The basic units of heredity that direct the structure and function of all body cells are genes.

Here's why:

* Genes contain the instructions: Genes are segments of DNA that hold the code for building and maintaining an organism. They contain the instructions for creating proteins, which are the workhorses of the body.

* Proteins determine traits: Proteins carry out a vast array of functions, from building tissues to controlling chemical reactions. This means that genes indirectly determine many of an organism's traits, including physical characteristics, disease susceptibility, and even behavior.

* Passed down through generations: Genes are passed down from parents to offspring, ensuring that each generation inherits a unique combination of traits.

So, while DNA is the molecule that holds all the genetic information, genes are the specific units within DNA that provide the blueprints for building and running our bodies.