What is a Dominant Allele?


In genetics, alleles are alternative forms of genes that determine specific traits or characteristics of an organism. Alleles can be either dominant or recessive, with dominant alleles exerting their effects over recessive alleles in certain genetic contexts. This article aims to provide a detailed explanation of dominant alleles, their inheritance patterns, and their significance in genetics.

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Genetic inheritance involves the transfer of traits from parents to offspring. These traits are determined by genes, which are specific segments of DNA. Genes can exist in different forms, known as alleles. Dominant alleles are those that are expressed in the phenotype of an organism, while recessive alleles have their effects masked by dominant alleles.

Definition of a Dominant Allele

A dominant allele is an allele that, when present in an organism’s genotype, will be expressed in the phenotype, regardless of whether the other allele is dominant or recessive. This means that even if an organism has only one copy of a dominant allele, it will exhibit the corresponding trait or characteristic.

In genetics, dominant alleles are represented by uppercase letters, while recessive alleles are represented by lowercase letters. For example, the allele for brown eyes is dominant (B), while the allele for blue eyes is recessive (b). If an individual has the genotype BB or Bb, they will have brown eyes.

Dominant Allele Examples

There are numerous examples of dominant alleles in humans and other organisms. Some common examples include:

  • Brown hair color (dominant) vs. blonde hair color (recessive)
  • Attached earlobes (dominant) vs. unattached earlobes (recessive)
  • Widow’s peak hairline (dominant) vs. straight hairline (recessive)
  • Normal vision (dominant) vs. color blindness (recessive)

These examples demonstrate how the presence of a dominant allele can override the effects of a recessive allele, resulting in the dominant trait being expressed.

Inheritance Patterns of Dominant Alleles

The inheritance patterns of dominant alleles depend on whether the organism is homozygous dominant (has two copies of the dominant allele) or heterozygous (has one copy of the dominant allele and one copy of the recessive allele).

Homozygous Dominant: In this case, when both alleles are dominant, the organism will always exhibit the dominant trait in its phenotype. For example, if an individual has the genotype BB for brown eyes, they will have brown eyes.

Heterozygous: When an organism has one dominant allele and one recessive allele, the dominant allele will be expressed in the phenotype, while the recessive allele remains hidden. This is known as the law of dominance. For example, if an individual has the genotype Bb for eye color, they will have brown eyes due to the presence of the dominant allele.

Homozygous Dominant vs. Heterozygous

Homozygous dominant and heterozygous genotypes have different implications for the inheritance and expression of dominant alleles.

Homozygous Dominant (BB): Individuals with a homozygous dominant genotype have two copies of the dominant allele. This means that they will always pass on the dominant allele to their offspring, ensuring the expression of the dominant trait in subsequent generations.

Heterozygous (Bb): Heterozygous individuals carry one dominant allele and one recessive allele. When they reproduce, there is a 50% chance of passing on the dominant allele and a 50% chance of passing on the recessive allele. This results in a mix of individuals with the dominant trait and individuals who are carriers of the recessive allele but do not exhibit the trait themselves.

Co-Dominance and Incomplete Dominance

While dominant and recessive alleles are the two main types of alleles, there are also other forms of allele interaction known as co-dominance and incomplete dominance.

Co-Dominance: In co-dominance, both alleles in a heterozygous individual are fully expressed in the phenotype. This means that both traits associated with each allele are visible. An example of co-dominance is seen in blood types, where the A and B alleles are co-dominant, resulting in individuals with AB blood type.

Incomplete Dominance: In incomplete dominance, neither allele is completely dominant over the other, resulting in a blending of traits in the phenotype. For example, when a red flower (RR) is crossed with a white flower (WW), the resulting offspring will have pink flowers (RW).

Dominant Allele and Genetic Disorders

While dominant alleles are responsible for the expression of many desirable traits, they can also be associated with genetic disorders when the allele carries a harmful mutation. These disorders are known as dominant genetic disorders.

Examples of dominant genetic disorders include Huntington’s disease, Marfan syndrome, and neurofibromatosis. In these cases, individuals who inherit the mutated dominant allele from one parent will develop the disorder, even if they also have a normal allele from the other parent.

Mendelian Laws and Dominant Alleles

Dominant alleles play a crucial role in the principles of inheritance established by Gregor Mendel, known as Mendelian laws. Mendel’s laws describe the patterns of inheritance for dominant and recessive alleles.

Law of Segregation: According to this law, individuals inherit two alleles for each trait, one from each parent. These alleles separate during gamete formation, with each gamete carrying only one allele for each trait. This law explains why offspring have a 50% chance of inheriting a dominant allele.

Law of Independent Assortment: This law states that the inheritance of one trait is independent of the inheritance of other traits. It explains how alleles segregate independently during gamete formation, leading to the random combination of alleles in offspring.

Evolutionary Significance of Dominant Alleles

Dominant alleles can have significant implications for the process of evolution. As dominant alleles are expressed in the phenotype, they can lead to the rapid spread of advantageous traits within a population.

Additionally, dominant alleles can determine an organism’s ability to survive and reproduce in specific environments. If a dominant allele confers a survival advantage, individuals with that allele are more likely to pass it on to future generations, leading to an increase in the frequency of the dominant allele within the population.


Dominant alleles are an essential concept in genetics, determining which traits are expressed in an organism’s phenotype. They can have a profound impact on inheritance patterns, genetic disorders, and the process of evolution. Understanding the nature and inheritance of dominant alleles is crucial for comprehending the complexity of genetic traits and their transmission from one generation to the next.

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