Genetic traits are determined by genes, which are segments of DNA inherited from parents. These genes come in different versions called alleles, and the concept of genetic dominance describes how these different allele versions interact within an individual to produce an observable characteristic, known as a phenotype.
Understanding Genetic Dominance
Genetic dominance manifests in several distinct ways, each describing a specific interaction between alleles at a single gene location. Complete dominance occurs when one allele completely masks the presence of another allele in a heterozygous individual. For instance, in pea plants, the allele for purple flowers will entirely obscure the allele for white flowers, resulting in purple blooms even if only one purple allele is present.
Incomplete dominance presents a different scenario where neither allele is fully dominant, leading to a blended phenotype in heterozygotes. A classic example involves snapdragon flowers, where a cross between red-flowered and white-flowered plants produces offspring with pink flowers. The red and white alleles combine to create an intermediate color. Codominance, conversely, involves both alleles being fully and simultaneously expressed in the heterozygote. Human ABO blood groups illustrate this, where individuals with both A and B alleles express both A and B antigens on their red blood cells, resulting in AB blood type.
Classifying Principles of Dominance
The “primary principle of dominance” refers to complete dominance, a foundational concept first described by Gregor Mendel. This forms the basis of many simple inherited traits.
“Secondary principles of dominance” are considered variations or extensions of this fundamental complete dominance. Incomplete dominance and codominance fall into this category.
Concepts Distinct from Secondary Principles
While related to gene expression, several genetic concepts are distinct from the secondary principles of dominance because they involve interactions beyond alleles at a single gene locus. Epistasis, for example, describes a situation where the expression of one gene modifies or completely masks the expression of a separate gene located elsewhere on the genome. This interaction is between different genes, not between different alleles of the same gene, which is the focus of dominance. Labrador retriever coat color provides an example, where one gene determines pigment presence and another gene dictates the color of that pigment.
Pleiotropy is another distinct concept, referring to a single gene influencing multiple, seemingly unrelated phenotypic traits. Here, one gene mutation can lead to a cascade of diverse effects throughout an organism. This differs from dominance, which focuses on how two alleles of one gene determine a single trait. Phenylketonuria (PKU), a human genetic disorder, illustrates pleiotropy as a single gene defect can cause intellectual disability, light skin, and eczema.
Polygenic inheritance also stands apart from dominance principles, as it involves multiple genes collectively contributing to a single complex trait. Traits like human height, skin color, or intelligence are often influenced by many genes, each contributing a small additive effect. This cumulative action of several genes contrasts with dominance, which specifically describes the interaction between alleles at a single gene position to shape a particular characteristic.