The appearance and physiological makeup of every living organism are determined by biological instructions passed down through heredity. The basic rules of how these inherited instructions are expressed determine the differences between a dominant and a recessive trait. The distinction lies in how many copies of a specific instruction are needed to produce a visible effect.
The Vocabulary of Inheritance
The fundamental unit of heredity is the gene, a specific section of DNA containing instructions for a particular characteristic. For most traits, an organism inherits two copies of this instruction, one from each parent. These variations of the gene are called alleles. The specific combination of alleles an individual possesses is their genotype, the internal genetic code. The observable physical manifestation of that code, such as eye color, is the phenotype.
How Dominant Traits Are Expressed
A dominant trait requires only a single copy of its corresponding allele to be expressed in the phenotype. When a dominant allele is present, it effectively masks the instruction from any other allele for that specific gene. For instance, the allele for brown eyes is dominant over the allele for blue eyes. If an individual inherits one brown and one blue eye allele, their eyes will be brown because the dominant instruction is followed.
How Recessive Traits Are Expressed
A recessive trait can only be expressed physically if an individual inherits two copies of the corresponding allele. The trait remains hidden if even a single dominant allele is present. For example, the allele for blue eyes is recessive, requiring a person to inherit the allele from both parents to have blue eyes. Individuals who possess one dominant and one recessive allele are considered carriers. Carriers do not show the recessive trait but can still pass the instruction on to their offspring.
Predicting Outcomes: Genotype and Phenotype
Dominant and recessive alleles allow for predictable patterns in how traits are passed through generations. When two individuals who are both carriers (Aa) for a recessive trait reproduce, their offspring inherit a random combination of alleles. If the dominant allele is (A) and the recessive is (a), the four possible genotypes are AA, Aa, aA, and aa, resulting in a 1:2:1 genotypic ratio. This translates into a 3:1 phenotypic ratio, a hallmark of simple Mendelian inheritance. Since three combinations contain at least one dominant allele, the dominant trait is expressed three times more often than the recessive trait (aa).
Beyond Simple Dominance
While the dominant/recessive relationship provides a foundational understanding, not all traits follow this simple pattern. Some traits exhibit non-Mendelian inheritance, where the expression of one allele does not completely mask the other. One exception is incomplete dominance, where the resulting phenotype is a blend of the two alleles, such as pink flowers resulting from red and white parents. Another pattern is codominance, where both alleles are fully and separately expressed at the same time. The human ABO blood group system is a classic example, as individuals with type AB blood express both the A and B alleles equally.