Genetics studies how characteristics are passed from one generation to the next. Biological features, from flower color to blood type, are determined by specific instructions encoded in DNA. Understanding the expression of these instructions, particularly dominance, is fundamental to grasping how an individual’s physical makeup is determined.
The Core Definition of Allelic Dominance
Dominance describes the relationship between the two versions of a gene, known as alleles, that an individual inherits for a specific trait. Since most organisms inherit two copies of every gene—one from each parent—they possess a pair of alleles for each trait.
The dominant allele is the one whose effect is expressed in the observable trait (phenotype), even when only one copy is present in the genetic makeup (genotype). This allele can effectively mask the presence of a different, non-expressed allele. Geneticists use a capital letter, such as ‘R’, to denote a dominant allele.
An organism’s genotype is either homozygous (two identical alleles, e.g., RR) or heterozygous (two different alleles, e.g., Rr). In a heterozygous state, the single dominant allele is sufficient to determine the phenotype. For instance, a pea plant with one allele for round seeds (R) and one for wrinkled seeds (r) will still have round seeds, demonstrating the dominance of the ‘R’ allele.
The Necessary Contrast: Understanding Recessiveness
Recessiveness is the counterpart to dominance, describing an allele that is only expressed when the dominant version is entirely absent. A lowercase letter, such as ‘r’, is used to represent the recessive allele in genetic notation.
The trait associated with a recessive allele only becomes visible in the phenotype if the individual inherits two copies of that allele, creating a homozygous recessive genotype (e.g., rr). Returning to the pea plant example, a plant will only develop wrinkled seeds if its genotype is ‘rr’.
In a heterozygous individual (Rr), the dominant allele provides enough functional instruction to produce the dominant phenotype, leaving the recessive allele’s effect hidden. Certain inherited conditions in humans, such as cystic fibrosis, follow this pattern, requiring a person to inherit two copies of the recessive allele to exhibit the disease.
Beyond Simple Dominance: Incomplete and Codominance
Not all genetic traits follow the straightforward pattern of one allele completely overriding the other. This leads to more nuanced inheritance patterns like incomplete dominance and codominance. Incomplete dominance is a scenario where the heterozygous genotype results in a phenotype that is an intermediate blend of the two parental traits.
A common example of incomplete dominance is the flower color of snapdragons. When a plant with red flowers is crossed with a plant with white flowers, the offspring are pink. The single allele for red pigment in the heterozygous plant is not enough to produce the full red color, resulting in the blended phenotype.
Codominance occurs when both alleles in the heterozygous state are fully and separately expressed at the same time. Instead of blending, the characteristics of both alleles are distinctly visible in the phenotype. A clear illustration of this is the human ABO blood group system, specifically the AB blood type.
A person with type AB blood has inherited one allele for the A antigen and one allele for the B antigen. Both the ‘A’ and ‘B’ alleles are equally expressed, meaning both A and B antigens are present on the surface of the red blood cells simultaneously.
Another example is the roan coat color in cattle. Here, both the alleles for red hair and white hair are expressed, resulting in a coat that has distinct red and white hairs interspersed, rather than a single blended color.