* Genes are located on chromosomes: Genes are segments of DNA that carry the instructions for building and maintaining an organism. These DNA segments are organized into structures called chromosomes.
* Almost all cells have chromosomes: Every cell in your body, with the exception of mature red blood cells, contains a complete set of chromosomes.
* Red blood cells lose their nucleus: Red blood cells lose their nucleus (and therefore their chromosomes) during development to make room for more hemoglobin, the protein that carries oxygen. This is why they don't contain genes.
So, while red blood cells are essential for oxygen transport, they are the only human cell type that doesn't have genes.
