Genetics is the study of heredity, exploring how traits are passed from parents to their offspring. This field examines the mechanisms of inheritance, revealing how biological characteristics are transmitted. Among various inheritance patterns, complete dominance is a fundamental concept for understanding trait expression. It describes a specific interaction between different gene versions that dictates an organism’s observable features.
The Concept of Complete Dominance
Complete dominance describes an inheritance pattern where one allele completely masks the effect of another allele in a heterozygous individual. The trait associated with the dominant allele will be visible, while the recessive allele’s trait will be hidden.
The dominant trait manifests whenever the dominant allele is present, whether in one or two copies. Conversely, the recessive trait appears only when an individual inherits two copies of the recessive allele.
Alleles, Genotypes, and Phenotypes
Genes are the fundamental units of heredity, carrying the instructions for building and maintaining an organism. Different versions of a gene are called alleles. For any given gene, an individual inherits two alleles, one from each parent.
The specific combination of alleles an individual possesses for a gene is known as their genotype. For example, if ‘A’ represents a dominant allele and ‘a’ represents a recessive allele, possible genotypes include AA (homozygous dominant), Aa (heterozygous), and aa (homozygous recessive). The observable trait or characteristic that results from this genetic makeup is called the phenotype. In complete dominance, both the homozygous dominant (AA) and heterozygous (Aa) genotypes will display the same dominant phenotype, while only the homozygous recessive (aa) genotype will show the recessive phenotype.
Illustrative Examples in Nature
Complete dominance is widely observed in nature, providing clear examples of how traits are inherited. A classic instance is Gregor Mendel’s pea plants, where the allele for tall stems (T) is dominant over the allele for short stems (t). A pea plant with at least one tall allele (TT or Tt) will grow tall, while only plants with two short alleles (tt) will be short.
Another common example involves human earlobe attachment. The allele for unattached earlobes is dominant, while the allele for attached earlobes is recessive. Individuals with at least one dominant allele will have unattached earlobes, even if they also carry the recessive allele. Brown eye color in humans is dominant over blue eyes, meaning a person with at least one brown eye allele will have brown eyes.
Distinguishing Complete Dominance from Other Patterns
Complete dominance is distinct from other inheritance patterns like incomplete dominance and codominance. In incomplete dominance, the heterozygous individual displays a phenotype that is an intermediate blend of the two parental traits. For instance, a cross between a red-flowered plant and a white-flowered plant might produce pink-flowered offspring. Here, neither allele completely masks the other; instead, they combine to create a new, blended phenotype.
Codominance occurs when both alleles in a heterozygous individual are fully and separately expressed, without blending. A well-known example is the human ABO blood group system, where individuals with both A and B alleles express both A and B antigens on their red blood cells, resulting in AB blood type. In this case, both traits are visible simultaneously.