True or False: Dominant Alleles Can Be Rare, Recessive Common?

The idea that dominant traits are always common and recessive traits are always rare is a misconception. While dominance determines how a trait is expressed, it does not dictate how frequently an allele appears in a population. Many factors influence allele frequency, including mutation rates, genetic drift, natural selection, and historical events like population bottlenecks.

Understanding the distinction between dominance and allele frequency clarifies why some dominant traits remain rare while certain recessive traits are widespread.

Allele Frequency Basics

The prevalence of an allele in a population results from evolutionary forces rather than dominance or recessiveness. Allele frequency, expressed as a proportion or percentage, indicates how common a genetic variant is within a gene pool. This frequency is shaped by mutation, genetic drift, natural selection, and gene flow, which determine whether an allele spreads or remains rare.

A dominant allele may be rare if it arises from a recent mutation or confers a disadvantage that limits its spread. Conversely, a recessive allele can be common if it persists due to neutral selection or heterozygote advantage. Recessive alleles often remain unnoticed in heterozygous carriers, allowing them to persist even if they cause disease in homozygous individuals. This explains why conditions like cystic fibrosis, caused by recessive alleles, are relatively common despite their harmful effects when inherited from both parents.

Genetic drift, which refers to random fluctuations in allele frequencies, has a stronger effect in small populations. A rare dominant allele may disappear entirely if affected individuals fail to reproduce, while a recessive allele can remain stable if carried silently in heterozygotes. Additionally, historical events such as population bottlenecks or founder effects can amplify certain alleles, making them disproportionately common in specific groups. For instance, the high prevalence of Tay-Sachs disease among Ashkenazi Jews is attributed to genetic drift and historical isolation rather than the recessive nature of the allele itself.

Dominance and Phenotypic Expression

Dominance describes how alleles influence observable traits, but it does not determine allele frequency. A dominant allele expresses its associated trait in heterozygous individuals, requiring only one copy for the phenotype to manifest. In contrast, recessive traits require two copies—one from each parent—to be expressed. This principle of Mendelian genetics explains why dominant traits do not necessarily become widespread.

Dominance affects phenotypic expression based on molecular mechanisms. A dominant allele may produce a functional protein that overrides a recessive allele, which may encode a nonfunctional or absent protein. For example, in achondroplasia, a single copy of the FGFR3 mutation alters bone growth. In contrast, recessive traits often result from loss-of-function mutations, where only individuals inheriting two defective copies exhibit the full effect. Carriers with one normal allele typically produce enough functional protein to prevent the trait from appearing.

Not all dominant traits are fully expressed in individuals carrying the allele. Incomplete penetrance occurs when some individuals with the dominant allele do not display the expected phenotype due to interactions with other genes or environmental influences. Polydactyly, for example, may not manifest even in those who inherit the dominant allele. Variable expressivity can also lead to differences in severity, as seen in Marfan syndrome, where symptoms range from mild to life-threatening. These complexities highlight that dominance is not a binary concept but rather a spectrum influenced by genetic and external factors.

Examples of Rare Dominant Traits

Some dominant traits remain rare due to negative selection, spontaneous mutations, or limited inheritance opportunities. These traits may persist because they arise from de novo mutations or have mild enough effects to allow reproduction.

Polydactyly

Polydactyly, characterized by extra fingers or toes, follows an autosomal dominant inheritance pattern, meaning individuals with one copy of the mutation can exhibit the trait. Despite this, polydactyly remains relatively rare, occurring in approximately 1 in 500 to 1 in 1,000 live births worldwide. The condition can result from mutations in several genes, including GLI3, which influences limb development. Incomplete penetrance contributes to its rarity, as some individuals carrying the allele do not express the trait. Additionally, cultural and medical practices, such as surgical removal of extra digits in infancy, reduce its visibility in adult populations.

Achondroplasia

Achondroplasia, the most common form of dwarfism, is caused by a dominant mutation in the FGFR3 gene, which regulates bone growth. Despite its dominant inheritance, the condition remains rare, affecting approximately 1 in 25,000 live births. The mutation leads to abnormal cartilage formation, resulting in shortened limbs and other skeletal differences. Most cases arise from spontaneous mutations rather than being inherited. Additionally, homozygosity for the FGFR3 mutation (inheriting two copies) is typically lethal in infancy, preventing further transmission. While individuals with achondroplasia can lead full lives, associated health risks, such as spinal stenosis and respiratory complications, contribute to its limited prevalence.

Huntington’s Condition

Huntington’s condition is a neurodegenerative disorder caused by a dominant mutation in the HTT gene, leading to the progressive breakdown of nerve cells in the brain. It remains rare, affecting approximately 5 to 10 per 100,000 people worldwide. Its late onset—typically between ages 30 and 50—allows the allele to persist, as affected individuals may unknowingly pass it on before symptoms develop. However, the disease’s severity, which includes cognitive decline and motor dysfunction, often reduces reproductive success in later generations. Genetic testing and family planning decisions have contributed to a gradual decline in the allele’s prevalence in some populations.

Examples of Common Recessive Traits

Recessive traits can be widespread because individuals carrying a single copy of the allele do not exhibit symptoms. These alleles persist across generations, particularly when they provide a selective advantage in heterozygous carriers or are neutral in terms of survival and reproduction.

Blood Type O

Blood type is determined by the ABO gene, which encodes enzymes that modify red blood cell antigens. The O blood type results from a recessive allele that produces a nonfunctional enzyme, meaning individuals must inherit two copies to express this blood type. Despite being recessive, blood type O is the most common worldwide, with prevalence varying by region—approximately 45% of the U.S. population and over 60% of South American populations have type O blood. Some studies suggest that individuals with type O blood may have a lower risk of severe malaria, which could have contributed to its persistence.

Attached Earlobes

Earlobe attachment is commonly used as an example in genetics education, though its inheritance is more complex than a simple dominant-recessive model. Attached earlobes—where the earlobe connects directly to the side of the head—are considered a recessive trait. Individuals must inherit two copies of the allele to express the trait. While prevalence varies, attached earlobes occur in an estimated 30-40% of individuals. The trait persists due to its neutral impact on survival and reproduction. Additionally, variations in earlobe shape are influenced by multiple genes, making it more complex than a single-gene recessive characteristic.

Certain Enzyme Deficiencies

Several enzyme deficiencies follow a recessive inheritance pattern and remain common due to their persistence in heterozygous carriers. One example is glucose-6-phosphate dehydrogenase (G6PD) deficiency, affecting an estimated 400 million people worldwide. This condition results from mutations in the G6PD gene, reducing enzyme activity crucial for protecting red blood cells from oxidative damage. While individuals with two defective copies may experience hemolytic anemia under certain conditions, heterozygous carriers often have no symptoms. Interestingly, G6PD deficiency is more prevalent in regions where malaria is endemic, as carriers may have some resistance to severe malaria. This selective advantage has contributed to the high frequency of the recessive allele in populations across Africa, the Mediterranean, and parts of Asia.

Genetic Factors Influencing Frequencies

The distribution of dominant and recessive alleles in a population is shaped by genetic and evolutionary forces beyond inheritance patterns. While an allele’s effects on survival and reproduction influence its presence in a gene pool, other mechanisms such as mutation, genetic drift, and selective pressures also determine its persistence or decline.

Natural selection plays a key role in allele frequency. Dominant alleles linked to severe genetic disorders are often selected against. For example, the Huntington’s allele persists because symptoms typically appear after reproduction. In contrast, recessive alleles can remain common if they provide an advantage in heterozygous carriers. Sickle cell trait, for instance, grants resistance to malaria, allowing the allele to persist in regions where the disease is prevalent.

Historical events also shape allele frequencies through genetic drift, population bottlenecks, and founder effects. When a small group establishes a new population, certain alleles may become overrepresented by chance. The Amish population in the United States has a higher prevalence of certain rare genetic conditions due to their small founding population and genetic isolation. Similarly, population bottlenecks caused by disease outbreaks or migrations can drastically reduce genetic diversity, amplifying certain alleles while eliminating others. These factors illustrate that allele frequency is driven by evolutionary pressures and historical contingencies rather than dominance alone.