From ISOGG Wiki
The gene is the basic physical unit of inheritance. Genes are passed from parents to offspring and contain the information needed to specify traits. Genes are arranged, one after another, on structures called chromosomes. A chromosome contains a single, long DNA molecule, only a portion of which corresponds to a single gene. Humans have approximately 23,000 genes arranged on their chromosomes.
This article uses material in the public domain from the Talking Glossary of Genetic Terms and is reproduced courtesy of the National Human Genome Research Institute.