Many people assume that if a trait is dominant, it must be common within a population. This idea stems from the basic understanding of genetics, where a dominant gene’s effect masks that of a recessive one. However, this common assumption is incorrect; dominant traits are not always widespread, and recessive traits are not necessarily rare. The prevalence of a trait in a population involves more than just its dominance or recessiveness, but rather population-level genetic dynamics.
Understanding Genetic Dominance
In genetics, “dominant” refers to how an allele is expressed when two different versions are present in an individual. If an individual inherits one dominant and one recessive allele for a trait, the dominant allele’s characteristic will be observed, masking the recessive allele’s effect. For example, brown eyes are dominant over blue eyes; a person with one allele for each will have brown eyes.
Conversely, a recessive trait only appears when an individual inherits two copies of the recessive allele, one from each parent. If even one dominant allele is present, the recessive trait remains unexpressed. Genetic dominance describes the pattern of expression at the individual level, indicating which trait will be visible when different alleles are combined. It does not imply how frequently that trait appears across a population.
How Allele Frequencies Determine Commonality
The commonness of any trait in a population is primarily determined by the frequency of its underlying alleles, not by whether the trait is dominant or recessive. Allele frequency refers to the proportion of a specific allele within the gene pool of a population. A gene pool encompasses all the genetic information present in a population. If a particular allele is rare, the trait it codes for will also be rare, regardless of its dominance.
For instance, a dominant allele can be quite uncommon if its frequency in the population is low. Consider a hypothetical dominant allele (D) that is present in only a small percentage of the population’s gene pool. Even though an individual needs only one copy of D to express the trait, the trait will be rare simply because the D allele itself is rare. Conversely, a recessive allele (r) can be very common if its frequency is high, even though two copies (rr) are needed for the trait to be observed. Many people might carry one copy of the common recessive allele without showing the trait, but the high frequency ensures the trait appears frequently in the population.
Forces That Influence Trait Prevalence
Allele frequencies and trait prevalence are dynamic, shaped by several evolutionary forces.
Natural Selection
Natural selection favors alleles providing a survival or reproductive advantage in a given environment. If a dominant allele causes a severe disorder that prevents individuals from reproducing, its frequency will likely remain very low. Conversely, if a recessive allele confers an advantage or is simply neutral, its frequency can increase over generations.
Genetic Drift
Genetic drift also influences allele frequencies, particularly in smaller populations. This force involves random fluctuations in allele proportions from one generation to the next, which can lead to an allele becoming more or less common by chance. Events like the “founder effect,” where a new population is established by a small number of individuals, or a “bottleneck effect,” where a population undergoes a drastic reduction in size, can significantly alter allele frequencies.
Mutation
Mutation is the source of new genetic variation, introducing new alleles into a population. While mutations are rare, they can introduce either dominant or recessive alleles. A new dominant allele, even if beneficial, starts at a very low frequency and may take a long time to become common, if it ever does.
Gene Flow
Gene flow, the movement of alleles between populations through migration, can alter existing allele frequencies. This process can introduce new alleles or change the proportions of existing ones, highlighting the complex interplay of factors influencing trait prevalence.
Examples of Rare Dominant and Common Recessive Traits
Several real-world examples illustrate that dominant traits are not always common, and recessive traits can be widespread. Huntington’s disease, a neurodegenerative disorder, is caused by a dominant allele. Despite its dominant inheritance, it is rare, affecting about 3 to 7 people per 100,000 in European populations. The allele’s low frequency keeps the trait uncommon, partly because it manifests later in life, often after individuals have had children, allowing the allele to persist.
Conversely, blue eyes are a recessive trait common in many human populations. The blue eye allele is recessive to brown, yet its high frequency in certain populations, such as those of Northern European descent, makes the trait widely observed. Similarly, attached earlobes are a common recessive trait in various populations where its recessive allele is prevalent.