Heredity is the process by which traits are passed from parents to offspring. The instructions for these traits are encoded in segments of DNA called genes. Genes exist in different versions, known as alleles, which determine an organism’s physical and functional characteristics. An individual typically inherits two alleles for every gene, one from each parent. The interaction between these two alleles dictates the resulting trait.
What Defines a Dominant Allele
A dominant allele is a version of a gene whose trait is expressed even when only a single copy is present. This means the effect of the allele will manifest visually or functionally regardless of the second allele it is paired with. In classical genetics, the presence of one dominant allele is sufficient to determine the observable characteristic, or phenotype.
The mechanism behind dominance involves one allele effectively “masking” or overriding the presence of the other allele. This masking occurs because the dominant allele produces enough functional protein to create the full trait. Geneticists use a standardized notation where a capital letter, such as ‘A’, represents the dominant allele. Therefore, an individual with the combination ‘AA’ or ‘Aa’ will display the dominant trait.
The Contrast: Recessive Alleles
Understanding a dominant allele requires contrasting it with the recessive allele. A recessive allele is one whose associated trait is only expressed when two copies of it are present. If a dominant allele is also present, the recessive allele’s effects are completely hidden, or masked, from the phenotype.
The combination of alleles an individual possesses is called the genotype, while the resulting physical trait is the phenotype. When both inherited alleles are the same, the genotype is described as homozygous (e.g., ‘AA’ or ‘aa’). The heterozygous state (‘Aa’) involves one dominant and one recessive allele. In this pairing, the recessive allele is carried but remains unexpressed, meaning the organism exhibits the dominant trait.
Illustrative Examples of Dominant Traits
Dominant inheritance patterns are responsible for many common human physical characteristics. For instance, detached earlobes are governed by a dominant allele, while attached earlobes are recessive. Similarly, the ability to roll the tongue into a tube is a simple dominant trait, requiring only one copy of the responsible allele for expression.
In terms of appearance, a widow’s peak, a distinctive V-shaped hairline, is often determined by a dominant allele. Brown eye color is also dominant over blue eye color, though eye color is influenced by multiple genes. These examples highlight that a trait being dominant does not imply it is more common, but rather that it only requires one copy of the allele for expression.
Dominant inheritance also applies to certain genetic conditions, where inheriting a single copy of the mutated allele is enough to cause the disorder. Huntington’s Disease, a progressive neurodegenerative disorder, is a well-known example of an autosomal dominant condition. Similarly, achondroplasia, a common form of dwarfism, follows a dominant inheritance pattern. The presence of one affected allele overrides the normal allele, leading to the condition’s expression.
When Dominance Isn’t Complete
The simple dominant and recessive relationship provides a foundational model for inheritance, but not all gene interactions follow this strict pattern. Some alleles exhibit a relationship known as incomplete dominance. In these cases, the heterozygous individual displays a phenotype that is a blend or intermediate of the two homozygous traits. For example, crossing a plant with red flowers and one with white flowers might result in offspring with pink flowers.
A different variation is called codominance, where both alleles are fully expressed simultaneously, with neither one masking the other. The human ABO blood group system provides a classic example. An individual who inherits both the A allele and the B allele will have type AB blood, meaning both the A and B antigens are present on their red blood cells. These examples illustrate that genetic inheritance can be complex and involve multiple forms of allelic interaction.