The phenomenon in which a single gene may express more than one effect is known as:

Question

Accepted Answer

The correct option is (2):
Pleiotropy

The correct option is **Pleiotropy**. This term describes a fundamental genetic phenomenon where a single gene influences two or more distinct and seemingly unrelated phenotypic traits. The core concept is that a gene's primary product—which is typically a protein, such as an enzyme or a structural molecule—does not act in isolation but is essential for several different biological functions, pathways, or cellular components throughout the body. For instance, in the classic example of Phenylketonuria (PKU) in humans, a single defective gene for the enzyme phenylalanine hydroxylase leads not just to a buildup of phenylalanine but also results in intellectual disability, reduced hair and skin pigmentation, and various neurological problems. These diverse effects stem from the failure to produce one crucial enzyme, which is involved in different metabolic and developmental processes. Another excellent example is Sickle Cell Anemia, where a single mutation in the beta-globin gene affects the shape of red blood cells, leading to anemia, joint pain, increased susceptibility to infection, and damage to various organs, all stemming from that one initial gene defect. This complexity demonstrates how the failure of one gene can ripple across an organism's entire physiology to create a complex syndrome of multiple effects.

The incorrect option is (1):
Multiple allelism

Multiple allelism is a condition in which a population possesses more than two alternative forms of a gene (alleles) that can occupy the same locus on a chromosome. This phenomenon focuses on the *diversity* of gene *forms* within a population, not on the *range of effects* that a single, specific allele might have on an individual. The best-known example is the human ABO blood group system, where three common alleles—$I^A$, $I^B$, and $i$—exist in the population, though any single diploid individual only carries two of these alleles. Therefore, multiple allelism is a mechanism of genetic variation that results in multiple forms of a trait, while the question asks about the multifaceted phenotypic expression of one gene.

The incorrect option is (3):
Mosaicism

Mosaicism is a condition where an individual organism contains two or more populations of cells with different genotypes, all of which originated from a single fertilized egg (zygote). This difference arises due to a mutation or chromosomal change that occurs after fertilization in one of the somatic cells, leading to a lineage of genetically distinct cells alongside the original cell line. For example, a person with a mosaic disorder might have some body cells with a normal number of chromosomes and other cells with an abnormal number, such as in mosaic Down syndrome. This phenomenon is about genotypic heterogeneity *within* an individual's body tissues, not about the complex phenotypic effects resulting from a single, consistent gene variation.

The incorrect option is (4):
Polygeny

The term Polygeny, or polygenic inheritance, describes the inheritance of a single phenotypic trait that is determined by the cumulative action of two or more independent genes. In this type of inheritance, multiple genes located at different loci all work together to contribute to one overall characteristic, often leading to a continuous range of phenotypes. Classic polygenic traits include characteristics such as human height, skin color, and human intelligence. Because the question asks about *one gene* having *multiple effects*, while polygeny describes *multiple genes* determining *one trait*, the two concepts are the opposite of one another in terms of the gene-to-phenotype relationship.