Multiple alleles are located on:

Question

Accepted Answer

The correct option is 3: Same locus of homologous chromosomes

Alleles are fundamentally defined as the alternative forms or versions of a single gene. The specific, fixed physical position of any gene on a chromosome is known as its locus (plural: loci). In a diploid organism, chromosomes are present in homologous pairs—one chromosome from this pair is inherited from the mother and the other from the father. Critically, these two homologous chromosomes carry the same set of genes, and the gene for a particular trait is always found at the *same locus* on both chromosomes of the pair. The concept of "multiple alleles" refers to the presence of three or more variants of a single gene existing within a population (a classic example is the IA, IB, and i alleles governing the human ABO blood group). Even though many alleles may exist in the gene pool of a species, any one individual can only carry a maximum of two alleles for that gene—one on each chromosome of the homologous pair. Since all these variants are forms of the *same* gene, they must, by definition, all occupy the identical, corresponding locus on the homologous chromosomes.

The incorrect option is 1: Different loci on same chromosome

This option incorrectly mixes the definition of alleles with that of linked genes. Alleles are variants of a *single* gene, and thus must occupy a single locus. Conversely, different loci on the same chromosome are occupied by completely *separate* genes, which are referred to as linked genes. For instance, the gene for eye color and the gene for wing shape in fruit flies are linked genes, each at a distinct locus on the same chromosome. The multiple alleles (e.g., blue, brown, green) for the eye color gene itself are all located at the *same* locus.

The incorrect option is 2: Randomly on the chromosome

The location of genes on chromosomes is not random; it is a fixed and highly conserved characteristic for every species. This precise, unchanging position is what defines the gene's locus. Genetic mapping is based on the fact that gene order and location are stable features of the genome. If alleles were distributed randomly, there would be no predictable pattern of inheritance, and the fundamental processes of meiosis, such as the pairing of homologous chromosomes and crossing over, could not occur accurately.

The incorrect option is 4: On a single chromatid

While an allele *is* physically located on a chromatid, this answer is incomplete and misrepresents the core principle of allele location. A chromatid is merely one replicated strand of a chromosome, and a chromosome (before replication) or a pair of homologous chromosomes (after replication) carries the genetic information. The most significant feature of allele location is the *pair*—the relationship between the two alleles inherited from both parents. Stating that they are "on a single chromatid" ignores the existence and necessity of the homologous partner carrying the second allele at the same specific locus.