The common understanding of genetics suggests a dominant trait should be widespread because it requires only one copy of the gene to appear. However, many human traits determined by a dominant allele, such as having an extra digit (polydactyly) or certain genetic disorders, are surprisingly rare in the general population. This apparent contradiction arises from confusing the mechanism of inheritance within a family with the overall prevalence of a gene in a large population. To resolve this paradox, it is necessary to examine the two distinct concepts of dominance and allele frequency, and then explore the evolutionary and biological factors that suppress a dominant trait’s visibility and spread.
Defining Dominance and Allele Frequency
The term dominance describes the relationship between two versions of a gene, called alleles, within a single individual. A dominant allele produces its associated trait, or phenotype, even when only one copy is inherited from a parent. For example, the gene variant responsible for Polydactyly (extra fingers or toes) is dominant, meaning a person only needs one copy to show the trait.
Allele frequency, in contrast, refers to the proportion of a specific allele within the entire gene pool of a population. Dominance status does not determine how common an allele is across the population; an allele can be dominant in its expression but still exist at a very low frequency.
This separation of concepts explains why some recessive traits are quite common. For instance, the allele for Type O blood is recessive, meaning an individual must inherit two copies to have the blood type. However, the recessive ‘O’ allele is the most frequent ABO allele in most human populations, making Type O the most common blood type globally. Therefore, the rarity of a trait like polydactyly is not due to its dominant nature, but rather the low frequency of the dominant allele in the population’s gene pool.
How Natural Selection Reduces Dominant Traits
The primary mechanism that keeps harmful dominant alleles rare is the immediate action of natural selection. If a dominant allele reduces an individual’s ability to survive or reproduce, the trait is immediately exposed to selection pressure, even in a heterozygote. Any negative impact prevents that allele from being passed on as often as its non-harmful counterpart, maintaining it at a very low frequency in the gene pool.
Consider a debilitating condition like Huntington’s disease, which is caused by a dominant allele. Although the disease typically manifests later in life, often after reproductive age, selection pressure still works to keep its frequency low. When a new dominant, harmful allele arises through a spontaneous mutation, it is almost immediately expressed and swiftly eliminated from the population over just a few generations.
This process differs fundamentally from how selection acts on harmful recessive alleles. A harmful recessive allele can be “hidden” in the genomes of heterozygous carriers, who do not express the trait and therefore do not face selection pressure. This protective mechanism allows harmful recessive alleles to persist in a population at higher frequencies than equally harmful dominant alleles. Random chance events, known as genetic drift, can also cause a dominant allele to be lost entirely in smaller populations.
Incomplete Expression and Trait Visibility
Beyond population-level frequency, a dominant allele may appear rare because its presence in the genotype does not always guarantee its visibility in the phenotype. This phenomenon is described through the concepts of penetrance and expressivity. Penetrance is the probability that an individual who possesses a dominant genotype will actually express the associated physical trait.
The dominant allele for polydactyly, for example, often displays incomplete penetrance. This means a person can inherit the gene variant but still have the typical number of five digits. Such an individual can unknowingly pass the dominant allele to their children, who may then express the trait, creating the appearance that the trait skipped a generation. This reduces the number of people who display the dominant trait, even though the dominant allele is present in the population.
Expressivity further complicates visibility by describing the degree to which a trait is expressed among individuals who display it. If a dominant allele has variable expressivity, it can manifest in many forms, from a mild, unnoticeable version to a severe one. For polydactyly, the extra digit might be a fully formed finger in one person, but only a small, fleshy stub in another.
Environmental factors and other modifying genes also interact with the dominant allele to influence the final outcome. Reduced penetrance and variable expressivity ultimately obscure the true frequency of the dominant allele by keeping its associated trait from being consistently visible.