Genes located on the same locus but having different expressions areA. Multiple allelesB. OncogenesC. PolygenesD. Codominants

The correct answer is A. Multiple alleles.

Explanation: Genes that are located on the same locus (position on a chromosome) but have different expressions are called multiple alleles. Multiple alleles refer to different forms of a gene that can exist at a single locus. While an individual can only have two alleles for each gene (one inherited from each parent), there can be more than two possible alleles in a population. For example, the ABO blood group system in humans has three alleles (IA, IB, and i) at the same locus, but any individual can carry at most two of them.

Here’s a breakdown of the other options:

B. Oncogenes: These are genes that have the potential to cause cancer when mutated or expressed at high levels. They are not related to allelic variations.
C. Polygenes: These are genes that contribute to a trait in a cumulative way, typically affecting traits like height or skin color, and are not related to different expressions of a single gene at the same locus.
D. Codominants: Codominance occurs when both alleles in a heterozygous individual are fully expressed, such as in the case of AB blood type, where both IA and IB alleles are expressed equally. However, this refers to allele interactions, not the presence of multiple alleles at the same locus.