Genes are the fundamental units of heredity. They are segments of DNA that contain the instructions for building and maintaining an organism. Here's a more detailed definition:

* Genes are specific sequences of nucleotides (A, T, C, G) within DNA. These sequences code for the production of proteins or RNA molecules, which perform various functions within the cell.

* Genes are located on chromosomes. Each chromosome contains many genes arranged linearly.

* Genes are passed down from parents to offspring. This inheritance pattern explains how traits are passed through generations.

* Genes can be altered through mutations. These changes in the DNA sequence can lead to different versions of a gene, which can affect the function of the protein it codes for.

In summary: Genes are like blueprints containing the instructions for building and maintaining an organism. They are the fundamental units of heredity, responsible for passing traits from parents to offspring.