The number of alleles a gene can have depends entirely on the perspective, whether looking at a single individual or an entire species. A gene is a segment of DNA that provides instructions for a functional product, typically a protein. Its physical location on a chromosome is called the locus. An allele is a variation in the DNA sequence of that gene.
The Standard Model: Alleles in Diploid Individuals
For a human or any other sexually reproducing organism, the standard number of alleles for a single gene is two. This two-allele rule stems from the condition of diploidy, where an organism inherits one complete set of chromosomes from each parent. Since chromosomes exist in homologous pairs, every gene locus is represented twice, with one copy coming from the maternal gamete and the other from the paternal gamete.
The combination of these two inherited alleles determines an individual’s genetic makeup, or genotype, for that trait. If both inherited alleles are identical in their DNA sequence, the individual is considered homozygous for that gene. If the two inherited alleles carry different DNA sequences, the individual is heterozygous.
Allelic Variation within a Population
While any single individual can possess only two alleles for a gene, the total number of distinct alleles available within the gene pool of a species can be much higher. This phenomenon is called multiple allelism, which describes the existence of three or more alternative forms of a gene at a specific locus across a population. Multiple alleles arise from accumulated mutations over many generations, creating a broad spectrum of genetic variants.
The human ABO blood group system is the most commonly cited example of multiple allelism, which is governed by a single gene with three common alleles: \(I^A\), \(I^B\), and \(i\). The \(I^A\) and \(I^B\) alleles code for distinct enzymes that produce specific antigens on red blood cells. Conversely, the \(i\) allele produces a non-functional enzyme, leading to Type O blood. The existence of these three common forms allows for six possible genotypes and four distinct blood types in the population.
Structural Exceptions to the Two-Allele Rule
The two-allele standard is a generality for diploid organisms, but exceptions can change the allele count within an individual. Haploid organisms, such as gametes or some fungi, possess only one set of chromosomes, meaning they carry only a single allele for every gene.
Another deviation occurs in sex-linked inheritance, particularly in human males, who have one X and one Y sex chromosome (XY). Genes located on the X chromosome are present in two copies in females (XX) but only one copy in males. For these X-linked genes, males are considered hemizygous, possessing only one allele. Conversely, some species, particularly plants, exhibit polyploidy, a condition where they possess three or more complete sets of chromosomes, resulting in three, four, or more alleles for every gene locus.