A gene is a basic unit of heredity in living organisms. It is made up of DNA and carries the instructions for building and maintaining an organism's cells and passing traits from parents to offspring. Genes determine various characteristics, such as eye color, height, and even certain behaviors. Genes are organized into structures called chromosomes, which are found in the nucleus of cells. Each species has a specific number of chromosomes; for example, humans have 46. Understanding genes is crucial for fields like genetics, medicine, and evolution, as they help explain how traits are inherited and how species change over time.