The biological blueprint that determines individual characteristics is passed down through generations in a process called heredity. These instructions are stored within the DNA in segments known as genes, the fundamental units of inheritance. Genes contain the coding information necessary for the body to build specific proteins, which influence a wide range of traits, from hair texture to blood type. Every gene exists in different versions, called alleles. An individual inherits two alleles for every gene—one from each biological parent—and their interaction determines the resulting physical trait.
Defining Dominance and Recessiveness
The difference between dominant and recessive traits lies in the functional relationship between a pair of alleles. A dominant allele expresses its characteristic trait even when only a single copy is present, essentially overruling the instruction from the other allele.
In contrast, a recessive allele only expresses its specific trait when two identical copies of that allele are inherited. This means the recessive instruction is only followed if no dominant allele is present to mask it.
If paired with a dominant allele, the recessive version is still present in the genetic code, but its effect is hidden. For instance, the allele for a straight hairline is recessive and only appears when inherited from both parents. Conversely, a dominant trait, like a widow’s peak hairline, requires only one allele to be expressed.
The biological reason for this masking effect relates to protein production. A dominant allele usually codes for a functional protein, and one copy is sufficient to display the trait. Recessive alleles frequently contain a mutation resulting in a non-functional or absent protein. If a person has one functional (dominant) allele, it compensates for the non-working (recessive) copy, preventing the recessive trait from showing.
From Genotype to Observable Trait
The terms dominant and recessive describe how the underlying genetic makeup (genotype) translates into the observable characteristic (phenotype). Since every person inherits one allele from each parent, there are three possible combinations of alleles for any given gene. The genotype is represented by letters, using a capital letter for the dominant allele (e.g., ‘A’) and a lowercase letter for the recessive allele (e.g., ‘a’).
An individual who inherits two identical alleles is called homozygous (AA or aa). The homozygous dominant genotype (AA) and the heterozygous genotype (Aa)—consisting of one dominant and one recessive allele—both result in the dominant phenotype. This occurs because the single dominant allele is sufficient to determine the trait.
The recessive phenotype is only observed in an individual with the homozygous recessive genotype (aa). An individual who is heterozygous (Aa) for a recessive trait does not display the trait but possesses the recessive allele and can pass it on to their children. These individuals are referred to as genetic carriers. If two carriers (Aa) reproduce, there is a one-in-four chance that their child will inherit the two recessive alleles (aa) and express the recessive trait.
Relatable Examples of Inheritance Patterns
Many physical traits are governed by this simple dominant-recessive relationship. For eye color, the allele for brown eyes is dominant over the allele for blue eyes. A person will have brown eyes if they possess at least one brown eye allele, but they must inherit two blue eye alleles to have blue eyes.
Another common example is the shape of the earlobe, where detached earlobes are dominant over attached earlobes. These patterns also govern the inheritance of certain genetic disorders.
Huntington’s disease, a progressive neurodegenerative disorder, is an example of an autosomal dominant condition. Because the allele for the disease is dominant, a child only needs to inherit one copy of the faulty allele from either parent to develop the disorder.
Conversely, cystic fibrosis (CF), which affects the body’s cells that produce mucus and sweat, follows an autosomal recessive pattern. A person must inherit a non-functional CFTR gene allele from both parents to have the condition. A person with one normal allele and one CF allele is a carrier, remaining healthy while still able to pass the recessive allele to the next generation. This mechanism explains why a condition can seemingly skip a generation, as carriers show no symptoms.