Here's why:

* Physical Unit: Genes are segments of DNA that contain the instructions for building and maintaining an organism. They are located on chromosomes.

* Functional Unit: Genes code for specific proteins or functional RNA molecules, which carry out a vast array of functions within the cell and organism. These functions include:

* Determining traits: Genes influence our physical characteristics (like eye color, height) and many other aspects of our biology.

* Controlling development: Genes guide the growth and development of an organism from a single fertilized egg.

* Regulating metabolism: Genes play a key role in how our bodies process energy and nutrients.

Therefore, a gene is the fundamental unit of heredity, carrying the blueprints for our physical and functional characteristics.