Traits are inherited characteristics, governed by genes passed down through generations. They are often categorized as “dominant” or “recessive.” There is a widespread assumption that dominant traits are inherently more common in the population. This article explores the mechanisms behind dominant and recessive traits and investigates whether dominant traits are, in fact, more prevalent than recessive ones.
Defining Dominant and Recessive Traits
Traits are determined by genes, which are segments of DNA that provide instructions for building and maintaining an organism. Genes exist in different versions called alleles. For most genes, an individual inherits two alleles, one from each parent.
A dominant allele expresses its associated characteristic, or phenotype, even when only one copy is present. If an individual inherits a dominant allele from one parent and a different allele from the other, the dominant trait will be observed.
In contrast, a recessive allele only expresses its phenotype when two copies of that allele are present. If an individual inherits one recessive allele and one dominant allele, the dominant one masks the recessive allele’s expression. The recessive trait will only be observed if an individual inherits two copies of the recessive allele.
Dominance and Trait Frequency in Populations
It is a common misconception that dominant traits are always more prevalent in a population than recessive traits. The terms “dominant” and “recessive” describe how alleles interact to produce a specific observable characteristic in an individual, not how frequently those alleles appear within a gene pool. The commonness of a trait within a population is a separate concept from its dominance.
Many common human traits are, in fact, recessive. For instance, blue eyes are a recessive trait, requiring two copies of the recessive allele for their expression. Similarly, attached earlobes are also a recessive characteristic. The ability to taste phenylthiocarbamide (PTC) is a dominant trait, yet non-tasters (a recessive trait) are common in some populations.
Conversely, some dominant traits are quite rare. Polydactyly, the condition of having extra fingers or toes, is a dominant trait, yet it is uncommon in the general population. Huntington’s disease, a severe neurological disorder, is another example of a rare dominant trait. Achondroplasia, a form of dwarfism, is also caused by a dominant allele and is uncommon in the general population. These examples illustrate that a trait’s dominance does not dictate its frequency within a population.
Mechanisms Driving Trait Prevalence
The prevalence of a trait in a population is influenced by several factors, independent of its dominance.
Natural selection favors traits providing a survival or reproductive advantage in a given environment. Individuals with advantageous traits are more likely to survive and pass on their genes, increasing their frequency. Disadvantageous traits become less common.
Genetic drift describes random fluctuations in allele frequencies, especially in smaller populations. Events like a “founder effect” (new population established by few individuals) or a “bottleneck effect” (drastic population reduction) cause unpredictable changes. This can result in certain alleles becoming more or less common by chance.
Mutation rates affect trait prevalence, as mutations are the ultimate source of new alleles. The rate a new allele arises influences its initial frequency. Most mutations are neutral or harmful, but beneficial ones can increase through natural selection.
Gene flow, the movement of individuals and their genes between populations, alters allele frequencies. Migration can introduce new alleles or change existing proportions, affecting trait prevalence.
Heterozygote advantage is another factor, where individuals carrying one recessive allele (heterozygotes) have higher fitness than homozygous dominant or recessive individuals. The sickle cell trait is a well-known example; heterozygotes show resistance to malaria, which helps maintain the recessive allele in affected populations.