetics questions can become confusing when the words “gene,” “allele,” and “genotype” are treated as interchangeable.
A gene is a stretch of DNA associated with a biological function or trait.
An allele is a particular version of that gene.
A genotype describes the allele combination an individual carries at a particular gene or set of genes.
A phenotype is an observable characteristic or biological state resulting from genetic and environmental influences.
Multiple alleles are therefore a population-level concept. Imagine a population containing five alleles of one gene. An individual does not normally inherit all five. Instead, that person’s genotype contains two alleles, and those two are drawn from the larger population-wide set.
This population-level variation is one reason genetic inheritance is more diverse than the simplest Mendelian examples suggest.
Multiple alleles still follow inheritance rules
Having multiple alleles does not make inheritance random. Parents pass one allele at a given autosomal gene to each child, and which allele is passed depends on the parent’s genotype and the process of chromosome segregation during reproduction.
For example, consider a parent with genotype Iᴬi. That parent can pass either Iᴬ or i to a child. A parent with genotype Iᴮi can pass either Iᴮ or i.
If these two parents have a child, the possible combinations are IᴬIᴮ, Iᴬi, Iᴮi, and ii. Those correspond to blood types AB, A, B, and O, respectively, assuming the standard ABO inheritance model.
The important lesson is that multiple alleles expand the set of possible inheritance outcomes, but the underlying principles of allele transmission remain the same.
Multiple alleles do not always produce obvious differences
Another important point is that an allele is not necessarily a completely separate “version” of a visible trait.
Two alleles may differ in DNA sequence without producing an obvious difference in appearance. Some variants alter a gene’s activity, some alter the structure of the protein it produces, and some have little detectable effect under ordinary conditions.
Likewise, when multiple alleles exist, their effects can depend on the biological context. The phenotype may reflect interactions among alleles, other genes, and environmental factors.
For that reason, multiple-allele inheritance is best understood as a pattern of genetic variation, rather than as a rule that every gene with several alleles must produce several visibly distinct traits.
Why multiple alleles matter in genetics
Multiple alleles provide a more realistic picture of how genetic variation works. The classic Mendelian examples often begin with two alleles because they are easier to analyze, but real populations contain many genes with numerous naturally occurring variants.
The ABO blood group makes the principle especially clear: one gene has three major alleles, individuals carry two at a time, and different allele pairs can have different relationships.
Once that distinction is understood, multiple alleles become straightforward. A gene can have many alleles across a population, while a diploid individual generally carries only two. The particular pair an individual inherits—and the biological relationship between those alleles—determines the genotype and helps determine the resulting phenotype.
