When students ask how many alleles in a gene, the answer depends on whether the question is about one individual or about a population of individuals. Day to day, a gene is a specific segment of DNA that contains instructions for making a particular product, such as a protein or RNA molecule. An allele is a version of that gene. In most diploid organisms, such as humans, an individual usually carries two alleles for each autosomal gene, one inherited from each parent. That said, in a population, a single gene can have many different alleles. This is why the phrase “how many alleles in a gene” can be confusing: a gene does not contain a fixed number of alleles. Instead, the number of alleles depends on the biological context.
The Short Answer
If the question is, “How many alleles does one person have for a typical gene?”, the usual answer is two.
If the question is, “How many alleles can exist for a gene in a population?Here's the thing — ”, the answer is: there is no fixed limit. A gene can have two, three, four, or even many alleles in a population.
As an example, the ABO blood group gene has at least three common alleles in humans: I<sup>A</sup>, I<sup>B</sup>, and i. Any one person usually has two of these alleles, but the human population contains more than two possible versions of that gene.
What Is an Allele?
An allele is a variant form of a gene. Genes are located at specific positions on chromosomes, called loci. The singular form is locus, and the plural form is loci. At a given locus, different DNA sequences can exist. These different DNA sequences are alleles.
Here's one way to look at it: a gene may control flower color in a plant. Even so, one allele may produce purple flowers, while another allele may produce white flowers. Because of that, both alleles occupy the same gene location, but they differ in their DNA sequence. These differences can affect the structure or function of the protein produced by the gene.
It is important to understand that an allele is not a separate gene. It is a version of the same gene. Simply put, the gene is the “slot” in the genome, and the allele is the specific “version” that fills that slot.
How Many Alleles Does an Individual Have?
In most animals, including humans, cells are diploid, meaning they contain two sets of chromosomes. One set comes from the mother, and one set comes from the father. Because of this, most individuals have two alleles for each autosomal gene.
As an example, a person may have:
- two copies of the same allele, such as AA or aa, which is called homozygous;
- two different alleles, such as Aa, which is called heterozygous.
In this case, the individual has two alleles for that gene, but only one type of allele if homozygous, or two types of alleles if heterozygous Practical, not theoretical..
Example: Eye Color Simplified
Although eye color is actually controlled by multiple genes, it is often simplified in basic genetics. Suppose one gene has two alleles:
- B = brown eye allele
- b = blue eye allele
A person could have:
- BB = two brown alleles
- bb = two blue alleles
- Bb = one brown allele and one blue allele
In all three cases, the person has two alleles for that gene. The difference is whether the alleles are the same or different That's the part that actually makes a difference..
How Many Alleles Can a Gene Have in a Population?
While an individual usually has only two alleles for a diploid gene, a population can contain many more. This is called multiple allelism.
A gene may have:
- two alleles,
- three alleles,
- five alleles,
- or many more, depending on mutations and natural variation.
Mutations can create new alleles. On top of that, if a DNA sequence changes in a way that affects a gene, a new allele may form. Over time, these alleles can spread through a population or remain rare.
Example: ABO Blood Type
The ABO blood group system is a classic example of multiple alleles. The ABO gene has three main alleles:
