A gene is a heritable section of DNA with a particular function, while an allele is one version of a gene or DNA sequence at a particular genomic location. Put simply, the gene identifies the unit of genetic information being considered; the allele identifies which version of that information an organism carries.
For example, ABO is a gene involved in determining blood group, while Iᴬ, Iᴮ, and i are alleles of that gene. This distinction is fundamental to IB Biology genetics because it connects DNA, chromosomes, genotype, phenotype, meiosis, mutation, and inheritance.
Allele vs gene: the essential difference
The most useful exam-ready comparison is:
| Feature | Gene | Allele |
|---|---|---|
| Definition | A heritable section of DNA with a particular function | A version of a gene or DNA sequence at a particular locus |
| Main idea | The genetic unit being considered | The particular version present |
| Location | Occupies a particular position, or locus, in the genome | Alternative alleles occur at the same corresponding locus |
| Number in a population | One gene may be discussed as a genetic locus | A gene can have two or many alleles in the population |
| Number in a diploid individual | Usually represented by two copies, one on each homologous chromosome | Usually up to two alleles at one autosomal locus, one inherited from each parent |
| Example | ABO blood-group gene | Iᴬ, Iᴮ, or i |
| Relevance to variation | Provides the functional unit | Sequence differences between alleles contribute to genetic variation |
A concise IB-style answer to “distinguish between a gene and an allele” could state:
A gene is a heritable DNA sequence with a particular function, whereas an allele is one of the alternative versions of that sequence found at the same locus.
That definition is more precise than saying that a gene “controls a characteristic” and an allele “controls a variation.” Traits frequently result from several genes and environmental influences, so the simpler wording can become misleading.
What exactly is a gene?
A gene is commonly described as the basic physical and functional unit of heredity. It is made of DNA and contains information used to produce a functional product, usually a polypeptide or a functional RNA molecule. Genes can also include sequences involved in controlling when and where that product is made.
This molecular definition matters because genes do not directly manufacture visible characteristics. Instead, their expression produces RNA or influences protein production, and those molecules contribute to cellular functions and phenotypes.
For example, a gene may encode an enzyme in a biochemical pathway. A difference in that gene's nucleotide sequence may change the enzyme's amino acid sequence, activity, or production, which can then affect the organism's phenotype.
Gene, locus, and chromosome
A locus is a physical position in the genome. A gene occupies a particular locus, rather like a specific entry occupying a fixed position in an ordered database.
In a diploid organism, homologous chromosomes normally carry the same genes at corresponding loci. However, the DNA sequences at one locus do not have to be identical because the two chromosomes may carry different alleles.
This gives an important relationship:
chromosome → locus → gene → allele
- A chromosome is a long DNA molecule containing many genes and other sequences.
- A locus is the location of a gene or another DNA sequence.
- A gene is the functional hereditary unit at that location.
- An allele is the particular version of the sequence present there.
For broader syllabus context, RevisionDojo's IB Biology inheritance explained notes connect this terminology with meiosis, genetic crosses, pedigrees, linkage, and variation without turning this single-concept explanation into a complete inheritance topic guide.
What exactly is an allele?
An allele is one of two or more versions of a DNA sequence at a given genomic location. In school genetics, it is usually defined as an alternative form of a gene found at the same locus.
Alleles arise when mutations change a DNA sequence. A substitution might alter one nucleotide, while an insertion or deletion may produce a larger difference. If the changed sequence can be inherited and persists in a population, it represents an allele of that locus.
Alleles of the same gene are not different genes merely because their sequences differ. They remain versions associated with the same genomic location and function. RevisionDojo's explanation of how DNA mutation creates new alleles develops this relationship further.
How many alleles can an organism have?
A population may contain many alleles of a gene, but a typical diploid individual can carry no more than two alleles at one autosomal locus. One is inherited through the chromosome from one biological parent and the other through the homologous chromosome from the other parent.
For example, the human ABO gene has three commonly discussed alleles in the population:
- Iᴬ
- Iᴮ
- i
A particular diploid individual has only two of them, such as IᴬIᴮ, Iᴬi, or ii. Saying that a person has all three alleles would therefore be incorrect under the standard diploid model.
There are exceptions to the simplified “two alleles per gene” rule. A haploid gamete contains one allele at each locus, and a person with one X chromosome is normally hemizygous for X-linked loci that have no corresponding allele on the Y chromosome. Chromosomal deletions, duplications, polyploidy, and unusual cell lineages can also complicate the pattern.
How genes and alleles work together
Genes and alleles are not competing categories. An allele is a version of a gene, so the relationship resembles that between a general document and a particular edition of it.
Consider a hypothetical flower-colour gene with alleles P and p:
- The gene is the DNA unit involved in flower pigmentation.
- P and p are alternative alleles at its locus.
- PP, Pp, and pp are possible diploid genotypes.
- Flower colour is the phenotype produced through expression of the genotype, potentially modified by the environment.
The genotype does not describe the gene itself. It identifies the combination of alleles carried by an organism at one or more loci. This distinction is developed in the RevisionDojo genotype notes.
Gene, allele, genotype, and phenotype compared
These four terms describe different levels of genetic reasoning:
| Term | Meaning | ABO example |
|---|---|---|
| Gene | A heritable DNA sequence with a function | ABO gene |
| Allele | A version of the sequence at that locus | Iᴬ, Iᴮ, or i |
| Genotype | The allele combination carried by an organism | Iᴬi |
| Phenotype | An observable or measurable characteristic resulting from genotype and environmental influences | Blood group A |
A phenotype is not always a direct label for an allele. The genotype Iᴬi produces blood group A because Iᴬ is dominant to i in this combination, while IᴬIᴮ produces blood group AB because Iᴬ and Iᴮ are codominant.
Dominance describes the relationship between alleles in a heterozygote. It does not mean that one allele is stronger, more common, healthier, or evolutionarily superior. It also does not turn a recessive allele into a different gene.
Why homologous chromosomes matter
Homologous chromosomes carry the same sequence of gene loci, although they may carry different alleles at those loci. Before DNA replication, a diploid cell normally has two homologues of each autosome, one of parental origin from each side.
Suppose one homologue carries allele A and the other carries allele a at the same locus. The organism is heterozygous, written Aa. If both homologues carry A, the organism is homozygous, written AA.
During meiosis I, homologous chromosomes separate. Consequently, the two alleles of a gene segregate into different gametes, so each normal haploid gamete receives one allele at that locus. Fertilization combines two gametes and restores a pair of alleles in the diploid zygote.
This chromosome behaviour provides the cellular basis for Mendel's law of segregation. RevisionDojo's guide to Mendel's laws and inheritance patterns explains how segregation is applied in genetic crosses.
A worked IB Biology genetics example
Assume B and b are two alleles of one autosomal gene. B is dominant and produces brown body colour, while b is recessive and produces black body colour when no dominant allele is present.
A cross between two heterozygous organisms is written:
Bb × Bb
Each parent can produce gametes carrying B or b. Combining these gametes gives:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
The predicted genotype ratio is:
1 BB : 2 Bb : 1 bb
If dominance is complete, the predicted phenotype ratio is:
3 brown : 1 black
In this example, B is not a gene for brown colour and b a separate gene for black colour. They are two alleles of the same gene. The letters inside the Punnett grid represent alleles, while pairs such as Bb represent genotypes.
Students can apply this distinction through the IB Biology D3.2 inheritance questionbank, where identifying alleles correctly is necessary before calculating probabilities.
Multiple alleles do not mean multiple genes
The ABO system demonstrates why population-level and individual-level language must remain separate. There is one ABO locus, but multiple alleles exist in the human gene pool.
The alleles produce different forms or activity levels of a glycosyltransferase enzyme:
- Iᴬ contributes to production of the A antigen.
- Iᴮ contributes to production of the B antigen.
- i typically does not produce a functional enzyme that adds either antigen.
The existence of three alleles does not mean there are three ABO genes. It means that the same locus has three commonly considered sequence versions in the population.
Multiple alleles also do not require an organism to carry more than two. A diploid individual's two homologous chromosomes still provide two allele positions at the autosomal locus.
Common allele vs gene mistakes
Calling each inherited copy a different gene
A student may write, “The offspring inherits one gene for brown colour and one gene for blue colour.” In a simple single-locus model, it is more accurate to say that the offspring inherits one allele of the gene from each parent.
Saying that every gene has only two alleles
A diploid individual usually carries up to two alleles at an autosomal locus, but many alleles can exist across a population. The ABO example is the standard IB illustration.
Treating dominant and recessive as types of genes
Dominant and recessive describe how alleles affect phenotype in a heterozygous genotype. These terms should not normally be used to classify an entire gene.
Assuming one gene always controls one visible trait
The “one gene, one trait” model is useful for simple crosses but is not a universal biological rule. Many characteristics are polygenic, while one gene may influence several features, and environmental conditions can modify phenotype.
Confusing an allele with a genotype
B is an allele; BB, Bb, and bb are genotypes. A genotype is a combination, not another word for a single allele.
How this distinction is assessed in IB Biology
The current IB Biology course, first assessed in 2025, includes inheritance within D3.2 under the theme Continuity and Change. The official course roadmap places inheritance alongside reproduction and related genetic processes, while the specimen papers show that students must interpret allele notation and inheritance information in unfamiliar contexts.
Questions may require you to:
- define or distinguish gene, allele, genotype, and phenotype;
- identify alleles from genetic notation;
- construct or interpret a Punnett grid;
- explain segregation during meiosis;
- distinguish individual genotypes from population allele diversity;
- apply dominance, codominance, or incomplete dominance;
- interpret pedigrees, blood groups, or mutation data.
When the command term is distinguish, state a clear difference rather than giving two disconnected definitions. When asked to explain, connect the sequence logically, such as mutation changing DNA, producing an allele, altering a gene product, and potentially affecting phenotype.
Use notation consistently. A capital and lowercase form of the same letter, such as A and a, normally indicates alleles of one gene, whereas two different letters may represent different loci. For codominant alleles, superscripts such as Iᴬ and Iᴮ prevent inaccurate dominant-recessive notation.
For focused preparation, use the IB Biology genetics questionbank after reviewing the RevisionDojo genes notes. Jojo AI can help identify where an answer has confused a gene, allele, or genotype, but always compare the explanation with the notation and biological context in the question.
Conclusion
A gene is a functional, heritable section of DNA at a particular genomic location. An allele is a particular version of the gene or sequence at that locus. The alleles inherited by an organism form its genotype, and gene expression contributes to phenotype in combination with environmental influences.
For IB exams, remember the simplest reliable formula: gene = hereditary DNA unit; allele = version of that unit. RevisionDojo's inheritance notes, Questionbank, Flashcards, and Jojo AI can then be used to practise applying the distinction in genetic crosses and data-based questions.
Sources and referenced URLs
Official and external sources
- IB Diploma Programme Biology subject brief, first assessment 2025
- Official IB Biology course roadmap
- Official IB Biology specimen papers
- National Human Genome Research Institute definition of a gene
- National Human Genome Research Institute definition of an allele
- OpenStax Biology 2e: characteristics, traits, genes, and alleles