In genetics, “polymorphic” and “polygenic” are two concepts that are fundamental to understanding trait diversity. While both relate to genetic variation, they refer to different aspects of how genes contribute to an organism’s characteristics. This article will clarify the meaning of each term and their unique contributions.
Understanding Polymorphic Traits
A polymorphic trait results from a single gene having two or more different forms, known as alleles, within a population. For a trait to be polymorphic, each of these alternative alleles must be present at a relatively high frequency, typically at least 1%. This genetic arrangement leads to distinct, non-continuous categories of traits, where individuals clearly fall into one group or another without intermediate forms.
The human ABO blood group system provides a clear example. This system is determined by a single gene, the ABO gene, located on chromosome 9. This gene has three primary alleles: IA, IB, and i. The IA allele leads to the production of A antigens, while the IB allele results in B antigens. The i allele does not produce either A or B antigens.
The combinations of these alleles determine a person’s blood type. For instance, individuals with genotypes IAIA or IAi have Type A blood, while those with IBIB or IBi have Type B blood. If both IA and IB alleles are inherited, both antigens are expressed, resulting in Type AB blood, demonstrating codominance. Only individuals inheriting two copies of the recessive i allele (ii) will have Type O blood, which lacks both A and B antigens.
Another illustration of a polymorphic trait is the sickle cell trait. This condition involves a single gene responsible for producing beta-globin, a component of hemoglobin. Individuals with sickle cell trait inherit one normal allele and one allele that codes for hemoglobin S, an abnormal form. While typically benign, this single gene variation can result in distinct physiological responses under specific conditions, such as low oxygen levels, where some red blood cells may adopt a sickle shape.
Understanding Polygenic Traits
Polygenic traits, in contrast, are characteristics influenced by the combined effects of multiple genes, rather than just one. These genes, often called polygenes, each contribute a small, additive effect to the overall trait. The involvement of numerous genes typically results in a continuous spectrum of phenotypes, meaning there is a wide range of variation rather than distinct, separate categories.
Human height is a prime example of a polygenic trait. It is influenced by the cumulative action of many genes; estimates suggest more than 400 genes can contribute to an individual’s final stature. Each of these genes contributes a small amount, and their combined effect creates the wide variation in height observed across the human population. This leads to a distribution that often resembles a bell curve, with most people falling into an average height range and fewer individuals at the extreme ends of very short or very tall.
Beyond genetics, environmental factors play a considerable role in the expression of polygenic traits like height. For instance, nutrition during childhood and overall health can significantly influence whether an individual reaches their full genetic potential for height. Similarly, human skin color is another polygenic trait, determined by multiple genes that control the amount and type of melanin produced. Environmental factors, particularly exposure to ultraviolet (UV) radiation from the sun, can also modify skin color through tanning, demonstrating the interplay between genes and environment.
The Key Differences Summarized
Polymorphic and polygenic traits differ fundamentally in the number of genes involved in their expression. Polymorphic traits are determined by variations, or alleles, within a single gene. The diversity observed for a polymorphic trait stems from the different forms a single gene can take within a population. In contrast, polygenic traits are influenced by the collective action of many different genes, often located at various positions across the genome.
The source of variation also distinguishes these two concepts. For polymorphic traits, the variation arises from the presence of two or more distinct alleles of one specific gene. Each allele contributes to a particular, discrete outcome. For polygenic traits, the variation results from the combined and often additive effects of numerous genes, where each gene contributes a small, incremental amount to the overall phenotype.
Regarding phenotypic expression, polymorphic traits typically exhibit discontinuous variation, meaning individuals fall into clearly defined categories. There are distinct forms with no smooth transitions between them, such as different blood types. Polygenic traits, conversely, display continuous variation, manifesting as a wide range or spectrum of phenotypes. Traits like human height or skin color demonstrate this continuous variability, where individuals are imperceptibly graded across a broad range of possibilities.
The Intersection of Polymorphic and Polygenic Inheritance
While polymorphic and polygenic describe different aspects of genetic influence, they are not mutually exclusive; they can, in fact, intersect. This occurs because the individual genes that contribute to a polygenic trait can themselves be polymorphic. This distinction highlights that “polymorphic” describes a characteristic of a gene (having multiple alleles), whereas “polygenic” describes the inheritance pattern of a trait (influenced by many genes).
Eye color serves as an excellent example of this intersection. It is considered a polygenic trait, meaning multiple genes contribute to its final appearance, resulting in a wide array of colors from blue to brown and green. Among the genes influencing eye color, OCA2 and HERC2 are particularly significant. The OCA2 gene affects the production of melanin, the pigment responsible for eye color, while the HERC2 gene acts as a regulator, influencing how much OCA2 is expressed.
Both OCA2 and HERC2 are polymorphic genes, possessing different alleles within the human population. For instance, a specific single nucleotide polymorphism (SNP) within the HERC2 gene, known as rs12913832, is a major determinant for blue versus brown eye color. The various alleles of these and other contributing genes combine in different ways, leading to the continuous spectrum of eye colors observed in people. Therefore, the polygenic nature of eye color is underpinned by the polymorphic variations of its constituent genes.