Genes are the fundamental units of heredity, containing the codes for traits, and alleles are the variant forms of these genes. While early genetics focused on simple inheritance where one gene had two versions, most observable characteristics are determined by more intricate genetic mechanisms. Understanding how multiple versions of a gene or multiple genes work together is necessary to appreciate biological variation. This complexity primarily manifests through two distinct patterns: multiple alleles and polygenic traits.
The Concept of Multiple Alleles: Variations within a Single Gene Locus
An allele is a specific version of a gene found at a fixed position, or locus, on a chromosome. While simple Mendelian inheritance involves only two alleles, the concept of multiple alleles describes a situation where a single gene has three or more possible alleles present within the overall population. A single diploid organism, such as a human, can still only possess a maximum of two alleles for that gene—one inherited from each parent.
The classic human example is the ABO blood group system, controlled by the I gene on chromosome 9. This gene has three principal alleles: IA, IB, and i (or IO). The IA allele codes for the A antigen, and the IB allele produces the B antigen on red blood cells. The i allele codes for a non-functional enzyme, meaning no A or B antigen is produced.
The interaction of these three alleles determines the four common blood types: A, B, AB, and O. The IA and IB alleles exhibit codominance; if an individual inherits both (IAIB), both antigens are equally expressed, resulting in type AB blood. Both IA and IB are dominant over the i allele, so IAi results in type A blood, and IBi results in type B blood. Only the homozygous ii genotype results in type O blood.
Polygenic Traits: Cumulative Influence of Many Genes
Polygenic traits are characteristics governed by the collective and often additive effects of two or more different genes. These controlling genes, called polygenes, are located at multiple separate loci across the chromosomes, each contributing a small, incremental amount to the final physical trait. This inheritance results in continuous variation, meaning the trait exists on a spectrum rather than in discrete categories.
Human height is a prime example, where hundreds of different gene variants, each with a small effect, combine to determine an individual’s final stature. Skin color is similarly controlled by at least three to six genes that regulate the amount and type of melanin pigment produced. The combination of alleles across these distinct gene loci creates the vast spectrum of skin tones observed in the human population.
The continuous nature of polygenic traits is often represented by a bell-shaped curve when graphed across a large population. Most individuals fall into the middle of the spectrum, with fewer individuals possessing the extreme expressions of the trait, such as being exceptionally tall or having very light skin. This pattern arises because it is statistically rare to inherit the maximum number of trait-increasing or trait-decreasing alleles across all involved genes.
Polygenic traits are frequently multifactorial, meaning their expression is significantly shaped by environmental factors in addition to genetics. For instance, while approximately 80% of an individual’s height is determined by genes, factors like childhood nutrition play a noticeable role. Sun exposure is another environmental factor that directly modifies the expression of genetically determined skin color.
Structural and Functional Differences in Genetic Expression
The distinction between multiple alleles and polygenic traits lies in the number of gene locations involved. Multiple alleles involve three or more different versions for a single gene occupying one specific locus on a chromosome. In contrast, polygenic traits are governed by the action of multiple separate genes, each residing at its own distinct chromosomal locus.
This structural difference dictates the phenotypic outcome. Multiple alleles, such as those in the ABO blood group, produce a limited number of discrete or categorical phenotypes (A, B, AB, or O). The additive effect of many genes in polygenic inheritance results in quantitative variation, meaning the trait is expressed as a continuous range or gradient, such as the spectrum of human height.
The mechanism of gene action also differs. In multiple allele systems, alleles at the single locus interact through dominance, recessiveness, or codominance. For polygenic traits, the effect is cumulative, with each separate gene contributing a small, often equal, amount to the final result. Finally, the influence of the external environment is minimal on traits determined by multiple alleles, but it is a substantial factor in the final expression of many polygenic traits.