In genetics, a common question is why traits determined by dominant alleles are not always the most prevalent in a population. While “dominant” describes how a gene’s characteristic is expressed, it does not dictate how frequently that gene variant appears. An allele’s commonness is influenced by several factors beyond its simple dominance or recessiveness. Understanding this distinction requires looking at the broader dynamics of genetic populations.
Dominance and Allele Frequency
The term “dominant” in genetics refers to how a trait is expressed when an individual inherits two different versions of a gene, known as alleles. If a dominant allele is present, its associated trait will be observed, masking the effect of a recessive allele. This means that individuals with one copy of the dominant allele and one copy of the recessive allele will display the dominant trait. However, this expression pattern is separate from the allele’s frequency, which is how often it occurs in the gene pool of a population. An allele’s dominance simply describes its phenotypic effect in a heterozygous state, not its numerical abundance.
For example, polydactyly, the condition of having extra fingers or toes, is often inherited as a dominant trait. Despite its dominance, it is relatively uncommon in the general population, with incidences ranging from approximately 1 in 500 to 1 in 1,000 newborns in the USA. Huntington’s disease, a severe neurological disorder, is caused by a dominant allele but is rare because it is a debilitating, late-onset condition that can affect reproductive fitness. Conversely, traits caused by recessive alleles, such as O blood type or blue eyes, can be quite common. Type O positive blood is the most common blood type globally, found in about 38% of the world’s population. Blue eyes, while recessive, are also common in certain human populations.
Evolutionary Forces Shaping Allele Frequencies
The actual frequency of an allele, regardless of its dominance, is shaped by several evolutionary forces that act on populations over time. These forces continuously interact, influencing which alleles become more or less common.
Natural Selection
Natural selection plays a significant role in determining allele frequencies by favoring traits that provide a survival or reproductive advantage in a given environment. If a dominant allele confers a disadvantageous trait, individuals carrying it may be less likely to survive and reproduce, leading to a decrease in its frequency over generations. Conversely, a recessive allele that offers an advantage, particularly in heterozygotes, can become more common. A notable instance is the sickle cell trait, where individuals carrying one copy of the recessive allele for sickle cell hemoglobin (HbS) are protected against severe malaria, especially in regions where the disease is prevalent. This selective advantage has led to a higher frequency of the HbS allele in malaria-endemic areas, despite the severe health issues associated with having two copies of the allele (sickle cell anemia).
Genetic Drift
Genetic drift refers to random fluctuations in allele frequencies that occur by chance, particularly impactful in small populations. In such populations, random events like individuals failing to reproduce or random sampling of gametes can lead to significant changes in allele frequencies from one generation to the next. This random process does not consider whether an allele is beneficial or harmful; it can cause a dominant allele to become rare or a recessive allele to become common purely by chance. The effects of genetic drift are stronger in smaller populations because chance events have a more pronounced impact on the overall gene pool.
Mutation
Mutations are the ultimate source of new alleles in a population, introducing novel genetic variations. When a new allele arises through mutation, its initial frequency is extremely low. Whether this new allele, regardless of being dominant or recessive, increases in frequency depends on other evolutionary forces. If the mutation creates a beneficial trait, natural selection might favor its spread. If it is harmful, it will likely be selected against and remain rare or be eliminated. The rate at which mutations occur is generally low, so mutation alone does not rapidly change allele frequencies.
Gene Flow (Migration)
Gene flow, also known as migration, involves the movement of individuals and their genetic material between populations. This transfer of alleles can introduce new alleles into a population or alter the frequencies of existing ones, regardless of their dominance. High rates of gene flow can reduce genetic differences between populations, making their allele frequencies more similar. For instance, if individuals carrying a specific dominant allele migrate from a population where it is common to one where it is rare, the frequency of that allele in the receiving population will increase.
Common Misconceptions and Real-World Examples
The prevalence of a trait in a population is not simply a function of whether its underlying allele is dominant or recessive. Instead, the interplay of evolutionary forces determines an allele’s commonness.
Some dominant traits are indeed rare. Huntington’s disease, for example, is caused by a dominant allele, but it is rare because its severe, progressive symptoms typically appear later in life, often after an individual has had children, limiting its spread over many generations. Polydactyly, characterized by extra digits, is another dominant trait that remains uncommon, often due to natural selection against less advantageous forms or a lack of strong selective pressure for its increase.
Conversely, many common traits are governed by recessive alleles. The O blood type, recessive to both A and B blood types, is the most common blood type globally. Its prevalence is due to a lack of strong negative selection and historical population dynamics. Similarly, blue eyes are a recessive trait widespread in certain populations, particularly in Northern Europe. This high frequency is influenced by genetic drift, founder effects, or potentially mild selective advantages in specific environments, rather than being linked to dominance. The concept of “wild type” alleles further clarifies this: a wild type allele is simply the most common allele for a gene in a natural population, irrespective of whether it is dominant or recessive.