Which of the following is a non-allelic gene interaction ?

Question

Accepted Answer

The correct option is 2: Epistasis

Epistasis is correctly identified as a non-allelic gene interaction because it fundamentally involves genes located at two or more **different loci** influencing the expression of a single phenotypic trait. In this type of genetic scenario, a gene at one locus (called the epistatic gene) suppresses, masks, or modifies the expression of a gene at a different, distinct locus (called the hypostatic gene). This demonstrates an interaction *between* different genes, not just between the alleles of a single gene, which is the definition of a non-allelic or intergenic interaction. A prime example is the inheritance of coat colour in Labrador retrievers, where one gene pair controls the colour pigment production (B/b), and a separate gene pair controls the deposition of the pigment (E/e). If a dog is homozygous recessive at the deposition locus (*ee*), it will have a yellow coat regardless of the colour genes (B or b), as the deposition gene masks the expression of the colour gene, demonstrating epistasis.

The incorrect option is 1: Incomplete dominance

Incomplete dominance is a classic example of an **allelic interaction** (or intralocus interaction), as it involves only the relationship between two alleles situated at the *same gene locus*. It occurs when the dominant allele does not completely mask the effects of the recessive allele, resulting in a heterozygous individual that exhibits an intermediate phenotype, which is a blend of the two homozygous phenotypes. For instance, in Mirabilis jalapa (four o'clock plant), a cross between red and white flowers produces pink flowers, which is a third, intermediate phenotype. The interaction is strictly contained to the two alleles of a single gene, not a gene-to-gene interaction involving separate loci.

The incorrect option is 3: Codominance

Codominance is also categorized as an **allelic interaction** (single-gene inheritance) because the interaction is confined to the alleles present at a *single gene locus*. Codominance describes the situation where both alleles in a heterozygote are fully and equally expressed, resulting in a phenotype that shows characteristics of both homozygous parents simultaneously. The human AB blood type, where both the A and B antigens are present on red blood cells, is the standard illustration of codominance (I$^A$I$^B$). Since the interaction is directly between the two inherited alleles of one particular gene, it does not involve non-allelic genes.

The incorrect option is 4: Multiple allelism

Multiple allelism is a term used to describe a condition where a *single gene* has more than two possible alleles within a population. Although there are multiple alternatives for the gene, all of these alleles, such as the I$^A$, I$^B$, and *i* alleles for human blood type, occupy the *same locus* on the homologous chromosomes. An individual still only inherits two alleles for that gene, and the interaction between those two alleles is allelic (either codominance or simple dominance). Therefore, multiple allelism is a population-level concept concerning the variation of a single gene, not an interaction between different genes at different loci.