Here's why:

* Genes are segments of DNA: They contain the instructions for building and maintaining an organism.

* They code for specific traits: Each gene carries the blueprint for a particular characteristic, like eye color or height.

* They are passed down from parents to offspring: This is how traits are inherited across generations.

While a gene is the fundamental unit, it's important to note that:

* Genes can be complex: They often have multiple versions (alleles) that contribute to a trait.

* Traits are often influenced by multiple genes: Many characteristics are determined by the interaction of several genes.

* Environment also plays a role: Factors like diet and lifestyle can affect how genes are expressed.

Therefore, while the gene is the smallest unit of inheritance, understanding the complexities of gene interactions and environmental influence is crucial for a complete picture of inheritance.