Definition — What is a gene? Choose the most complete description reflecting classical and molecular genetics.

Introduction / Context:
“Gene” has evolved from a conceptual unit of heredity to a molecular entity encoded in DNA. Modern definitions reconcile function (heritable trait), structure (DNA sequence), and genomic context (chromosomal locus).

Given Data / Assumptions:

• Genes are located on chromosomes.
• Genes are DNA sequences that encode functional RNA or protein products.
• Genes act as heritable units influencing phenotype.

Concept / Approach:
Each option captures a correct facet. Classical genetics emphasizes “unit of inheritance.” Molecular biology defines genes as DNA segments specifying products (protein or functional RNA). Cytogenetics places genes along chromosomes; thus “all of the above” integrates the perspectives.

Step-by-Step Solution:

Verification / Alternative check:
Genome annotation treats genes as distinct loci with coordinates, transcripts, and products; inheritance studies link alleles at these loci to traits.

Why Other Options Are Wrong:

• Each single statement is incomplete alone; only the combined statement encompasses function, structure, and location.

Common Pitfalls:
Assuming genes must code for proteins only; many genes produce functional RNAs (rRNA, tRNA, lncRNA, microRNA).

Final Answer:
All of the above