A gene is a segment of DNA that holds the instructions for building a specific product, usually a protein, which determines a particular trait. Every individual inherits two copies of each gene, known as alleles, one from each parent. In the simplest form of inheritance, described by Gregor Mendel, one allele is fully dominant, masking the effect of the recessive allele in a heterozygous pairing. Many biological traits do not follow this straightforward dominant/recessive pattern, revealing a more complex interplay between inherited factors. These alternative patterns are categorized as non-Mendelian inheritance, where allele expression or the interaction between multiple genes deviates from the baseline model.
Allele Expression at a Single Locus: Codominance and Incomplete Dominance
Codominance and incomplete dominance involve the interaction between a pair of alleles located at the same physical position, or locus, on a chromosome. These patterns are characterized by how the heterozygous genotype translates into an observable physical trait, or phenotype. They differ fundamentally in whether the resulting phenotype is a blend or a simultaneous, full display of both parental traits.
Incomplete dominance results in a blending effect where the heterozygous individual exhibits a phenotype intermediate between the two homozygous parents. For instance, crossing a red-flowered snapdragon plant with a white-flowered plant yields offspring with pink flowers. The red allele is not fully dominant over the white allele, meaning the heterozygote produces a reduced amount of pigment, leading to the pink coloration. This creates a third, distinct phenotype that represents a compromise between the two pure traits.
Codominance occurs when both alleles are fully and equally expressed in the heterozygote, with no blending. A classic example is the human ABO blood group system, where the A and B alleles are codominant. An individual inheriting both the \(I^A\) and \(I^B\) alleles will have blood type AB, meaning their red blood cells possess both the A and B antigens simultaneously. Roan cattle also demonstrate codominance; a cross between a red-coated animal and a white-coated animal produces offspring with a roan coat, which is a mixture of individual red and white hairs.
Gene Interaction Across Loci: The Mechanism of Epistasis
Epistasis represents a higher level of genetic complexity, involving the functional interaction between two or more separate genes located at different loci. This phenomenon occurs when the activity of one gene masks, modifies, or prevents the expression of a gene at a second locus. The gene that performs the masking action is termed the epistatic gene, while the gene whose expression is altered or suppressed is known as the hypostatic gene.
The coat color in Labrador Retrievers illustrates this two-gene interaction. Two different genes, the \(B\) gene and the \(E\) gene, determine the three common coat colors: black, chocolate, and yellow. The \(B\) gene determines the type of pigment produced; the dominant \(B\) allele results in black pigment (eumelanin), and the recessive \(b\) allele results in a less dense, chocolate-colored pigment.
The \(E\) gene acts upstream by controlling whether that pigment is deposited into the hair shaft. The dominant \(E\) allele allows for pigment deposition, resulting in either a black or chocolate coat, depending on the \(B\) gene genotype. If a dog inherits the homozygous recessive genotype, \(ee\), the gene for pigment deposition is non-functional.
The \(ee\) genotype acts as an “off switch” for the color production pathway, preventing any pigment from being deposited into the fur. A dog with the \(ee\) genotype will have a yellow coat, regardless of whether its \(B\) gene genotype is \(BB\), \(Bb\), or \(bb\). In this scenario, the \(E\) gene is epistatic to the \(B\) gene, demonstrating how the product of one gene can override the expression of another gene located elsewhere in the genome.
Distinguishing the Patterns: Phenotypic Ratios and Real-World Examples
Geneticists rely on phenotypic ratios from controlled crosses to distinguish between these complex inheritance patterns. The standard Mendelian monohybrid cross between two heterozygotes yields a 3:1 phenotypic ratio, while a dihybrid cross results in a 9:3:3:1 ratio. Non-Mendelian inheritance patterns produce predictable deviations from these baseline ratios.
In both incomplete dominance and codominance, a monohybrid cross between two heterozygous individuals results in a 1:2:1 phenotypic ratio. For example, crossing two pink snapdragons yields red, pink, and white flowers in a 1:2:1 ratio. This ratio is unique because the phenotype ratio is identical to the genotypic ratio. This occurs because the heterozygote expresses a visible trait distinct from both homozygotes, meaning the three genotypes are directly linked to three different phenotypes.
The presence of epistasis is revealed in the phenotypic ratios of a dihybrid cross involving two genes. Instead of the standard 9:3:3:1 ratio, the phenotypic classes are modified because two or more genotypic classes produce the same phenotype. For example, in Labrador Retriever coat color, the expected 9:3:3:1 ratio is modified to a 9:3:4 phenotypic ratio (9 black: 3 chocolate: 4 yellow). This occurs because the \(B\_ee\) and \(bbee\) genotypes are grouped together into the single yellow phenotype. Other forms of epistasis, such as dominant epistasis, can result in ratios like 12:3:1.